\\\Poise comes when you ignore excessive negative feedbacks///

Home insurance

2008-07-13T23:43:00.000-07:00

Home insurance, also commonly called hazard insurance or homeowners insurance (often abbreviated in the real estate industry as HOI), is the type of property insurance that covers private homes. It is an insurance policy that combines various personal insurance protections, which can include losses occurring to one's home, its contents, loss of its use (additional living expenses), or loss of other personal possessions of the homeowner, as well as liability insurance for accidents that may happen at the home.
The cost of homeowners insurance often depends on what it would cost to replace the house and which additional riders—additional items to be insured—are attached to the policy. The insurance policy itself is a lengthy contract, and names what will and what will not be paid in the case of various events. Typically, claims due to earthquakes, floods, "Acts of God", or war (whose definition typically includes a nuclear explosion from any source) are excluded. Special insurance can be purchased for these possibilities, including flood insurance and earthquake insurance. Insurance must be updated to the present and existing value at whatever inflation up or down, and an appraisal paid by the insurance company will be added on to the policy premium.
The home insurance policy is usually a term contract—a contract that is in effect for a fixed period of time. The payment the insured makes to the insurer is called the premium. The insured must pay the insurer the premium each term. Most insurers charge a lower premium if it appears less likely the home will be damaged or destroyed: for example, if the house is situated next to a fire station, or if the house is equipped with fire sprinklers and fire alarms. Perpetual insurance, which is a type of home insurance without a fixed term, can also be obtained in certain areas.
In the United States, most home buyers borrow money in the form of a mortgage loan, and the mortgage lender always requires that the buyer purchase homeowners insurance as a condition of the loan, in order to protect the bank if the home were to be destroyed. Anyone with an insurable interest in the property should be listed on the policy. In some cases the mortagagee will waive the need for the mortgagor to carry homeowner's insurance if the value of the land exceeds the amount of the mortgage balance. In a case like this even the total destruction of any buildings would not affect the ability of the lender to be able to foreclose and recover the full amount of the loan.
The insurance crisis in Florida has meant that some waterfront property owners in that state have had to make that decision due to the high cost of premiums.
As described in Wiening et al, prior to the 1950s, there were separate policies for the various perils that could affect a home. A homeowner would have had to purchase separate policies covering fire losses, theft, personal property, and the like. During the 1950s, policy forms were developed, allowing the homeowner to purchase all the insurance they needed on one complete policy. However, these policies varied by insurance company, and were difficult to comprehend. The need for standardization grew so great that a private company based in Jersey City, New Jersey, Insurance Services Office, also known as the ISO, was formed in 1971 to provide risk information and issued a simplified homeowners policy for resell to insurance companies. These policies have been amended over the years until currently, the ISO has seven standardized homeowners insurance forms in general and consistent use . Of these HO-3 is the most common policy followed by HO-4 and HO-6. Others that are less used, though still significant, are HO-1, HO-2, HO-5, and HO-8. Each is summarized below:
HO-1
A limited policy that offers varying degrees of coverage but only for items specifically outlined in the policy. These might be used to cover a valuable object found in the home, such as a painting.
HO-2
Similar to HO-1, HO-2 is a limited policy in that it covers specific portions of a house against damage. The coverage is usually a "named perils" policy, which lists the events that would be covered. As above, these factors must be spelled out in the policy.
HO-3
This policy is the most commonly written policy for a homeowner and is designed to cover all aspects of the home, structure and its contents as well as any liability that may arise from daily use, as well as any visitors who may encounter accident or injury on the premises. Covered aspects as well as limits of liability must be clearly spelled out in the policy to insure proper coverage. The coverage is usually called "all risk". Also called an "open perils" policy.
HO-4
This is commonly referred to as renters insurance or renter's coverage. Similar to HO-6, this policy covers those aspects of the apartment and its contents not specifically covered in the blanket policy written for the complex. This policy can also cover liabilities arising from accidents and intentional injuries for guests as well as passers-by up to 150' of the domicile. Common coverage areas are events such as lightning, riot, aircraft, explosion, vandalism, smoke, theft, windstorm or hail, falling objects, volcanic eruption, snow, sleet, and weight of ice.
HO-5
This policy, similar to HO-3, covers a home (not a condo or apartment), the homeowner and its possessions as well as any liability that might arise from visitors or passers-by. This coverage is differentiated in that it covers a wider breadth and depth of incidents and losses than an HO-3.
HO-6
As a form of supplemental homeowner's insurance, HO-6, also known as a Condominium Coverage, is designed especially for the owners of condos. It includes coverage for the part of the building owned by the insured and for the property housed therein of the insured. Designed to span the gap between what the homeowner's association might cover in a blanket policy written for an entire neighborhood and those items of importance to the insured, typically the HO-6 covers liability for residents and guests of the insured in addition to personal property. The liability coverage, depending on the underwriter, premium paid, and other factors of the policy, can cover incidents up to 150' from the insured property, all valuables within the home from theft, fire or water damage or other forms of loss. It is important to read the Associations By-laws to determine the total amount of insurance needed on your dwelling.
HO-8
It is usually called "older home" insurance. It lets house owners with higher replacement cost than the market value insure them at the lower market value rate.
In addition, a Dwelling Fire policy is generally available for non-commercial owners of rented houses, covering property damage to the structure, and sometimes to the owner's personal property (such as appliances and furnishings). The owner's liability is generally extended from their own primary home insurance, and does not comprise part of the Dwelling Fire policy. It is a counterpart to the HO-4 renter's policy.

Travel insurance

2008-07-01T03:16:00.000-07:00

Travel insurance is insurance that is intended to cover medical expenses, financial (such as money invested in nonrefundable pre-payments), and other losses incurred while traveling, either within one's own country, or internationally.

Travel insurance can usually be arranged at the time of booking of a trip to cover exactly the duration of that trip or a more extensive, continuous insurance can be purchased from (most often) travel insurance companies, travel agents or directly from travel suppliers such as cruiselines or tour operators. However, travel insurance purchased from travel suppliers tends to be less inclusive than insurance offered by insurance companies.

Travel insurance often offers coverage for a variety of travelers. Student travel, business travel, leisure travel, adventure travel, cruise travel, and international travel are all various options that can be insured.

The most common risks that are covered by travel insurance are:

Medical expenses
Emergency evacuation/repatriation
Overseas funeral expenses
Accidental death, injury or disablement benefit
Cancellation
Curtailment
Delayed departure
Loss, theft or damage to personal possessions and money (including travel documents)
Delayed baggage (and emergency replacement of essential items)
Legal assistance
Personal liability and rental car damage excess
Some travel policies will also provide cover for additional costs, although these vary widely between providers.

And in addition, often separate insurance can be purchased for specific costs such as:

pre-existing medical conditions (e.g. asthma, diabetes)
high risk sports (e.g. skiing, scuba-diving)
travel to high risk countries (e.g. due to war or natural disasters or acts of terrorism)
Common Exclusions:

pre-existing medical conditions
war or terrorism - but some plans may cover this risk
pregnancy related expenses
injury or illness caused by alcohol or drug use
Travel insurance can also provide helpful services, often 24 hours a day, 7 days a week that can include concierge services and emergency travel assistance.

Typically travel insurance for the duration of a journey costs approximately 5-7% of the cost of the trip.

Vehicle insurance

2008-06-13T03:49:00.000-07:00

is insurance purchased for cars, trucks, and other vehicles. Its primary use is to provide protection against losses incurred as a result of traffic accidents and against liability that could be incurred in an accident.In many jurisdictions it is compulsory to have vehicle insurance before using or keeping a motor vehicle on public roads. Most jurisdictions relate insurance to both the car and the driver, however the degree of each varies greatly.
A 1994 study by Jeremy Jackson and Roger Blackman showed, consistent with the risk homeostasis theory, that increased accident costs caused large and significant reductions in accident frequencies.
Vehicle insurance can cover some or all of the following items:
The insured party
The insured vehicle
Third parties (car and people)
Different policies specify the circumstances under which each item is covered. For example, a vehicle can be insured against theft, fire damage, or accident damage independently.
An excess payment, also known as a deductible, is the fixed contribution you must pay each time your car is repaired through your car insurance policy. Normally the payment is made directly to the accident repair "garage" (The term "garage" refers to an establishment where vehicles are serviced and repaired) when you collect the car. If one's car is declared to be a "write off" ("write off" is commonly used in motor insurance to describe a vehicle which is cheaper to replace than to repair), the insurance company will deduct the excess agreed on the policy from the settlement payment it makes to you.
If the accident was the other driver's fault, and this is accepted by the third party's insurer, you'll be able to reclaim your excess payment from the other person's insurance company. If the other driver is uninsured, a policy's minimum limits include coverage for the uninsured/underinsured motorist(s) at fault.
A compulsory excess is the minimum excess payment your insurer will accept on your insurance policy. Minimum excesses vary according to your personal details, driving record and insurance company.
In order to reduce your insurance premium, you may offer to pay a higher excess than the compulsory excess demanded by your insurance company. Your voluntary excess is the extra amount over and above the compulsory excess that you agree to pay in the event of a claim on the policy. As a bigger excess reduces the financial risk carried by your insurer, your insurer is able to offer you a significantly lower premium.
Basis of premium charges:-
Depending on the jurisdiction, the insurance premium can be either mandated by the government or determined by the insurance company in accordance to a framework of regulations set by the government. Often, the insurer will have more freedom to set the price on physical damage coverages than on mandatory liability coverages.
When the premium is not mandated by the government, it is usually derived from the calculations of an actuary based on statistical data. The premium can vary depending on many factors that are believed to have an impact on the expected cost of future claims.Those factors can include the car characteristics, the coverage selected (deductible, limit, covered perils), the profile of the driver (age, gender, driving history) and the usage of the car (commute to work or not, predicted annual distance driven).
Gender
Men average more miles driven per year than women do, and have a proportionally higher accident involvement at all ages. Insurance companies cite women's lower accident involvement in keeping the youth surcharge lower for young women drivers than for their male counterparts, but adult rates are generally unisex. Reference to the lower rate for young women as "the women's discount" has caused confusion that was evident in news reports on a recently defeated EC proposal to make it illegal to consider gender in assessing insurance premiums.Ending the discount would have made no difference to most women's premiums.

Age
Teenage drivers who have no driving record will have higher car insurance premiums. However young drivers are often offered discounts if they undertake further driver training on recognised courses, such as the Pass Plus scheme in the UK. In the U.S. many insurers offer a good grade discount to students with a good academic record and resident student discounts to those who live away from home. Generally insurance premiums tend to become lower at the age of 25. Senior drivers are often eligible for retirement discounts reflecting lower average miles driven by this age group.

Distance
Some car insurance plans do not differentiate in regard to how much the car is used. However, methods of differentiation would include:

Reasonable estimation
Several car insurance plans rely on a reasonable estimation of the average annual distance expected to be driven which is provided by the insured. This discount benefits drivers who drive their cars infrequently but has no actuarial value since it is unverified.

Odometer-based systems
Cents Per Mile Now(1986) advocates classified odometer-mile rates. After the company's risk factors have been applied and the customer has accepted the per-mile rate offered, customers buy prepaid miles of insurance protection as needed, like buying gallons of gasoline. Insurance automatically ends when the odometer limit (recorded on the car’s insurance ID card) is reached unless more miles are bought. Customers keep track of miles on their own odometer to know when to buy more. The company does no after-the-fact billing of the customer, and the customer doesn't have to estimate a "future annual mileage" figure for the company to obtain a discount. In the event of a traffic stop, an officer could easily verify that the insurance is current by comparing the figure on the insurance card to that on the odometer.
Critics point out the possibility of cheating the system by odometer tampering. Although the newer electronic odometers are difficult to roll back, they can still be defeated by disconnecting the odometer wires and reconnecting them later. However, as the Cents Per Mile Now website points out:
As a practical matter, resetting odometers requires equipment plus expertise that makes stealing insurance risky and uneconomical. For example, in order to steal 20,000 miles of continuous protection while paying for only the 2,000 miles from 35,000 miles to 37,000 miles on the odometer, the resetting would have to be done at least nine times to keep the odometer reading within the narrow 2,000-mile covered range. There are also powerful legal deterrents to this way of stealing insurance protection. Odometers have always served as the measuring device for resale value, rental and leasing charges, warranty limits, mechanical breakdown insurance, and cents-per-mile tax deductions or reimbursements for business or government travel. Odometer tampering—detected during claim processing—voids the insurance and, under decades-old state and federal law, is punishable by heavy fines and jail.
Under the cents-per-mile system, rewards for driving less are delivered automatically without need for administratively cumbersome and costly GPS technology. Uniform per-mile exposure measurement for the first time provides the basis for statistically valid rate classes. Insurer premium income automatically keeps pace with increases or decreases in driving activity, cutting back on resulting insurer demand for rate increases and preventing today's windfalls to insurers when decreased driving activity lowers costs but not premiums.

Life insurance

2008-06-10T02:48:00.000-07:00

Life insurance or life assurance is a contract between the policy owner and the insurer, where the insurer agrees to pay a sum of money upon the occurrence of the insured individual's or individuals' death or other event, such as terminal illness or critical illness. In return, the policy owner (or policy payer) agrees to pay a stipulated amount called a premium at regular intervals or in lump sums. There may be designs in some countries where bills and death expenses plus catering for after funeral expenses should be included in Policy Premium. In the United States, the predominant form simply specifies a lump sum to be paid on the insured's demise.
As with most insurance policies, life insurance is a contract between the insurer and the policy owner (policyholder) whereby a benefit is paid to the designated Beneficiary (or Beneficiaries) if an insured event occurs which is covered by the policy. To be a life policy the insured event must be based upon life (or lives) of the people named in the policy.
Insured events that may be covered include:
Sickness
Life policies are legal contracts and the terms of the contract describe the limitations of the insured events. Specific exclusions are often written into the contract to limit the liability of the insurer; for example claims relating to suicide, fraud, war, riot and civil commotion.
Life based contracts tend to fall into two major categories:
Protection policies - designed to provide a benefit in the event of specified event, typically a lump sum payment. A common form of this design is term insurance.
Investment policies - where the main objective is to facilitate the growth of capital by regular or single premiums. Common forms (in the US anyway) are whole life, universal life, and variable life policies.
Parties to contract
There is a difference between the insured and the policy owner (policy holder), although the owner and the insured are often the same person. For example, if Joe buys a policy on his own life, he is both the owner and the insured. But if Jane, his wife, buys a policy on Joe's life, she is the owner and he is the insured. The policy owner is the guarantee and he or she will be the person who will pay for the policy. The insured is a participant in the contract, but not necessarily a party to it.
The beneficiary receives policy proceeds upon the insured's death. The owner designates the beneficiary, but the beneficiary is not a party to the policy. The owner can change the beneficiary unless the policy has an irrevocable beneficiary designation. With an irrevocable beneficiary, that beneficiary must agree to any beneficiary changes, policy assignments, or cash value borrowing.
In cases where the policy owner is not the insured (also referred to as the cestui qui vit or CQV), insurance companies have sought to limit policy purchases to those with an "insurable interest" in the CQV. For life insurance policies, close family members and business partners will usually be found to have an insurable interest. The "insurable interest" requirement usually demonstrates that the purchaser will actually suffer some kind of loss if the CQV dies. Such a requirement prevents people from benefiting from the purchase of purely speculative policies on people they expect to die. With no insurable interest requirement, the risk that a purchaser would murder the CQV for insurance proceeds would be great. In at least one case, an insurance company which sold a policy to a purchaser with no insurable interest (who later murdered the CQV for the proceeds), was found liable in court for contributing to the wrongful death of the victim (Liberty National Life v. Weldon, 267 Ala.171 (1957)).
Contract terms
Special provisions may apply, such as suicide clauses wherein the policy becomes null if the insured commits suicide within a specified time (usually two years after the purchase date; some states provide a statutory one-year suicide clause). Any misrepresentations by the insured on the application is also grounds for nullification. Most US states specify that the contestability period cannot be longer than two years; only if the insured dies within this period will the insurer have a legal right to contest the claim on the basis of misrepresentation and request additional information before deciding to pay or deny the claim.
The face amount on the policy is the initial amount that the policy will pay at the death of the insured or when the policy matures, although the actual death benefit can provide for greater or lesser than the face amount. The policy matures when the insured dies or reaches a specified age (such as 100 years old).
Costs, insurability, and underwriting
The insurer (the life insurance company) calculates the policy prices with intent to fund claims to be paid and administrative costs, and to make a profit. The cost of insurance is determined using mortality tables calculated by actuaries. Actuaries are professionals who employ actuarial science, which is based in mathematics (primarily probability and statistics). Mortality tables are statistically-based tables showing expected annual mortality rates. It is possible to derive life expectancy estimates from these mortality assumptions. Such estimates can be important in taxation regulation.
The three main variables in a mortality table have been age, gender, and use of tobacco. More recently in the US, preferred class specific tables were introduced. The mortality tables provide a baseline for the cost of insurance. In practice, these mortality tables are used in conjunction with the health and family history of the individual applying for a policy in order to determine premiums and insurability. Mortality tables currently in use by life insurance companies in the United States are individually modified by each company using pooled industry experience studies as a starting point. In the 1980s and 90's the SOA 1975-80 Basic Select & Ultimate tables were the typical reference points, while the 2001 VBT and 2001 CSO tables were published more recently. The newer tables include separate mortality tables for smokers and non-smokers and the CSO tables include separate tables for preferred classes.
Recent US select mortality tables predict that roughly 0.35 in 1,000 non-smoking males aged 25 will die during the first year of coverage after underwriting.Mortality approximately doubles for every extra ten years of age so that the mortality rate in the first year for underwritten non-smoking men is about 2.5 in 1,000 people at age 65.Compare this with the US population male mortality rates of 1.3 per 1,000 at age 25 and 19.3 at age 65 (without regard to health or smoking status).
The mortality of underwritten persons rises much more quickly than the general population. At the end of 10 years the mortality of that 25 year-old, non-smoking male is 0.66/1000/year. Consequently, in a group of one thousand 25 year old males with a $100,000 policy, all of average health, a life insurance company would have to collect approximately $50 a year from each of a large group to cover the relatively few expected claims. (0.35 to 0.66 expected deaths in each year x $100,000 payout per death = $35 per policy). Administrative and sales commissions need to be accounted for in order for this to make business sense. A 10 year policy for a 25 year old non-smoking male person with preferred medical history may get offers as low as $90 per year for a $100,000 policy in the competitive US life insurance market.
The insurance company receives the premiums from the policy owner and invests them to create a pool of money from which it can pay claims and finance the insurance company's operations. Contrary to popular belief, the majority of the money that insurance companies make comes directly from premiums paid, as money gained through investment of premiums can never, in even the most ideal market conditions, vest enough money per year to pay out claims.[citation needed] Rates charged for life insurance increase with the insured's age because, statistically, people are more likely to die as they get older.
Given that adverse selection can have a negative impact on the insurer's financial situation, the insurer investigates each proposed insured individual unless the policy is below a company-established minimum amount, beginning with the application process. Group Insurance policies are an exception.
This investigation and resulting evaluation of the risk is termed underwriting. Health and lifestyle questions are asked. Certain responses or information received may merit further investigation. Life insurance companies in the United States support the Medical Information Bureau (MIB), which is a clearinghouse of information on persons who have applied for life insurance with participating companies in the last seven years. As part of the application, the insurer receives permission to obtain information from the proposed insured's physicians.
Underwriters will determine the purpose of insurance. The most common is to protect the owner's family or financial interests in the event of the insured's demise. Other purposes include estate planning or, in the case of cash-value contracts, investment for retirement planning. Bank loans or buy-sell provisions of business agreements are another acceptable purpose.
Life insurance companies are never required by law to underwrite or to provide coverage to anyone, with the exception of Civil Rights Act compliance requirements. Insurance companies alone determine insurability, and some people, for their own health or lifestyle reasons, are deemed uninsurable. The policy can be declined (turned down) or rated.[citation needed] Rating increases the premiums to provide for additional risks relative to the particular insured.[citation needed]
Many companies use four general health categories for those evaluated for a life insurance policy. These categories are Preferred Best, Preferred, Standard, and Tobacco.[citation needed] Preferred Best is reserved only for the healthiest individuals in the general population. This means, for instance, that the proposed insured has no adverse medical history, is not under medication for any condition, and his family (immediate and extended) have no history of early cancer, diabetes, or other conditions.[citation needed] Preferred means that the proposed insured is currently under medication for a medical condition and has a family history of particular illnesses.[citation needed] Most people are in the Standard category.[citation needed] Profession, travel, and lifestyle factor into whether the proposed insured will be granted a policy, and which category the insured falls. For example, a person who would otherwise be classified as Preferred Best may be denied a policy if he or she travels to a high risk country.[citation needed] Underwriting practices can vary from insurer to insurer which provide for more competitive offers in certain circumstances.
Life insurance contracts are written on the basis of utmost good faith. That is, the proposer and the insurer both accept that the other is acting in good faith. This means that the proposer can assume the contract offers what it represents without having to fine comb the small print and the insurer assumes the proposer is being honest when providing details to underwriter.
Death proceeds
Upon the insured's death, the insurer requires acceptable proof of death before it pays the claim. The normal minimum proof required is a death certificate and the insurer's claim form completed, signed (and typically notarized).[citation needed] If the insured's death is suspicious and the policy amount is large, the insurer may investigate the circumstances surrounding the death before deciding whether it has an obligation to pay the claim.
Proceeds from the policy may be paid as a lump sum or as an annuity, which is paid over time in regular recurring payments for either a specified period or for a beneficiary's lifetime.

The Health insurance

2008-06-01T22:52:00.000-07:00

The term health insurance is generally used to describe a form of insurance that pays for medical expenses. It is sometimes used more broadly to include insurance covering disability or long-term nursing or custodial care needs. It may be provided through a government-sponsored social insurance program, or from private insurance companies. It may be purchased on a group basis (e.g., by a firm to cover its employees) or purchased by individual consumers. In each case, the covered groups or individuals pay premiums or taxes to help protect themselves from high or unexpected healthcare expenses. Similar benefits paying for medical expenses may also be provided through social welfare programs funded by the government.
Health insurance works by estimating the overall risk of healthcare expenses and developing a routine finance structure (such as a monthly premium or annual tax) that will ensure that money is available to pay for the healthcare benefits specified in the insurance agreement. The benefit is administered by a central organization, most often either a government agency or a private or not-for-profit entity operating a health plan.
The concept of health insurance was proposed in 1694 by Hugh the Elder Chamberlen from the Peter Chamberlen family. In the late 19th century, "accident insurance" began to be available, which operated much like modern disability insurance.This payment model continued until the start of the 20th century in some jurisdictions (like California), where all laws regulating health insurance actually referred to disability insurance.
Accident insurance was first offered in the United States by the Franklin Health Assurance Company of Massachusetts. This firm, founded in 1850, offered insurance against injuries arising from railroad and steamboat accidents. Sixty organizations were offering accident insurance in the US by 1866, but the industry consolidated rapidly soon thereafter. While there were earlier experiments, the origins of sickness coverage in the US effectively date from 1890. The first employer-sponsored group disability policy was issued in 1911.
Before the development of medical expense insurance, patients were expected to pay all other health care costs out of their own pockets, under what is known as the fee-for-service business model. During the middle to late 20th century, traditional disability insurance evolved into modern health insurance programs. Today, most comprehensive private health insurance programs cover the cost of routine, preventive, and emergency health care procedures, and also most prescription drugs, but this was not always the case.
Hospital and medical expense policies were introduced during the first half of the 20th century. During the 1920s, individual hospitals began offering services to individuals on a pre-paid basis, eventually leading to the development of Blue Cross organizations.The predecessors of today's Health Maintenance Organizations (HMOs) originated beginning in 1929, through the 1930s and on during World War II.
A Health insurance policy is a contract between an insurance company and an individual. The contract can be renewable annually or monthly. The type and amount of health care costs that will be covered by the health plan are specified in advance, in the member contract or Evidence of Coverage booklet. The individual policy-holder's payment obligations may take several forms:-

Premium: The amount the policy-holder pays to the health plan each month to purchase health coverage.
Deductible: The amount that the policy-holder must pay out-of-pocket before the health plan pays its share. For example, a policy-holder might have to pay a $500 deductible per year, before any of their health care is covered by the health plan. It may take several doctor's visits or prescription refills before the policy-holder reaches the deductible and the health plan starts to pay for care.
Copayment: The amount that the policy-holder must pay out of pocket before the health plan pays for a particular visit or service. For example, a policy-holder might pay a $45 copayment for a doctor's visit, or to obtain a prescription. A copayment must be paid each time a particular service is obtained.
Coinsurance: Instead of paying a fixed amount up front (a copayment), the policy-holder must pay a percentage of the total cost. For example, the member might have to pay 20% of the cost of a surgery, while the health plan pays the other 80%. Because there is no upper limit on coinsurance, the policy-holder can end up owing very little, or a significant amount, depending on the actual costs of the services they obtain.
Exclusions: Not all services are covered. The policy-holder is generally expected to pay the full cost of non-covered services out of their own pocket.
Coverage limits: Some health plans only pay for health care up to a certain dollar amount. The policy-holder may be expected to pay any charges in excess of the health plan's maximum payment for a specific service. In addition, some plans have annual or lifetime coverage maximums. In these cases, the health plan will stop payment when they reach the benefit maximum, and the policy-holder must pay all remaining costs.
Out-of-pocket maximums: Similar to coverage limits, except that in this case, the member's payment obligation ends when they reach the out-of-pocket maximum, and the health plan pays all further covered costs. Out-of-pocket maximums can be limited to a specific benefit category (such as prescription drugs) or can apply to all coverage provided during a specific benefit year.
Capitation: An amount paid by an insurer to a health care provider, for which the provider agrees to treat all members of the insurer.
In-Network Provider: A health care provider on a list of providers preselected by the insurer. The insurer will offer discounted coinsurance or copayments, or additional benefits, to a plan member to see an in-network provider. Generally, providers in network are providers who have a contract with the insurer to accept rates further discounted from the "usual and customary" charges the insurer pays to out-of-network providers.

Prescription drug plans are a form of insurance offered through some employer benefit plans in the US, where the patient pays a copayment and the prescription drug insurance part or all of the balance for drugs covered in the formulary of the plan.
Some, if not most, health care providers in the United States will agree to bill the insurance company if patients are willing to sign an agreement that they will be responsible for the amount that the insurance company doesn't pay. The insurance company pays out of network providers according to "reasonable and customary" charges, which may be less than the provider's usual fee. The provider may also have a separate contract with the insurer to accept what amounts to a discounted rate or capitation to the provider's standard charges. It generally costs the patient less to use an in-network provider.

Historically, HMOs tended to use the term "health plan", while commercial insurance companies used the term "health insurance". A health plan can also refer to a subscription-based medical care arrangement offered through health maintenance organization, HMO, PPO, or POS plan. These plans are similar to pre-paid dental, pre-paid legal, and pre-paid vision plans. Pre-paid health plans typically pay for a fixed number of services (for instance, $300 in preventive care, a certain number of days of hospice care or care in a skilled nursing facility, a fixed number of home health visits, a fixed number of spinal manipulation charges, etc.) The services offered are usually at the discretion of a utilization review nurse who is often contracted through the managed care entity providing the subscription health plan. This determination may be made either prior to or after hospital admission (concurrent utilization review).

Comprehensive health insurance pays a percentage (may be 100, 90, 80, 70, 60, 50, percent) of the cost of hospital and physician charges after a deductible (usually applies to hospital charges) or a co-pay (usually applies to physician charges, but may apply to some hospital services) is met by the insured. These plans are generally expensive because of the high potential benefit payout — $1,000,000 to 5,000,000 is common — and because of the vast array of covered benefits.Scheduled health insurance plans are not meant to replace a traditional comprehensive health insurance plans and are more of a basic policy providing access to day-to-day health care such as going to the doctor or getting a prescription drug. In recent years, these plans have taken the name mini-med plans or association plans. These plans may provide benefits for hospitalization and surgical, but these benefits will be limited. Scheduled plans are not meant to be effective for catastrophic events. These plans cost much less then comprehensive health insurance. They generally pay limited benefits amounts directly to the service provider, and payments are based upon the plan's "schedule of benefits". Annual benefits maximums for a typical scheduled health insurance plan may range from $1,000 to $25,000.

Social health insurance:-

Social health insurance (SHI) is a method for financing health care costs through a social insurance program based on the collection of funds contributed by individuals, employers, and sometimes government subsidies.It is one of the five main ways that health care systems are funded.
SHI systems are characterized by the presence of sickness funds which usually receive a proportional contribution of their members' wages. With this insurance contributions these funds pay medical costs of their members, to the extent that the services are included in the, sometimes nationally defined, benefit package. Affiliation to such funds is usually based on professional, geographic, religious/political and/or non-partisan criteria. (Saltman 2004, p.8-9) Usually, there are user fees for several health care services to inhibit usage and to keep social health insurance affordable.
Otto von Bismarck was the first to make social health insurance mandatory on a national scale (in Germany), but social health insurance was already common for many centuries before among guilds mainly in continental Europe. Countries with SHI systems include Austria, Belgium, Germany, France, and Luxembourg. Generally, their per capita health expenditures is higher than in tax-based systems. Such predominantly tax-based systems tend to be called "National Health Systems" (or, "Beveridge systems", named after William Beveridge, who was in charge of writing the Beveridge report). Some see this label as inappropriate as the health care systems have been largely decentralized beyond the national level in these countries.

E-commerce

2008-05-23T04:29:00.000-07:00

Electronic commerce, commonly known as e-commerce or eCommerce, consists of the buying and selling of products or services over electronic systems such as the Internet and other computer networks. The amount of trade conducted electronically has grown extraordinarily since the spread of the Internet. A wide variety of commerce is conducted in this way, spurring and drawing on innovations in electronic funds transfer, supply chain management, Internet marketing, online transaction processing, electronic data interchange (EDI), inventory management systems, and automated data collection systems. Modern electronic commerce typically uses the World Wide Web at least at some point in the transaction's lifecycle, although it can encompass a wider range of technologies such as e-mail as well.
A large percentage of electronic commerce is conducted entirely electronically for virtual items such as access to premium content on a website, but most electronic commerce involves the transportation of physical items in some way. Online retailers are sometimes known as e-tailers and online retail is sometimes known as e-tail. Almost all big retailers have electronic commerce presence on the World Wide Web.
Electronic commerce that is conducted between businesses is referred to as Business-to-business or B2B. B2B can be open to all interested parties (e.g. commodity exchange) or limited to specific, pre-qualified participants (private electronic market).
Electronic commerce is generally considered to be the sales aspect of e-business. It also consists of the exchange of data to facilitate the financing and payment aspects of the business transactions.Early development
The meaning of electronic commerce has changed over the last 30 years. Originally, electronic commerce meant the facilitation of commercial transactions electronically, using technology such as Electronic Data Interchange (EDI) and Electronic Funds Transfer (EFT). These were both introduced in the late 1970s, allowing businesses to send commercial documents like purchase orders or invoices electronically. The growth and acceptance of credit cards, automated teller machines (ATM) and telephone banking in the 1980s were also forms of electronic commerce. From the 1990s onwards, electronic commerce would additionally include enterprise resource planning systems (ERP), data mining and data warehousing.
Perhaps it is introduced from the Telephone Exchange Office, or maybe not.The earliest example of many-to-many electronic commerce in physical goods was the Boston Computer Exchange, a marketplace for used computers launched in 1982. The first online information marketplace, including online consulting, was likely the American Information Exchange, another pre-Internet online system introduced in 1991.In the United States, some electronic commerce activities are regulated by the Federal Trade Commission (FTC). These activities include the use of commercial e-mails, online advertising and consumer privacy. The CAN-SPAM Act of 2003 establishes national standards for direct marketing over e-mail. The Federal Trade Commission Act regulates all forms of advertising, including online advertising, and states that advertising must be truthful and non-deceptive.Using its authority under Section 5 of the FTC Act, which prohibits unfair or deceptive practices, the FTC has brought a number of cases to enforce the promises in corporate privacy statements, including promises about the security of consumers’ personal information.As result, any corporate privacy policy related to e-commerce activity may be subject to enforcement by the FTC.Contemporary electronic commerce involves everything from ordering "digital" content for immediate online consumption, to ordering conventional goods and services, to "meta" services to facilitate other types of electronic commerce.
On the consumer level, electronic commerce is mostly conducted on the World Wide Web. An individual can go online to purchase anything from books, grocery to expensive items like real estate. Another example will be online banking like online bill payments, buying stocks, transferring funds from one account to another, and initiating wire payment to another country. All these activities can be done with a few keystrokes on the keyboard.
On the institutional level, big corporations and financial institutions use the internet to exchange financial data to facilitate domestic and international business. Data integrity and security are very hot and pressing issues for electronic commerce these days.

Financial of accountancy

2008-05-15T03:15:00.000-07:00

Financial accountancy (or financial accounting) is the field of accountancy concerned with the preparation of financial statements for decision makers, such as stockholders, suppliers, banks, employees, government agencies, owners, and other stakeholders. The fundamental need for financial accounting is to reduce principal-agent problem by measuring and monitoring agents' performance and reporting the results to interested users.
Financial accountancy is used to prepare accounting information for people outside the organization or not involved in the day to day running of the company. Managerial accounting provides accounting information to help managers make decisions to manage the business.
Financial accountancy is governed by both local and international accounting standards.
Basic accounting concepts:-
Financial accountants produce financial statements based on Generally Accepted Accounting Principles (GAAP) of a respective country.
Financial accounting serves following purposes:
producing general purpose financial statements
provision of information used by management of a business entity for decision making, planning and performance evaluation
for meeting regulatory requirements
Meaning of the accounting equation:-
The value of a company can be understood simply as the useful assets that ownership of a company entitles one to claim. This value is known as Owners' Equity. Some assets of a company, however, cannot be claimed as equity by the owners of a company because other people have legal claim to them - for example if the company has borrowed money from the bank. The value of a resource claimable by a non-owner is called a liability. All of the Assets of a company can be claimed by someone, whether owner or not, so the sum of a company's equity and its liabilities must equal the value of its Assets. Thus the accounting equation describes what portion of a company's assets can be claimed by the owners.
Various account types are classified as 'credit' or 'debit' depending on the role they play in the accounting equation.
Assets = Liabilities + Equity or Assets - Liabilities - Equity = 0
Another way of stating it is:
Equity = Assets - Liabilities
which can be interpreted as: "Equity is what is left if all assets have been sold and all liabilities have been paid".
There are several related professional qualifications in the field of financial accountancy including:
Qualified Accountant qualifications (Chartered Certified Accountant(ACCA), Chartered Accountant (CA) and Certified Public Accountant (CPA))
CCA Chartered Cost Accountant (cost control) designation offered by the American Academy of Financial Management.
Accounting analyst:-
An accounting analyst evaluates and interprets public company financial statements. Public companies issue these (10-k) annual financial statements as required by the Security and Exchange Commission. The statements include the balance sheet, the income statement, the statement of cash flows and the notes to the financial statements. Specifically, the notes to the financial statements contain considerable quantitative detail supporting the financial statements along with narrative information.
This individual has extensive training in understanding financial accounting principles for public companies based on generally accepted accounting principles as provided by the Financial Accounting Standards Board. Or, he/she may have additional experience in applying international accounting standards based on the rules put out by the International Accounting Standards Board.
As an example, the accounting analyst may work for a financial research company evaluating differing financial accounting principles and how they influence the company's reported wealth.
The accounting analyst will most likely hold a Masters Degree in Accounting MSAcc and will have specialized in the financial accounting area. Or, the analyst may have a MBA degree with an Accounting specialization.
In addition, the analyst may hold the Chartered Certified Accountant (ACCA) or Certified Public Accountant (CPA) or Chartered Accountant (CA or ACA) designation.

Types of accounts

2008-05-15T03:07:00.000-07:00

In accountancy, an account is a label used for recording and reporting a quantity of almost anything. Most often it is a record of an amount of money owned or owed by or to a particular person or entity, or allocated to a particular purpose. It may represent amounts of money that have actually changed hands, or it may represent an estimate of the values of assets, or it may be a combination of these.
Types of accounts:-

Asset accounts: represent the different types of economic resources owned by a business, common examples of Asset accounts are cash, cash in bank, building, inventory, prepaid rent, goodwill, accounts receivable.
Liability accounts: represent the different types of economic obligations by a business, such as accounts payable, bank loan, bonds payable, accrued interest.
Equity accounts: represent the residual equity of a business (after deducting from Assets all the liabilities) including Retained Earnings and Appropriations.
Revenue or Income accounts: represent the company's gross earnings and common examples include Sales, Service revenue and Interest Income.
Expense accounts: represent the company's expenditures to enable itself to operate. Common examples are electricity and water, rentals, depreciation, doubtful accounts, interest, insurance.
Contra-accounts: from the term contra, meaning to deduct, the value of which are opposite the 5 above mentioned types of accounts. For instance, a contra-asset account is Accumulated depreciation. This label represent deductions to a relatively permanent asset like Building.

Chart of accounts:-

A chart of accounts (COA) is a list of all accounts tracked by a single accounting system, and should be designed to capture financial information to make good financial decisions. Each account in the chart is assigned a unique identifier, typically an account number. Each account in the Anglo-Saxon chart is classified into one of the five categories: assets, liabilities, equity, income and expenses.

finance

2008-05-07T01:57:00.000-07:00

An entity whose income exceeds its expenditure can lend or invest the excess income. On the other hand, an entity whose income is less than its expenditure can raise capital by borrowing or selling equity claims, decreasing its expenses, or increasing its income. The lender can find a borrower, a financial intermediary, such as a bank or buy notes or bonds in the bond market. The lender receives interest, the borrower pays a higher interest than the lender receives, and the financial intermediary pockets the difference.
A bank aggregates the activities of many borrowers and lenders. A bank accepts deposits from lenders, on which it pays the interest. The bank then lends these deposits to borrowers. Banks allow borrowers and lenders, of different sizes, to coordinate their activity. Banks are thus compensators of money flows in space.
A specific example of corporate finance is the sale of stock by a company to institutional investors like investment banks, who in turn generally sell it to the public. The stock gives whoever owns it part ownership in that company. If you buy one share of XYZ Inc, and they have 100 shares outstanding (held by investors), you are 1/100 owner of that company. Of course, in return for the stock, the company receives cash, which it uses to expand its business in a process called "equity financing". Equity financing mixed with the sale of bonds (or any other debt financing) is called the company's capital structure.
Finance is used by individuals (personal finance), by governments (public finance), by businesses (corporate finance), as well as by a wide variety of organizations including schools and non-profit organizations. In general, the goals of each of the above activities are achieved through the use of appropriate financial instruments, with consideration to their institutional setting.
Finance is one of the most important aspects of business management. Without proper financial planning a new enterprise is unlikely to be successful. Managing money (a liquid asset) is essential to ensure a secure future, both for the individual and an organization.
Corporate finance
Managerial or corporate finance is the task of providing the funds for a corporation's activities. For small business, this is referred to as SME finance. It generally involves balancing risk and profitability, while attempting to maximize an entity's wealth and the value of its stock.
Long term funds are provided by ownership equity and long-term credit, often in the form of bonds. The balance between these forms the company's capital structure. Short-term funding or working capital is mostly provided by banks extending a line of credit.
Another business decision concerning finance is investment, or fund management. An investment is an acquisition of an asset in the hope that it will maintain or increase its value. In investment management -- in choosing a portfolio -- one has to decide what, how much and when to invest. To do this, a company must:
Identify relevant objectives and constraints: institution or individual goals, time horizon, risk aversion and tax considerations;
Identify the appropriate strategy: active v. passive -- hedging strategy
Measure the portfolio performance
Financial management is duplicate with the financial function of the Accounting profession. However, financial accounting is more concerned with the reporting of historical financial information, while the financial decision is directed toward the future of the firm.
Personal finance
Questions in personal finance revolve around
How much money will be needed by an individual (or by a family) at various points in the future?
Where will this money come from (e.g. savings or borrowing)?
How can people protect themselves against unforeseen events in their lives, and risk in financial markets?
How can family assets be best transferred across generations (bequests and inheritance)?
How do taxes (tax subsidies or penalties) affect personal financial decisions?
How does credit affect an individual's financial standing?
How can one plan for a secure financial future in an environment of economic instability?
Personal financial decisions may involve paying for education, financing durable goods such as real estate and cars, buying insurance, e.g. health and property insurance, investing and saving for retirement.
Personal financial decisions may also involve paying for a loan.
Capital
Capital, in the financial sense, is the money which gives the business the power to buy goods to be used in the production of other goods or the offering of a service.
Sources of capital
Long Term - usually above 7 years
Share Capital
Mortgage
Retained Profit
Venture Capital
Debenture
Sale & Leaseback
Project Finance
Medium Term - usually between 2 and 7 years
Term Loans
Leasing
Hire Purchase
Short Term - usually under 2 years
Bank Overdraft
Trade Credit
Deferred Expenses
Factoring
Capital market
Long-term funds are bought and sold:
Shares
Debentures
Long-term loans, often with a mortgage bond as security
Reserve funds
Euro Bonds
Money market
Financial institutions can use short-term savings to lend out in the form of short-term loans:
Credit on open account
Bank overdraft
Short-term loans
Bills of exchange
Factoring of debtors
Borrowed capital
This is capital which the business borrows from institutions or people, and includes debentures:
Redeemable debentures
Irredeemable debentures
Debentures to bearer
Hardcore debentures
Own capital
This is capital that owners of a business (shareholders and partners, for example) provide:
Preference shares/hybrid source of finance
Ordinary preference shares
Cumulative preference shares
Participating preference share
Ordinary shares
Bonus shares
Founders' shares
Differences between shares and debentures
Shareholders are effectively owners; debenture-holders are creditors.
Shareholders may vote at AGMs and be elected as directors; debenture-holders may not vote at AGMs or be elected as directors.
Shareholders receive profit in the form of dividends; debenture-holders receive a fixed rate of interest.
If there is no profit, the shareholder does not receive a dividend; interest is paid to debenture-holders regardless of whether or not a profit has been made.
In case of dissolution of firms debenture holders are paid first as compared to shareholder.
Fixed capital
This is money which is used to purchase assets that will remain permanently in the business and help it to make a profit.
Factors determining fixed capital requirements
Nature of business
Size of business
Stage of development
Capital invested by the owners
location of that area
Working capital
This is money which is used to buy stock, pay expenses and finance credit.
Factors determining working capital requirements
Size of business
Stage of development
Time of production
Rate of stock turnover ratio
Buying and selling terms
Seasonal consumption
Seasonal production

Transcode

2008-04-28T04:40:00.000-07:00

Transcoding is the direct digital-to-digital conversion from one (usually lossy) codec to another. It involves decoding/decompressing the original data to a raw intermediate format (i.e. PCM for audio or YUV for video), in a way that mimics standard playback of the lossy content, and then re-encoding this into the target format. The simplest way to do transcoding is to decode a bitstream into YUV format using a compatible decoder and then encode the data using an encoder of a different standard. A better way to transcode is to change the bitstream format from one standard to another without its undergoing the complete decoding and encoding process. Many algorithms exist to achieve this.
Transrating is a process similar to transcoding in which files are coded to a lower bitrate without changing video formats. Need for transrating arises from the fact that the bitrate requirement varies from channel to channel because of vastness in the compression standards in use. Changing the picture size of video is known as transsizing.
Transcoding is the direct digital-to-digital conversion from one (usually lossy) codec to another. It involves decoding/decompressing the original data to a raw intermediate format (i.e. PCM for audio or YUV for video), in a way that mimics standard playback of the lossy content, and then re-encoding this into the target format. The simplest way to do transcoding is to decode a bitstream into YUV format using a compatible decoder and then encode the data using an encoder of a different standard. A better way to transcode is to change the bitstream format from one standard to another without its undergoing the complete decoding and encoding process. Many algorithms exist to achieve this.
Transrating is a process similar to transcoding in which files are coded to a lower bitrate without changing video formats. Need for transrating arises from the fact that the bitrate requirement varies from channel to channel because of vastness in the compression standards in use. Changing the picture size of video is known as transsizing.
Compression artifacts are cumulative; therefore transcoding between lossy codecs causes a progressive loss of quality with each successive generation. For this reason, it is generally discouraged unless unavoidable. For instance, if an individual owns a digital audio player that does not support a particular format (e.g., Apple iPod and Ogg Vorbis), then the only way for the owner to use content encoded in that format is to transcode it to a supported format. It is better to retain a copy in a lossless format (such as TTA, FLAC or WavPack), and then encode directly from the lossless source file to the lossy formats required.
Data transformation
Data transformation can be divided into two steps:
data mapping maps data elements from the source to the destination and captures any transformation that must occur
code generation that creates the actual transformation program
Data element to data element mapping is frequently complicated by complex transformations that requires one-to-many and many-to-one transformation rules.
The code generation step takes the data element mapping specification and creates an executable program that can be run on a computer system. Code generation can also create transformation in easy-to-maintain computer languages such as Java or XSLT.
When the mapping is indirect via a mediating data model, the process is also called data mediation.There are numerous languages available for performing data transformation. Many transformational languages require a grammar to be provided. In many cases the grammar is structured using something closely resembling Backus–Naur Form (BNF). There are numerous languages available for such purposes varying in their accessibility (cost) and general usefulness. Examples of such languages include:
XSLT - the XML transformation language
TXL - prototyping language-based descriptions using source transformation
It should be noted that though transformational languages are typically best suited for transformation, something as simple as regular expressions can be used to achieve useful transformation. Textpad supports the use of regular expressions with arguments. This would allow all instances of a particular pattern to be replaced with another pattern using parts of the original pattern.Another advantage to using regular expressions is that they will not fail the null transform test. That is, using your transformational language of choice, run a sample program through a transformation that doesn't perform any transformations. Many transformational languages will fail this test.In other words, all instances of a function invocation of foo with three arguments, followed by a function invocation with two invocations would be replaced with a single function invocation using some or all of the original set of arguments.A really general solution to handling this is very hard because such preprocessor directives can essentially edit the underlying language in arbitrary ways. However, because such directives are not, in practice, used in completely arbitrary ways, one can build practical tools for handling preprocessed languages. The DMS Software Reengineering Toolkit is capable of handling structured macros and preprocessor conditionals.

Multimeter

2008-04-21T22:08:00.000-07:00

A multimeter or a multitester, also known as a volt/ohm meter or VOM, is an electronic measuring instrument that combines several functions in one unit. A standard multimeter may include features such as the ability to measure voltage, current and resistance. There are two categories of multimeters, analog multimeters and digital multimeters (often abbreviated DMM.)
A multimeter can be a hand-held device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as batteries, motor controls, appliances, power supplies, and wiring systems.
Multimeters are available in a wide ranges of features and prices. Cheap multimeters can cost less than US$10, while the top of the line multimeters can cost more than US$5000.
The resolution of a multimeter is often specified in "digits" of resolution. For example, the term 5½ digits refers to the number of digits displayed on the readout of a multimeter.
By convention, a half digit can display either a zero or a one, while a three-quarters digit can display a numeral higher than a one but not nine. Commonly, a three-quarters digit refers to a maximum count of 3 or 5. The fractional digit is always the most significant digit in the displayed value. A 5½ digit multimeter would have five full digits that display values from 0 to 9 and one half digit that could only display 0 or 1.Such a meter could show positive or negative values from 0 to 199,999. A 3¾ digit meter can display a quantity from 0 to 3,999 or 5,999, depending on the manufacturer.
Resolution of analog multimeters is limited by the width of the scale pointer, vibration of the pointer, parallax observation errors, and the accuracy of printing of scales. Resistance measurements, in particular, are of low precision due to the typical resistance measurement circuit which compresses the scale at the higher resistance values. Mirrored scales and larger meter movements are used to improve resolution; two and a half to three digits equivalent resolution is usual (and may be adequate for the limited precision actually necessary for most measurements).
While a digital display can easily be extended in precision, the extra digits are of no value if not accompanied by care in the design and calibration of the analog portions of the multimeter. Meaningful high-resolution measurements require a good understanding of the instrument specifications, good control of the measurement conditions, and traceability of the calibration of the instrument.
Digital multimeters generally take measurements with superior accuracy to their analog counterparts. Analog multimeters typically measure with three to five percent accuracy.[citation needed] Standard portable digital multimeters claim to be capable of taking measurements with an accuracy of 0.5% on DC voltage and current scales. Mainstream bench-top multimeters make claim es to have as great accuracy as ±0.01%. Laboratory grade instruments can have accuracies in the parts per million figures.
Manufacturers can provide calibration services so that new meters may be purchased with a certificate of calibration indicating the meter has been adjusted to standards traceable to the National Institute of Standards and Technology. Such manufacturers usually provide calibration services after sales, as well, so that older equipment may be recertified. Multimeters used for critical measurements may be part of a metrology program to assure calibration.
The current load, or how much current is drawn from the circuit being tested may affect a multimeter's accuracy. A small current draw usually will result in more precise measurements. With improper usage or too much current load, a multimeter may be damaged therefore rendering its measurements unreliable and substandard.
Meters with electronic amplifiers in them, such as all digital multimeters and transistorized analog meters, have a standardized input impedance usually considered high enough not to disturb the circuit tested. This is often one million ohms, or ten million ohms. The standard input impedance allows use of external probes to extend the direct-current measuring range up to tens of thousands of volts.
Analog multimeters of the moving pointer type draw current from the circuit under test to deflect the meter pointer. The impedance of the meter varies depending on the basic sensitivity of the meter movement and the range which is selected. For example, a meter with a 20,000 ohms/volt sensitivity will have an input resistance of two million ohms on the 100 volt range (100 V * 20,000 ohms/volt = 2,000,000 ohms). Low-sensitivity meters are useful for general purpose testing especially in power circuits, where source impedances are low compared to the meter impedance. Measurements in signal circuits generally require higher sensitivity so as not to load down the circuit under test with the meter impedance.
The sensitivity of a meter is also a measure of the lowest voltage, current or resistance that can be measured with it. For general-purpose digital multimeters, a full-scale range of several hundred millivolts AC or DC is common, but the minimum full-scale current range may be several hundred milliamps. Since general-purpose mulitmeters have only two-wire resistance measurements, which do not compensate for the effect of the lead wire resistance, measurements below a few tens of ohms will be of low accuracy. The upper end of multimeter measurement ranges varies considerably by manufacturer; generally measurements over 1000 volts, over 10 amperes, or over 100 megohms would require a specialized test instrument, as would accurate measurement of currents on the order of microamperes or less.
Since the basic indicator system in either an analog or digital meter responds to DC only, a multimeter includes an AC to DC conversion circuit for making alternating current measurements. Basic multimeters may utilize a rectifier circuit, calibrated to evaluate the average value of a rectified sine wave. User guides for such meters will give correction factors for some simple waveforms, to allow the correct root mean square (RMS) equivalent value to be calculated for the average-responding meter. More expensive multimeters will include an AC to DC converter that responds to the RMS value of the waveform for a wide range of possible waveforms; the user manual for the meter will indicate the limits of the crest factor and frequency for which the meter calibration is valid. RMS sensing is necessary for measurement s of non-sinusoidal quantities, such as found in audio signals, or in variable-frequency drives.
Modern multimeters are often digital due their accuracy, durability and extra features.
In a DMM the signal under test is converted to a voltage and an amplifier with an electronically controlled gain preconditions the signal.
A DMM displays the quantity measured as a number, which prevents parallax errors.
The inclusion of solid state electronics, from a control circuit to small embedded computers, has provided a wealth of convenience features in modern digital meters. Commonly available measurement enhancements include:
Auto-ranging, which selects the correct range for the quantity under test so that the most significant digits are shown. For example, a four-digit multimeter would automatically select an appropriate range to display 1.234 instead of 0.012, or overloading. Auto-ranging meters usually include a facility to 'freeze' the meter to a particular range, because a measurement that causes frequent range changes is distracting to the user.
Auto-polarity for direct-current readings, shows if the applied voltage is positive (agrees with meter lead labels) or negative (opposite polarity to meter leads).
Sample and hold, which will latch the most recent reading for examination after the instrument is removed from the circuit under test.
Current-limited tests for voltage drop across semiconductor junctions. While not a replacement for a transistor tester, this facilitates testing diodes and a variety of transistor types.
A graphic representation of the quantity under test, as a bar graph. This makes go/no-go testing easy, and also allows spotting of fast-moving trends.
A low-bandwidth oscilloscope.
Automotive circuit testers, including tests for automotive timing and dwell signals.
Simple data acquisition features to record maximum and minimum readings over a given period, or to take a number of samples at fixed intervals.
Modern meters may be interfaced with a personal computer by IrDA links, RS-232 connections, USB, or an instrument bus such as IEEE-488. The interface allows the computer to record measurements as they are made. Some DMM's can store measurements and upload them to a computer.The first digital multimeter was manufactured in 1955 by Non Linear Systems.
Modern multimeters are often digital due their accuracy, durability and extra features.
In a DMM the signal under test is converted to a voltage and an amplifier with an electronically controlled gain preconditions the signal.
A DMM displays the quantity measured as a number, which prevents parallax errors.
The inclusion of solid state electronics, from a control circuit to small embedded computers, has provided a wealth of convenience features in modern digital meters. Commonly available measurement enhancements include:
Auto-ranging, which selects the correct range for the quantity under test so that the most significant digits are shown. For example, a four-digit multimeter would automatically select an appropriate range to display 1.234 instead of 0.012, or overloading. Auto-ranging meters usually include a facility to 'freeze' the meter to a particular range, because a measurement that causes frequent range changes is distracting to the user.
Auto-polarity for direct-current readings, shows if the applied voltage is positive (agrees with meter lead labels) or negative (opposite polarity to meter leads).
Sample and hold, which will latch the most recent reading for examination after the instrument is removed from the circuit under test.
Current-limited tests for voltage drop across semiconductor junctions. While not a replacement for a transistor tester, this facilitates testing diodes and a variety of transistor types.A graphic representation of the quantity under test, as a bar graph. This makes go/no-go testing easy, and also allows spotting of fast-moving trends.
A low-bandwidth oscilloscope.
Automotive circuit testers, including tests for automotive timing and dwell signals.
Simple data acquisition features to record maximum and minimum readings over a given period, or to take a number of samples at fixed intervals.
Modern meters may be interfaced with a personal computer by IrDA links, RS-232 connections, USB, or an instrument bus such as IEEE-488. The interface allows the computer to record measurements as they are made. Some DMM's can store measurements and upload them to a computer.The first digital multimeter was manufactured in 1955 by Non Linear Systems.
A multimeter can utilize a variety of test probes to connect to the circuit or device under test. Crocodile clips, retractable hook clips, and pointed probes are the three most common attachments. The connectors are attached to flexible, thickly-insulated leads that are terminated with connectors appropriate for the meter. Handheld meters typically use shrouded or recessed banana jacks, while benchtop meters may use banana jacks or BNC connectors.
Meters which measure high voltages or current may use non-contact attachment mechanism to trade accuracy for safety. Clamp meters provide a coil that clamps around a conductor in order to measure the current flowing through it.
Almost every multimeter includes a fuse, which will generally prevent damage to the multimeter if it is overloaded. A common error when operating a multimeter is to set the meter to measure resistance or current and then connect it directly to a low-impedance voltage source; meters without protection are quickly damaged by such errors and may cause injury to the operator.
Digital meters are category rated based on their intended application, as set forth by the CEN EN61010 standard.There are four categories:
Category I: used where current levels are low.
Category II: used on residential branch circuits.
Category III: used on permanently installed loads such as distribution panels, motors, and appliance outlets.
Category IV: used on locations where current levels are high, such as service entrances, main panels, and house meters.
Each category also specifies maximum transient voltages for selected measuring ranges in the meter.Category-rated meters also feature protections from over-current faults.
Multimeters were invented in the early 1920's as radio receivers and other vacuum tube electronic devices became more common. As modern systems become more complicated, the multimeter is becoming more complex or may be supplemented by more specialized equipment in a technician's toolkit. For example, where a general-purpose multimeter might only test for short-circuits, conductor resistance and some coarse measure of insulation quality, a modern technician may use a hand-held analyzer to test several parameters in order to validate the performance of a network cable.

The Mind

2008-04-14T05:22:00.000-07:00

Mind collectively refers to the aspects of intellect and consciousness manifested as combinations of thought, perception, memory, emotion, will and imagination; mind is the stream of consciousness. It includes all of the brain's conscious processes. This denotation sometimes includes, in certain contexts, the working of the human unconscious or the conscious thoughts of animals. "Mind" is often used to refer especially to the thought processes of reason.
There are many theories of the mind and its function. The earliest recorded works on the mind are by Zarathushtra, the Buddha, Plato, Aristotle, Adi Shankara and other ancient Greek, Indian and Islamic philosophers. Pre-scientific theories, based in theology, concentrated on the relationship between the mind and the soul, the supposed supernatural, divine or god-given essence of the person. Modern theories, based on scientific understanding of the brain, theorise that the mind is a phenomenon of the brain and is synonymous with consciousness.
The question of which human attributes make up the mind is also much debated. Some argue that only the "higher" intellectual functions constitute mind: particularly reason and memory. In this view the emotions - love,hate, fear, joy - are more "primitive" or subjective in nature and should be seen as different from the mind. Others argue that the rational and the emotional sides of the human person cannot be separated, that they are of the same nature and origin, and that they should all be considered as part of the individual mind.
In popular usage mind is frequently synonymous with thought: It is that private conversation with ourselves that we carry on "inside our heads." Thus we "make up our minds," "change our minds" or are "of two minds" about something. One of the key attributes of the mind in this sense is that it is a private sphere to which no one but the owner has access. No-one else can "know our mind." They can only know what we communicate.
Mental faculties
Thought is a mental process which allows beings to model the world, and so to deal with it effectively according to their goals, plans, ends and desires. Words referring to similar concepts and processes include cognition, sentience, consciousness, idea, and imagination. Thinking involves the cerebral manipulation of information, as when we form concepts, engage in problem solving, reason and make decisions. Thinking is a higher cognitive function and the analysis of thinking processes is part of cognitive psychology.
Memory is an organism's ability to store, retain, and subsequently recall information. Although traditional studies of memory began in the realms of philosophy, the late nineteenth and early twentieth century put memory within the paradigms of cognitive psychology. In recent decades, it has become one of the principal pillars of a new branch of science called cognitive neuroscience, a marriage between cognitive psychology and neuroscience.
Imagination is accepted as the innate ability and process to invent partial or complete personal realms within the mind from elements derived from sense perceptions of the shared world. The term is technically used in psychology for the process of reviving in the mind percepts of objects formerly given in sense perception. Since this use of the term conflicts with that of ordinary language, some psychologists have preferred to describe this process as "imaging" or "imagery" or to speak of it as "reproductive" as opposed to "productive" or "constructive" imagination. Imagined images are seen with the "mind's eye". One hypothesis for the evolution of human imagination is that it allowed conscious beings to solve problems (and hence increase an individual's fitness) by use of mental simulation.
Consciousness is a quality of the mind generally regarded to comprise qualities such as subjectivity, self-awareness, sentience, sapience, and the ability to perceive the relationship between oneself and one's environment. It is a subject of much research in philosophy of mind, psychology, neuroscience, and cognitive science. Some philosophers divide consciousness into phenomenal consciousness, which is subjective experience itself, and access consciousness, which refers to the global availability of information to processing systems in the brain.Phenomenal consciousness is a state with qualia. Phenomenal consciousness is being something and access consciousness is being conscious of something.
Philosophy of mind
Main article: Philosophy of mind
Philosophy of mind is the branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body. The mind-body problem, i.e. the relationship of the mind to the body, is commonly seen as the central issue in philosophy of mind, although there are other issues concerning the nature of the mind that do not involve its relation to the physical body.Dualism and monism are the two major schools of thought that attempt to resolve the mind-body problem. Dualism is the position that mind and body are in some way separate from each other. It can be traced back to Plato,Aristotle and the Samkhya and Yoga schools of Hindu philosophy,but it was most precisely formulated by René Descartes in the 17th century.Substance dualists argue that the mind is an independently existing substance, whereas Property dualists maintain that the mind is a group of independent properties that emerge from and cannot be reduced to the brain, but that it is not a distinct substance.
Monism is the position that mind and body are not ontologically distinct kinds of entities. This view was first advocated in Western Philosophy by Parmenides in the 5th Century BC and was later espoused by the 17th Century rationalist Baruch Spinoza.Physicalists argue that only the entities postulated by physical theory exist, and that the mind will eventually be explained in terms of these entities as physical theory continues to evolve. Idealists maintain that the mind is all that exists and that the external world is either mental itself, or an illusion created by the mind. Neutral monists adhere to the position that there is some other, neutral substance, and that both matter and mind are properties of this unknown substance. The most common monisms in the 20th and 21st centuries have all been variations of physicalism; these positions include behaviorism, the type identity theory, anomalous monism and functionalism.
Many modern philosophers of mind adopt either a reductive or non-reductive physicalist position, maintaining in their different ways that the mind is not something separate from the body.These approaches have been particularly influential in the sciences, particularly in the fields of sociobiology, computer science, evolutionary psychology and the various neurosciences.Other philosophers, however, adopt a non-physicalist position which challenges the notion that the mind is a purely physical construct. Reductive physicalists assert that all mental states and properties will eventually be explained by scientific accounts of physiological processes and states.Non-reductive physicalists argue that although the brain is all there is to the mind, the predicates and vocabulary used in mental descriptions and explanations are indispensable, and cannot be reduced to the language and lower-level explanations of physical science.Continued neuroscientific progress has helped to clarify some of these issues. However, they are far from having been resolved, and modern philosophers of mind continue to ask how the subjective qualities and the intentionality (aboutness) of mental states and properties can be explained in naturalistic terms.
Science of mind
Psychology the scientific study of human behaviour; Noology, the study of thought. As both an academic and applied discipline, Psychology involves the scientific study of mental processes such as perception, cognition, emotion, personality, as well as environmental influences, such as social and cultural influences, and interpersonal relationships, in order to devise theories of human behaviour. Psychology also refers to the application of such knowledge to various spheres of human activity, including problems of individuals' daily lives and the treatment of mental health problems.
Psychology differs from the other social sciences (e.g., anthropology, economics, political science, and sociology) due to its focus on experimentation at the scale of the individual, as opposed to groups or institutions. Historically, psychology differed from biology and neuroscience in that it was primarily concerned with mind rather than brain, a philosophy of mind known as dualism. Modern psychological science incorporates physiological and neurological processes into its conceptions of perception, cognition, behaviour, and mental disorders.
See Sigmund Freud,Carl Jung, and Unconscious mind
A new scientific initiative, the Decade of the Mind, seeks to advocate for the U.S. Government to invest $4 billion over the next ten years in the science of the mind.
Mental health
By analogy with the health of the body, one can speak metaphorically of a state of health of the mind, or mental health. Merriam-Webster defines mental health as "A state of emotional and psychological well-being in which an individual is able to use his or her cognitive and emotional capabilities, function in society, and meet the ordinary demands of everyday life." According to the World Health Organization (WHO), there is no one "official" definition of mental health. Cultural differences, subjective assessments, and competing professional theories all affect how "mental health" is defined. In general, most experts agree that "mental health" and "mental illness" are not opposites. In other words, the absence of a recognized mental disorder is not necessarily an indicator of mental health.
One way to think about mental health is by looking at how effectively and successfully a person functions. Feeling capable and competent; being able to handle normal levels of stress, maintaining satisfying relationships, and leading an independent life; and being able to "bounce back," or recover from difficult situations, are all signs of mental health.
Psychotherapy is an interpersonal, relational intervention used by trained psychotherapists to aid clients in problems of living. This usually includes increasing individual sense of well-being and reducing subjective discomforting experience. Psychotherapists employ a range of techniques based on experiential relationship building, dialogue, communication and behavior change and that are designed to improve the mental health of a client or patient, or to improve group relationships (such as in a family). Most forms of psychotherapy use only spoken conversation, though some also use various other forms of communication such as the written word, art, drama, narrative story, or therapeutic touch. Psychotherapy occurs within a structured encounter between a trained therapist and client(s). Purposeful, theoretically based psychotherapy began in the 19th century with psychoanalysis; since then, scores of other approaches have been developed and continue to be created.

Time-domain reflectometer

2008-04-07T01:46:00.000-07:00

In telecommunication, an optical time domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber.
An OTDR injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber, light that is scattered back and reflected back from points in the fiber where the index of refraction changes. (This is equivalent to the way that an electronic TDR measures reflections caused by changes in the impedance of the cable under test.) The intensity of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length.
An OTDR may be used for estimating the fiber's length and overall attenuation, including splice and mated-connector losses. It may also be used to locate faults, such as breaks.

A time-domain reflectometer (TDR) is an electronic instrument used to characterize and locate faults in metallic cables (for example, twisted wire pairs, coaxial cables) and, in the OTDR domain: optical fibers.A TDR transmits a fast rise time pulse along the conductor. If the conductor is of a uniform impedance and properly terminate, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected back to the TDR. But where impedance discontinuities exist, each discontinuity will create an echo that is reflected back to the reflectometer (hence the name). Increases in the impedance create an echo that reinforces the original pulse while decreases in the impedance create an echo that opposes the original pulse. The resulting reflected pulse that is measured at the output/input to the TDR is displayed or plotted as a function of time and, because the speed of signal propagation is relatively constant for a given transmission medium, can be read as a function of cable length. This is similar in principle to radar.
Because of this sensitivity to impedance variations, a TDR may be used to verify cable impedance characteristics, splice and connector locations and associated losses, and estimate cable lengths, as every nonhomogenity in the impedance of the cable will reflect some signal back in the form of echoes.

Consider the case where the far end of the cable is shorted (that is, it is terminated into zero ohms impedance). When the rising edge of the pulse is launched down the cable, the voltage at the launching point "steps up" to a given value instantly and the pulse begins propagating down the cable towards the short. When the pulse hits the short, no energy is absorbed at the far end. Instead, an opposing pulse reflects back from the short towards the launching end. It is only when this opposing reflection finally reaches the launch point that the voltage at this launching point abruptly drops back to zero, signaling the fact that there is a short at the end of the cable. That is, the TDR had no indication that there is a short at the end of the cable until its emitted pulse can travel down the cable at roughly the speed of light and the echo can return back up the cable at the same speed. It is only after this round-trip delay that the short can be perceived by the TDR. Assuming that one knows the signal propagation speed in the particular cable-under-test, then in this way, the distance to the short can be measured.
A similar effect occurs if the far end of the cable is an open circuit (terminated into an infinite impedance). In this case, though, the reflection from the far end is polarized identically with the original pulse and adds to it rather than cancelling it out. So after a round-trip delay, the voltage at the TDR abruptly jumps to twice the originally-applied voltage.
Note that a theoretical perfect termination at the far end of the cable would entirely absorb the applied pulse without causing any reflection. In this case, it would be impossible to determine the actual length of the cable. Luckily, perfect terminations are very rare and some small reflection is nearly always caused. (This property was employed by a now-defunct audio cable company to design unusual high-end audio cables, and while those cables can no longer be purchased, the site remains an excellent introduction to the principles of the technology.)
The magnitude of the reflection is referred to as the reflection coefficient or ρ. The coefficient ranges from 1 (open circuit) to -1 (short circuit). The value of zero means that there is no reflection. The reflection coefficient is calculated as follows:

Where Zo is defined as the characteristic impedance of the transmission medium and Zt is the impedance of the termination at the far end of the transmission line.
Any discontinuity can be viewed as a termination impedance and substituted as Zt. This includes abrupt changes in the characteristic impedance. As an example, a trace width on a printed circuit board doubled at its midsection would constitute a discontinuity. Some of the energy will be reflected back to the driving source; the remaining energy will be transmitted. This is also known as a scattering junction.

Time domain reflectometers are commonly used for in-place testing of very long cable runs, where it is impractical to dig up or remove what may be a kilometers-long cable. They are indispensable for preventive maintenance of telecommunication lines, as they can reveal growing resistance levels on joints and connectors as they corrode, and increasing insulation leakage as it degrades and absorbs moisture long before either leads to catastrophic failures. Using a TDR, it is possible to pinpoint a fault to within centimetres.
TDRs are also very useful tools for Technical Surveillance Counter-Measures, where they help determine the existence and location of wire taps. The slight change in line impedance caused by the introduction of a tap or splice will show up on the screen of a TDR when connected to a phone line.
TDR equipment is also an essential tool in the failure analysis of today's high-speed printed circuit boards. The signal traces on these boards are carefully crafted to emulate a transmission line. By observing reflections, any unsoldered pins of a ball grid array device can be detected. Additionally, short circuited pins can also be detected in a similar fashion.
The TDR principle is used in industrial settings, in situations as diverse as the testing of integrated circuit packages to measuring liquid levels. In the former, the time domain reflectometer is used to isolate failing sites in the same. The latter is primarily limited to the process industry.

TDR in level measurement
In a TDR-based level measurement device, a low-energy electromagnetic impulse generated by the sensor’s circuitry is propagated along a thin wave guide (also referred to as a probe) – usually a metal rod or a steel cable. When this impulse hits the surface of the medium to be measured, part of the impulse energy is reflected back up the probe to the circuitry which then calculates the fluid level from the time difference between the impulse sent and the impulse reflected (in nanoseconds). The sensors can output the analyzed level as a continuous analog signal or switch output signals. In TDR technology, the impulse velocity is primarily affected by the permittivity of the medium through which the pulse propagates, which can vary greatly by the moisture content and temperature of the medium. In most cases, this can be corrected for without undue difficulty. However, in complex environments, such as in boiling and/or high temperature environments, this can be a significant signal processing dilemma. In particular, determining the froth height and true collapsed liquid level in a frothy / boiling medium can be very difficult.

TDR used in the Earth and Agricultural Sciences
TDR is used to determine moisture content in soil and porous media, where over the last two decades substantial advances have been made; including in soils, grains and food stuffs, and in sediments. The key to TDR’s success is its ability to accurately determine the permittivity (dielectric constant) of a material from wave propagation, and the fact that there is a strong relationship between the permittivity of a material and its water content, as demonstrated in the pioneering works of Hoekstra and Delaney (1974) and Topp et al. (1980). Recent reviews and reference work on the subject include, Topp and Reynolds (1998), Noborio (2001), Pettinellia et al. (2002), Topp and Ferre (2002) and Robinson et al. (2003). The TDR method is a transmission line technique, and determines an apparent TDR permittivity (Ka) from the travel time of an electromagnetic wave that propagates along a transmission line, usually two or more parallel metal rods embedded in a soil or sediment. TDR probes are usually between 10 and 30 cm in length and connected to the TDR via a coaxial cable.

TDR in Geotechnical Usage
Time Domain Reflectometry (TDR) has also been utilized to monitor slope movement in a variety of geotechnical settings including highway cuts, rail beds, and open pit mines (Dowding & O'Connor, 1984, 2000a, 2000b; Kane & Beck, 1999). In stability monitoring applications using TDR, a coaxial cable is installed in a vertical borehole passing through the region of concern. The electrical impedance at any point along a coaxial cable changes with deformation of the insulator between the conductors. A brittle grout surrounds the cable to translate earth movement into an abrupt cable deformation that shows up as a detectable peak in the reflectance trace. Until recently, the technique was relatively insensitive to small slope movements and could not be automated because it relied on human detection of changes in the reflectance trace over time. Farrington and Sargand (2004) developed a simple signal processing technique using numerical derivatives to extract reliable indications of slope movement from the TDR data much earlier than by conventional interpretation.

TDR in Semiconductor Device Analysis
Time Domain Reflectometry is used in semiconductor failure analysis as a non-destructive method for the location of defects in semiconductor device packages. The TDR provides an electrical signature of individual conductive traces in the device package, and is useful for determining the location of opens and shorts.

Photonics system

2008-04-01T02:19:00.000-07:00

Photonics is the science of generating, controlling, and detecting photons, particularly in the visible and near infra-red spectrum, but also extending to the ultraviolet (0.2 - 0.35 µm wavelength), long-wave infrared (8 - 12 µm wavelength), and far-infrared/THz portion of the spectrum (e.g., 2-4 THz corresponding to 75-150 µm wavelength) where today quantum cascade lasers are being actively developed. Photonics is an outgrowth of the first practical semiconductor light emitters invented in the early 1960s at General Electric, MIT Lincoln Laboratory, IBM, and RCA and made practical by Zhores Alferov and Dmitri Z. Garbuzov and collaborators working at the Ioffe Physico-Technical Institute and almost simultaneously by Izuo Hayashi and Mort Panish working at Bell Telephone Laboratories. Photonics most typically operates at frequencies on the order of hundreds of terahertz.
Just as applications of electronics have expanded dramatically since the first transistor was invented in 1948, the unique applications of photonics continue to emerge. Those which are established as economically important applications for semiconductor photonic devices include optical data recording, fiber optic telecommunications, laser printing (based on xerography), displays, and optical pumping of high-power lasers. The potential applications of photonics are virtually unlimited and include chemical synthesis, medical diagnostics, on-chip data communication, laser defense, and fusion energy to name several interesting additional examples.
Relationship to other fields:
Classical optics
Photonics is closely related to optics. However optics preceded the discovery that light is quantized (when the photoelectric effect was explained by Albert Einstein in 1905). The tools of optics are the refracting lens, the reflecting mirror, and various optical components which were known prior to 1900. The key tenets of classical optics, such as Huygens Principle, the Maxwell Equations, and wave equations, do not depend on quantum properties of light.
Modern optics
Photonics is approximately synonymous with quantum optics, quantum electronics, electro-optics, and optoelectronics. However each is used with slightly different connotations by scientific and government communities and in the marketplace. Quantum optics often connotes fundamental research, whereas photonics is used to connote applied research and development.
The term photonics more specifically connotes:
the particle properties of light,
the potential of creating signal processing device technologies using photons,
those quantum optical technologies which are manufacturable and can be low-cost, and
an analogy to electronics.
The term optoelectronics eponymously connotes devices or circuits comprising both electrical and optical functions, i.e., a thin-film semiconductor device. The term electro-optics came into earlier use and specifically encompasses nonlinear electrical-optical interactions applied, e.g, as bulk crystal modulators such as the Pockels cell, but also includes advanced imaging sensors typically used for surveillance by civilian or government organizations.
Emerging fields
Photonics also relates to the emerging science of quantum information in those cases where it employs photonic methods. Other emerging fields include opto-atomics in which devices integrate both photonic and atomic devices for applications such as precision timekeeping, navigation, and metrology. Another emerging field is polaritonics which differs with photonics in that the fundamental information carrier is a phonon-polariton, which is a mixture of photons and phonons, and operates in the range of frequencies from 300 gigahertz to approximately 10 terahertz.
Overview of photonics research:
The science of photonics includes the emission, transmission, amplification, detection, modulation, and switching of light.
Photonic devices include optoelectronic devices such as lasers and photodetectors, as well as optical fiber, photonic crystals, planar waveguides, and other passive optical elements.
Applications of photonics include light detection, telecommunications, information processing, illumination, metrology, spectroscopy, holography, medicine (surgery, vision correction, endoscopy, health monitoring), military technology, laser material processing, visual art, biophotonics, agriculture and robotics.
History of photonics
Photonics as a field really began in 1960, with the invention of the laser, and the laser diode followed in the 1970s by the development of optical fibers as a medium for transmitting information using light beams, and the Erbium-doped fiber amplifier. These inventions formed the basis for the telecommunications revolution of the late 20th century, and provided the infrastructure for the internet.
Historically , the term photonics only came into common use among the scientific community in the 1980s as fiber optic transmission of electronic data was adopted widely by telecommunications network operators (although it had earlier been coined). At that time, the term was adopted widely within Bell Laboratories. Its use was confirmed when the IEEE Lasers and Electro-Optics Society established an archival journal named Photonics Technology Letters at the end of the 1980s.
During the period leading up to the dot-com crash circa 2001, photonics as a field was largely focused on telecommunications. However, photonics covers a huge range of science and technology applications, including:
laser manufacturing,
biological and chemical sensing,
medical diagnostics and therapy,
display technology,
optical computing.
Various non-telecom photonics applications exhibit a strong growth particularly since the dot-com crash, partly because many companies have been looking for new application areas quite successfully. A huge further growth of photonics can be expected for the case that the current development of silicon photonics will be successful.
Applications of Photonics:

Consumer Equipment: Barcode scanner, printer, CD/DVD/Blu-ray devices, remote control devices
Telecommunications: Optical fiber communications , Optical Down converter to Microwave
Medicine: correction of poor eyesight, laser surgery, surgical endoscopy, tattoo removal
Industrial manufacturing: the use of lasers for welding, drilling, cutting, and various kinds of surface modification
Construction: laser levelling, laser rangefinding, smart structures
Aviation: photonic gyroscopes lacking any moving parts
Military: IR sensors, command and control, navigation, search and rescue, mine laying and detection
Entertainment: laser shows, beam effects, holographic art
Information processing
Metrology: time and frequency measurements, rangefinding
Photonic computing: clock distribution and communication between computers, circuit boards, or within optoelectronic integrated circuits; in the future: quantum computing.

Broadband & Digital subscriber line

2008-03-20T18:04:00.000-07:00

Broadband in telecommunications is a term that refers to a signaling method that includes or handles a relatively wide range of frequencies, which may be divided into channels or frequency bins. Broadband is always a relative term, understood according to its context. The wider the bandwidth, the greater the information-carrying capacity. In radio, for example, a very narrow-band signal will carry Morse code; a broader band will carry speech; a still broader band is required to carry music without losing the high audio frequencies required for realistic sound reproduction. A television antenna described as "normal" may be capable of receiving a certain range of channels; one described as "broadband" will receive more channels. In data communications a modem will transmit a bandwidth of 64 kilobits per seconds (kbit/s) over a telephone line; over the same telephone line a bandwidth of several megabits per second can be handled by ADSL, which is described as broadband (relative to a modem over a telephone line, although much less than can be achieved over a fibre optic circuit, for example).[citation needed]
Broadband in data communications can refer to Broadband Networks or Broadband Internet and may have the same meaning as above, so that data transmission over a fiber optic cable would be referred to as broadband as compared to a telephone modem operating at 600 bits per second.[citation needed]
However, broadband in data communications is frequently used in a more technical sense to refer to data transmission where multiple pieces of data are sent simultaneously to increase the effective rate of transmission, regardless of actual data rate. In network engineering this term is used for methods where two or more signals share a medium.[citation needed]
The various forms of Digital Subscriber Line (DSL) services are broadband in the sense that digital information is sent over a high-bandwidth channel above the baseband voice channel on a single pair of wires.[citation needed]
A baseband transmission sends one type of signal using a medium's full bandwidth, as in 100BASE-T Ethernet. Ethernet, however, is the common interface to broadband modems such as DSL data links, and has a high data rate itself, so is sometimes referred to as broadband. Ethernet provisioned over cable modem is a common alternative to DSL.
Digital subscriber line
DSL or xDSL, is a family of technologies that provide digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, although in recent years, many[attribution needed] have adopted digital subscriber line as a more marketing-friendly term for the most popular version of consumer-ready DSL, ADSL. DSL uses high frequency; regular telephone uses low frequency.
Typically, the download speed of consumer DSL services ranges from 512 kilobits per second (kbit/s) to 24,000 kbit/s, depending on DSL technology, line conditions and service level implemented. Typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line (ADSL) and equal to download speed for Symmetric Digital Subscriber Line (SDSL).
Voice and data
Some variants of DSL connections, like ADSL and very high speed DSL (VDSL), typically work by dividing the frequencies used in a single phone line into two primary 'bands'. The ISP data is carried over the high frequency band (25 kHz and above) whereas the voice is carried over the lower frequency band (4 kHz and below). (See the ADSL article on how the high frequency band is sub-divided). The user typically installs a DSL filter on each phone. This filters out the high frequencies from the phone line, so that the phone only sends or receives the lower frequencies (the human voice). The DSL modem and the normal telephone equipment can be used simultaneously on the line without interference from each other.
History and science
Digital subscriber line technology was originally implemented as part of the ISDN specification, which is later reused as IDSL. Higher speed DSL connections like HDSL and SDSL have been developed to extend the range of DS1 services on copper lines. Consumer oriented ADSL is designed to operate also on a BRI ISDN line, which itself is a form of DSL, as well as on an analog phone line.
DSL, like many other forms of communication, stems directly from Claude Shannon's seminal 1948 scientific paper: A Mathematical Theory of Communication. Employees at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone company central offices and customers on conventional twisted pair cabling.
U.S. telephone companies promote DSL to compete with cable modems. DSL service was first provided over a dedicated "dry loop", but when the FCC required the incumbent local exchange carriers ILECs to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over Unbundled Network Element , this allows a single pair to carry data (via a digital subscriber line access multiplexer [DSLAM]) and analog voice (via a circuit switched telephone switch) at the same time. Inline low-pass filter/splitters keep the high frequency DSL signals out of the user's telephones. Although DSL avoids the voice frequency band, the nonlinear elements in the phone would otherwise generate audible intermodulation products and impair the operation of the data modem.
Older ADSL standards can deliver 8 Mbit/s to the customer over about 2 km (1.25 miles) of unshielded twisted pair copper wire. The latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM. Distances greater than 2 km (1.25 miles) significantly reduce the bandwidth usable on the wires, thus reducing the data rate. By using an ADSL loop extender, these distances can be increased substantially.
In 2007, Dr. John Papandriopoulos, a University of Melbourne engineering researcher, patented algorithms that can potentially boost DSL line speeds to a maximum of 250 Mbit/s.
Operation
The local loop of the public switched telephone network (PSTN) was initially designed to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as voiceband or commercial bandwidth.
At the local telephone exchange (United Kingdom) or central office (United States) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore, according to the Nyquist theorem, any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).
The laws of physics, specifically the Shannon limit, cap the speed of data transmission. For a long time, it was believed that a conventional phone line couldn't be pushed beyond the low speed limits (typically under 9600 bit/s). In the 1950s, 4 MHz television signals were often carried between studios on ordinary twisted pair telephone cable, suggesting that the Shannon Limit would allow transmitting many megabits per second. However, these cables had other impairments besides Gaussian noise, preventing such rates from becoming practical in the field. In the 1980s techniques were developed for broadband communications that allowed the limit to be greatly extended.
The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor (the higher frequencies used by DSL travel only short distances). The pool of usable channels is then split into two different frequency bands for upstream and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable.
One of Lechlider's greatest contributions to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of synchronous DSL. This allowed Internet Service Providers to offer efficient service to consumers, who benefitted greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where transmission errors are impermissible, even though resending packets may increase latency.
Because DSL operates at above the 3.4 kHz voice limit, it cannot be passed through a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a properly installed and working load coil, nor can voice service be maintained past a certain distance without such coils. Some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this phone companies are endeavoring to remove load coils on copper loops that can operate without them, and conditioning lines not to need them through the use of fiber to the neighborhood or node FTTN.
The commercial success of DSL and similar technologies largely reflects the advances made in electronics, that, over the past few decades, have been getting faster and cheaper even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. Several factors contributed to the popularization of DSL technology:
Until the late 1990s, the cost of digital signal processors for DSL was prohibitive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of VLSI technology, the cost of the equipment associated with a DSL deployment (a DSLAM at one end and a DSL "modem" at the other end) lowered significantly.
A DSL line can be deployed over existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cable over the same route and distance. This is true both for ADSL and SDSL variations.
In the case of ADSL, competition in Internet access caused subscription fees to drop significantly over the years, thus making ADSL more economical when compared to dial up access. Telephone companies were pressured into moving to ADSL largely due to competition from cable companies, which use DOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to popularize ADSL technology.
Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that with suitable filters and/or splitters the existing voice service continues to operate independent of the DSL service. Thus POTS-based communications, including fax machines and analog modems, can share the wires with DSL. Only one DSL "modem" can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection is to use a router that establishes a connection between the DSL modem and a local Ethernet, Powerline, or Wi-Fi network on the customer's premises.
Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as an Internet service provider.
Dry-loop DSL or "naked DSL," which does not require the subscriber to have traditional land-line telephone service, started making a comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC, to consumers.
Even without the regulatory mandate, however, many ILECs offer naked DSL to consumers. The number of telephone landlines in the US has dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has grown to 195 million. This lack of demand for landline service has resulted in the expansion of naked DSL availability.

Optical communication

2008-03-06T23:07:00.000-08:00

An optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal.
Forms of optical communication
There are many forms of non-technological optical communication, including body language and sign language.
Techniques such as semaphore lines, ship flags, smoke signals, and beacon fires were the earliest form of technological optical communication.
The heliograph uses a mirror to reflect sunlight to a distant observer. By moving the mirror the distant observer sees flashes of light that can be used to send a prearranged signaling code. Navy ships often use a signal lamp to signal in Morse code in a similar way.
Distress flares are used by mariners in emergencies, while lighthouses and navigation lights are used to communicate navigation hazards.
Aircraft use the landing lights at airports to land safely, especially at night. Aircraft landing on an aircraft carrier use a similar system to land correctly on the carrier deck. The light systems communicate the correct position of the aircraft relative to the best landing glideslope.
Optical fiber is the most common medium for modern digital optical communication.
Free-space optical communication is also used today in a variety of applications.
Optical fiber communication
Optical fiber is the most common type of channel for optical communications, however, other types of optical waveguides are used within communications gear, and have even formed the channel of very short distance (e.g. chip-to-chip, intra-chip) links in laboratory trials. The transmitters in optical fiber links are generally light-emitting diodes (LEDs) or laser diodes. Infrared light, rather than visible light is used more commonly, because optical fibers transmit infrared wavelengths with less attenuation and dispersion. The signal encoding is typically simple intensity modulation, although historically optical phase and frequency modulation have been demonstrated in the lab. The need for periodic signal regeneration was largely superseded by the introduction of the erbium-doped fiber amplifier, which extended link distances at significantly lower cost.
Free-space optical communication
IrDA is an example of low-data-rate, short distance free-space optical communications using LEDs. RONJA is an example of 10Mbit/s 1.4 km full-duplex optical point-to-point link.

Laser

The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation.A typical laser emits light in a narrow, low-divergence monochromatic (single-coloured, if the laser is operating in the visible spectrum), beam with a well-defined wavelength. In this way, laser light is in contrast to a light source such as the incandescent light bulb, which emits light over a wide area and over a wide spectrum of wavelengths.
The first working laser was demonstrated in May 1960 by Theodore Maiman at Hughes Research Laboratories. Recently, lasers have become a multi-billion dollar industry. The most widespread use of lasers is in optical storage devices such as compact disc and DVD players, in which the laser (a few millimeters in size) scans the surface of the disc. Other common applications of lasers are bar code readers and laser pointers.
In industry, lasers are used for cutting steel and other metals and for inscribing patterns (such as the letters on computer keyboards). Lasers are also commonly used in various fields in science, especially spectroscopy, typically because of their well-defined wavelength or short pulse duration in the case of pulsed lasers. Lasers are used by the military for target identification and illumination for weapons delivery. Lasers used in medicine are used for internal surgery and cosmetic applications.
Design
Although the word light in the acronym Light Amplification by Stimulated Emission of Radiation is typically used in the expansive sense, as photons of any electromagnetic energy; it is not limited to photons in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. For example, a source of atoms in a coherent state can be called an atom laser.
A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material (gas, liquid, solid or free electrons) with appropriate optical properties. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically, one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, stimulating the gain material continuously. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. A typical pump source is a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.
Laser physics
A laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The gain medium transfers external energy into the laser beam. It is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission. The gain medium is energized, or pumped, by an external energy source. Examples of pump sources include electricity and light, for example from a flash lamp or from another laser. The pump energy is absorbed by the laser medium, placing some of its particles into high-energy ("excited") quantum states. Particles can interact with light both by absorbing photons or by emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. Strictly speaking, these are the essential ingredients of a laser. However, usually the term laser is used for devices where the light that is amplified is produced as spontaneous emission from the same gain medium as where the amplification takes place. Devices where light from an external source is amplified are normally called optical amplifiers.
The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and often monochromaticity established by the optical cavity design.
The optical cavity, a type of cavity resonator, contains a coherent beam of light between reflective surfaces so that the light passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. But each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be saturated. The balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the chosen pump power is too small, the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons aligned with the cavity manage to pass more than once through the medium and so have significant amplification.
The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are, at best, low order Gaussian beams. However this is rarely the case with powerful lasers. If the beam is not a low-order Gaussian shape, the transverse modes of the beam can be described as a superposition of Hermite-Gaussian or Laguerre-Gaussian beams (for stable-cavity lasers). Unstable laser resonators on the other hand, have been shown to produce fractal shaped beams.The beam may be highly collimated, that is being parallel without diverging. However, a perfectly collimated beam cannot be created, due to diffraction. The beam remains collimated over a distance which varies with the square of the beam diameter, and eventually diverges at an angle which varies inversely with the beam diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometers (1 mile) diameter if shone from the Earth to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost as soon as it leaves the aperture, at an angle of anything up to 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well.
The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, modelocking, or gain-switching. In pulsed operation, much higher peak powers can be achieved.
Some types of lasers, such as dye lasers and vibronic solid-state lasers can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few femtoseconds (10-15 s).
Although the laser phenomenon was discovered with the help of quantum physics, it is not essentially more quantum mechanical than other light sources. The operation of a free electron laser can be explained without reference to quantum mechanics.
Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser. This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser.

Optical fiber

2008-02-21T02:00:00.000-08:00

An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communication, which permits transmission over longer distances and at higher data rates than other forms of communications. Fibers are used instead of metal wires because signals travel along them with less loss, and they are immune to electromagnetic interference. Optical fibers are also used to form sensors, and in a variety of other applications.
Light is kept in the "core" of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide. Fibers which support many propagation paths or transverse modes are called multimode fibers (MMF). Fibers which support only a single mode are called singlemode fibers (SMF). Multimode fibers generally have a large-diameter core, and are used for short-distance communication links or for applications where high power must be transmitted. Singlemode fibers are used for most communication links longer than 200 meters.
Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced together either mechanically or by fusing them together with an electric arc. Special connectors are used to make removable connections.

The light-guiding principle behind optical fibers was first demonstrated by Daniel Colladon and Jacques Babinet in the 1840s, with Irish inventor John Tyndall offering public displays using water-fountains ten years later.Practical applications, such as close internal illumination during dentistry, appeared early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. The principle was first used for internal medical examinations by Heinrich Lamm in the following decade. In 1952, physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber, based on Tyndall's earlier studies; modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared later in the decade.Development then focused on fiber bundles for image transmission. The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of Michigan, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material. A variety of other image transmission applications soon followed. The advent of ultrapure silicon for semiconductor devices made low-loss silica fiber practical.
In 1965, Charles K. Kao and George A. Hockham of the British company Standard Telephones and Cables were the first to suggest that attenuation of contemporary fibers was caused by impurities, which could be removed, rather than fundamental physical effects such as scattering. They speculated that optical fiber could be a practical medium for communication, if the attenuation could be reduced below 20 dB per kilometer.This attenuation level was first achieved in 1970, by researchers Robert D. Maurer, Donald Keck, Peter C. Schultz, and Frank Zimar working for American glass maker Corning Glass Works, now Corning Ins. They demonstrated a fiber with 17 dB optic attenuation per kilometer by doping silica glass with titanium. A few years later they produced a fiber with only 4 dB/km using germanium oxide as the core dopant. Such low attenuations ushered in optical fiber telecommunications and enabled the Internet. Nowadays, attenuations in optical cables are far less than those in electrical copper cables, leading to long-haul fiber connections with repeater distances of 500–800 km.
The erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by reducing or even in many cases eliminating the need for optical-electrical-optical repeaters, was co-developed by teams led by David Payne of the University of Southampton, and Emmanuel Desurvire at Bell Laboratories in 1986. The more robust optical fiber commonly used today utilizes glass for both core and sheath and is therefore less prone to aging processes. It was invented by Gerhard Bernsee in 1973 by Schott Glass in Germany.In 1991, the emerging field of photonic crystals led to the development of photonic crystal fiber (Science (2003), vol 299, page 358), which guides light by means of diffraction from a periodic structure, rather than total internal reflection. The first photonic crystal fibers became commercially available in 1996.Photonic crystal fibers can be designed to carry higher power than conventional fiber, and their wavelength dependent properties can be manipulated to improve their performance in certain applications.

Optical fiber communication
Main article: Fiber-optic communication
Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the light signals propagating in the fiber can be modulated at rates as high as 40 Gb/s, and each fiber can carry many independent channels, each by a different wavelength of light (wavelength-division multiplexing). Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable. Fiber is also immune to electrical interference, which prevents cross-talk between signals in different cables and pickup of environmental noise. Also, wiretapping is more difficult compared to electrical connections, and there are concentric dual core fibers that are said to be tap-proof. Because they are non-electrical, fiber cables can bridge very high electrical potential differences and can be used in environments where explosive fumes are present, without danger of ignition.
Although fibers can be made out of transparent plastic, glass, or a combination of the two, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers (core diameter about 10 micrometers), single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.
Fiber optic sensors
Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters. The small size and the fact that no electrical power is needed at the remote location gives the fiber optic sensor an advantage over a conventional electrical sensor in certain applications.
Optical fibers are used as hydrophones for seismic or SONAR applications. Hydrophone systems with more than 100 sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries' navies. Both bottom mounted hydrophone arrays and towed streamer systems are in use. The German company Sennheiser developed a microphone working with a laser and optical fibers.
Optical fiber sensors for temperature and pressure have been developed for downhole measurement in oil wells. The fiber optic sensor is well suited for this environment as it is functioning at temperatures too high for semiconductor sensors (Distributed Temperature Sensing).
Another use of the optical fiber as a sensor is the optical gyroscope which is in use in the Boeing 767 and in some car models (for navigation purposes) and the use in Hydrogen microsensors.
Fiber-optic sensors have been developed to measure co-located temperature and strain simultaneously with very high accuracy. This is particularly useful when acquiring information from small complex structures.

Other uses of optical fibers

Fibers are widely used in illumination applications. They are used as light guides in medical and other applications where bright light needs to be shone on a target without a clear line-of-sight path. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building (see non-imaging optics). Optical fiber illumination is also used for decorative applications, including signs, art, and artificial Christmas trees. Swarovski boutiques use optical fibers to illuminate their crystal showcases from many different angles while only employing one light source. Optical fiber is an intrinsic part of the light-transmitting concrete building product, LiTraCon.Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes along with lenses, for a long, thin imaging device called an endoscope, which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures (endoscopy). Industrial endoscopes (see fiberscope or borescope) are used for inspecting anything hard to reach, such as jet engine interiors.
An optical fiber doped with certain rare-earth elements such as erbium can be used as the gain medium of a laser or optical amplifier. Rare-earth doped optical fibers can be used to provide signal amplification by splicing a short section of doped fiber into a regular (undoped) optical fiber line. The doped fiber is optically pumped with a second laser wavelength that is coupled into the line in addition to the signal wave. Both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification is stimulated emission.
Optical fibers doped with a wavelength shifter are used to collect scintillation light in physics experiments.
Optical fiber can be used to supply a low level of power (around one watt) to electronics situated in a difficult electrical environment. Examples of this are electronics in high-powered antenna elements and measurement devices used in high voltage transmission equipment.

Principle of operation

An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.

Multimode fiber

Fiber with large (greater than 10 μm) core diameter may be analyzed by geometric optics. Such fiber is called multimode fiber, from the electromagnetic analysis (see below). In a step-index multimode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. A low numerical aperture may therefore be desirable.In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

Singlemode fiber

Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The behavior of larger-core multimode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes.
The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.
The most common type of single-mode fiber has a core diameter of 8 to 10 μm and is designed for use in the near infrared. The mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 micrometres and as large as hundreds of micrometres.

Special-purpose fiber
Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation.
Photonic crystal fiber is made with a regular pattern of index variation (often in the form of cylindrical holes that run along the length of the fiber). Such fiber uses diffraction effects instead of or in addition to total internal reflection, to confine light to the fiber's core. The properties of the fiber can be tailored to a wide variety of applications.

Water heating

2008-01-12T02:05:00.000-08:00

Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating. In industry both hot water and water heated to steam have many uses.
Domestically, water is traditionally heated in vessels known as kettles, cauldrons, pots or coppers. These vessels heat a batch of water but do not produce a continual supply. Appliances for providing a more-or-less constant supply of hot water are variously known as water heaters, boilers, heat exchangers, calorifiers or geysers depending on whether they are heating Potable or non-potable water, in domestic or industrial use, their energy source, and in which part of the world they are found. In domestic installations, potable water heated for uses other than space heating is sometimes known as Domestic Hot Water (DHW).
In many countries the most common energy sources for heating water are fossil fuels: natural gas, liquefied petroleum gas, oil or sometimes solid fuels. These fuels may be consumed directly or by the use of electricity (which may derive from any of the above fuels or from nuclear or renewable sources). Alternative energy such as solar energy, heat pumps, hot water heat recycling, and sometimes geothermal heating, may also be used as available, usually in combination with backup systems supplied by gas, oil or electricity.
In some countries district heating is a major source of water heating. This is especially the case in Scandinavia. District heating systems make it possible to supply all of the energy for water heating as well as space heating from waste heat from industries, power plants, incinerators, geothermal heating and central solar heating. The actual heating of the tap water is performed in heat exchangers at the consumers premises. Generally the consumer needs no backup system due to the very high availability of district heating systems.

Types of water heating appliance

Water for space heating can be heated by fossil fuels in a boiler. Potable water may be heated in a separate appliance: this is common practice in the USA where warm-air space heating is usually employed.
Gravity system
Where a space-heating water boiler is employed the traditional arrangement in the UK is to use boiler-heated ("primary") water to heat ("secondary") water in a cylindrical vessel (usually made of copper) containing potable water supplied from a cold water storage tank, usually in the roof space of the building. This produces a fairly steady supply of DHW at low pressure but usually with a good flow. Water heating appliances in most other parts of the world do not use cold water storage tanks but heat water at pressures close to that of the incoming mains water supply.
Instantaneous water heaters
Stand-alone appliances for instantaneously heating water for DHW (Domestic Hot Water) are known in North America as tankless heaters, elsewhere as multipoint heaters, geysers or Ascots. In Australia and New Zealand there was a similar wood fired appliance known as the chip heater.
A common arrangement where hot-water space heating is employed is for the boiler to also heat potable water giving a continuous supply of DHW without any extra equipment required. Appliances capable of supplying both space-heating and DHW are known as combination (or "combi") boilers.
Although instantaneous heaters can give a continuous supply of DHW the rate at which they can produce it is limited by the thermodynamics of heating water from the available fuel supplies.
Storage systems
Another popular arrangement where higher flow rates are required (although for limited periods) is to heat water in a pressure vessel capable of withstanding a hydrostatic pressure close to that of the incoming mains supply. (A pressure reducing valve is usually employed to limit the pressure to a safe level for the vessel.)
In North America these vessels are known as tanks and may incorporate a gas or oil burner heating the water directly.
Where hot-water space heating boilers are used DHW cylinders are usually heated indirectly by primary water from the boiler, or by an electric immersion heater (often as backup to the boiler). In the UK these vessels are known as unvented cylinders (or commonly as Megaflos after the brand name of a widely-used model). In the US, when connected to a boiler they are known as indirect-fired water heaters.

Thermodynamics and economics

Water enters residences in the US at about 10 °C (50 °F) (varies with latitude and season). Adults generally prefer shower temperatures of 40–49 °C (105–120 °F), requiring the water temperature to be raised about 30 °C (55 °F) or more, if the hot water is later mixed with cold water. The Uniform Plumbing Code reference shower flow rate is 2.5 gpm (gallons per minute); sink and dishwasher usages range from 1–3 gpm.
Natural gas in the U.S. is measured in CCF (100 cubic feet), which is converted to a standardized heat content unit called the therm, equal to 100,000 British thermal units. A BTU is the energy required to raise one pound of water by one degree Fahrenheit. A U.S. gallon of water weighs 8.3 pounds. So, to raise a 40-gallon tank of 55 °F water up to 105 °F would require 40 x 8.3 x (105 − 55) / 100,000 BTU, or approximately 0.17 CCF, at 100% efficiency. A 40,000 BTU (per hour) heater would take 25 minutes to do this, at 100% efficiency. At $1 per therm, the cost of the gas would be about 17 cents.
In comparison, a typical electric water heater has a 4500 watt heating element, which if 100% efficient results in a heating time of about 1.1 hours. Since 16,600 BTU is roughly 4.9 kWh, at 10 cents/kWh the electricity would cost $0.49. Operating a shower at 2.5 gpm and 104 degrees Fahrenheit is equivalent to operating a 13.2 kW appliance.
Energy efficiencies of tank water heaters in residential use can vary greatly, particularly based on manufacturer and model. However, electric heaters tend to be slightly more efficient with recovery efficiency (how efficient energy is transferred to the water) reaching about 98%. Gas fired heaters have maximum recovery efficiencies of only about 86% (the remaining heat is lost with the flue gasses). Overall energy factors can be as low as 80% for electric and 50% for gas systems.
A tankless water heater operating at those same power levels (at 100% efficiency) would be able to supply 1.6 gpm continuously, raising the temperature by 50°F. The same unit could supply 1.3 gpm while raising the temperature by 60°F. To be able to handle a full house load of multiple uses (at least 5 gpm) with a centralized tankless water heater would require three to four times this power level — somewhat difficult to achieve with natural gas, and very difficult to achieve with electricity. Many tankless water heaters can use over 100,000 BTU/hr during high flow, and so require especially large power supplies.
Unfortunately, it takes a great deal of energy to heat water, as one may experience when attempting to boil a gallon of water on a stove. For this reason, tankless on-demand water heaters need to have a very large energy source to be usable. A wall outlet, by comparison, can only source enough energy to warm a disappointingly small amount of water: about .17 gpm at 40°C temperature elevation.

Tank-type water heaters

In household and commercial usage, most water heaters in North America are of the tank type. Also called storage water heaters, these consist of a cylindrical tank in which water is kept continuously hot and ready for use. Typical sizes for household use range from 75 to 400 litres (20 to 100 U.S. gallons). These may use electricity, natural gas, propane, heating oil, solar, or other energy sources. Natural gas heaters are most popular in the United States and most European countries, since the gas is often conveniently piped throughout cities and towns and currently is the cheapest to use. Compared to tankless heaters, storage water heaters have the advantage of using energy (gas or electricity) at a relatively slow rate, storing the heat for later use. Larger tanks tend to provide hot water with less temperature fluctuation at moderate flow rates.
Storage water heaters in the United States and New Zealand are typically vertical, cylindrical tanks, usually standing on the floor or on a platform raised a short distance above the floor. Storage water heater tanks in Spain are typically horizontal. In India, they are mainly vertical. In apartments they can be mounted in the ceiling space over laundry-utility rooms.
In western countries, where ambient temperature is colder, tiny point-of-use electric storage water heaters with capacities ranging from 8 to 32 litres (2 to 6 gallons) are made for installation in kitchen and bath cabinets or on the wall above a sink. They typically use low power heating elements, about 1 kW to 1.5 kW, and can provide hot water long enough for hand washing, or, if plumbed into an existing hot water line, until hot water arrives from a remote high capacity water heater. They are sometimes used when retrofitting a pump and recirculating plumbing in a building is too costly or impractical. Since they maintain water temperature thermostatically, they will supply hot water at extremely low flow rates, unlike tankless heaters.
In tropical countries, like Singapore, India: An ideal storage water heater may vary from 10 lit to 35 lit. Usage of 6 ltr tanks is not uncommon. Smaller tanks are sufficient as ambient weather and water temperature are moderate.
The inner tank of the Water heater is the single most important feature of a water heater. The best heaters have a copper container. The second most important feature may be the type of heating element. The cartridge elements score over tubular elements.
Insulation and other improvements
In general, the more tank insulation the better, since it reduces standby heat loss. Tanks are available with insulation ratings ranging from R-6 to R-24. It may be possible to add an extra insulating blanket or jacket on the outside of a poorly insulated tank to reduce heat loss.The most common type of water heater blanket is fiberglass insulation with a vinyl film on the outside. The insulation is wrapped around the tank and the ends are taped together. It is important that the blanket be the right size for the tank and not block air flow or cover safety and drainage valves, the controls, or block airflow through the exhaust vent, if any. In very humid locations, adding insulation to an already well-insulated tank may cause condensation problems, potentially causing rust, mold, or operational problems.
Modern water heaters have PUF (Polyurethane Foam) insulation. In countries where serviceability is very important, PUF capsules are kept between the inner tank and the outer body. Depending upon the insulation efficiency, star rating is given in India.
Other improvements include check valve devices at their inlet and outlet, cycle timers, electronic ignition in the case of fuel-using models, sealed air intake systems in the case of fuel-using models, and pipe insulation. The sealed air-intake system types are sometimes called "band-joint" intake units. "High efficiency" condensing units can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cooled and are mechanically ventilated either through the roof or through an exterior wall. At high combustion efficiencies a drain must be supplied to handle the water condensed out of the combustion products which are primarily carbon dioxide and water vapor.
In traditional plumbing in the United Kingdom the space-heating boiler is set up to heat a separate hot water cylinder or hot water tank for potable hot water. Such tanks are often fitted with an auxiliary electrical immersion heater for a quick temperature boost. Heat from the space-heating boiler is transferred to the potable water tank by means of a heat exchanger, and the boiler operates at a higher temperature than the potable hot water supply. Most potable water heaters in the United States are completely separate from the space heating units.
Residential combustion water heaters manufactured since 2003 in the United States have been redesigned to resist ignition of flammable vapors and incorporate a thermal cutoff switch, per ANSI Z21.10.1. The first feature attempts to prevent vapors from flammable liquids and gasses in the vicinity of the heater from being ignited and thus causing a house fire or explosion. The second feature prevents tank overheating due to unusual combustion conditions. These safety requirements were made based on homeowners placing, and sometimes spilling, gasoline and other flammable gases near their water heaters and causing fires. Since most of the new designs incorporate some type of flame arrestor screen, they require monitoring to make sure they don't become clogged with lint or dust, reducing the availability of air for combustion. If the flame arrestor becomes clogged, the thermal cutoff may act to shut down the heater.
A wetback stove or wetback heater is the name (used in New Zealand at least) for a simple household secondary water-heater using incidental heat. It typically consists of a hot water pipe running behind a fireplace or stove (rather than hot water storage), and has no facility to limit the heating. Modern wetbacks may run the pipe in a more sophisticated design to assist heat-exchange.
Maintenance
It is important that a water heater, particularly a combustion type, be properly installed to avoid safety hazards.The area around it must be kept clear. The T&P (Temperature and Pressure) relief valve should be tested at least yearly. Sediment that accumulates in the bottom of the tank should be drained out at least yearly, to maintain heating efficiency and to make the tank last longer.
In the past, tanks were made out of copper and were very corrosion resistant; they could last for decades. Now tanks are made out of steel lined with glass or porcelain.A steel tank can rust and leak. To prevent this, sacrificial anode rods of magnesium or aluminum alloy themselves undergo galvanic corrosion to reduce corrosion of the steel tank. Tanks with longer guarantees have increased anode material — when the anode is used up, the tank will rust. It may be possible to check the anode condition, and replace it if it is wearing out.Modern US water heater tanks last about ten years.The tank should have a tray underneath to contain leaks and channel them into a drain. Simple, battery-powered alarms are available for $10-$30 that alert the homeowner to leaks.All water heaters will eventually leak, so it is important to know where the water supply shutoff valve is located!
Electric water heaters made with plastic tanks (e.g, polybutylene) are available. Designed with corrosion-free and corrosion-resistant materials, they can conceivably last the life of a dwelling. The only maintenance requirement would be to periodically clean sediments from the bottom of the tank and clean mineral scale off the heating elements.
Electric water heating power load shifting
Because tank-type water heaters store heat, electrical water heaters can be a good match for an intelligent electrical power distribution system, heating when the electrical grid load is low and turning off when the load is high. This could be implemented by allowing the power supplier to send load-shedding requests, or by the use of real-time energy pricing.
Tankless heaters
Tankless water heaters, also called combination or "combi" boilers, instantaneous, continuous flow, inline, flash, on-demand or instant-on water heaters, are also available and gaining in popularity. These water heaters heat the water as the water flows through the device, and do not retain any water internally except for what is in the heat exchanger coil. Tankless heaters are often installed throughout a household at more than one point-of-use (POU), far from the central water heater, or larger models may still be used to provide all the hot water requirements for an entire house. The chief advantages of tankless water heaters are a continuous flow of hot water and energy savings (as compared to a limited flow of continuously heating hot water from conventional tank water heaters).
Various types and their advantages
Point of use tankless water heaters are located right where the water is being used, so the water is almost instantly hot, which saves water. They also save even more energy than centrally installed tankless water heaters because no hot water is left in the pipes after the water is shut off. However, point of use tankless water heaters are usually used in combination with a central water heater since they are usually limited to under 6 litres/minute (1.5 U.S. gallons/minute), as the expense of buying a heater for every kitchen, laundry room, bathroom, or sink can outweigh the money saved in water and energy bills. In addition, point of use water heaters until recently were almost always electrical, and electricity is often substantially more expensive than natural gas.
Tankless heaters can ideally be somewhat more efficient than storage water heaters. In both kinds of installation (centralized and POU) the absence of a tank saves energy as conventional water heaters have to reheat the water in the tank as it cools off, called standby loss. With a central water heater of any type, water is wasted waiting for water to heat up because of the cold water in the pipes between the faucet and the water heater.
Tankless water heaters can be divided into two categories: "full on/full off" and "modulated". Full on/full off units do not have a variable power output level; the unit is either on or off. Modulated tankless water heaters base the heat output on the flow of water running through the unit. This is usually done through the use of a 'flow sensor', modulating gas valve, inlet water temperature sensor and an outlet water temperature sensor-choke valve and means that the occupants should receive the same output temperature of water at differing velocities, usually within a close range of ±2°C.
The high efficiency condensing combination boiler provides both space heating and water heating, an increasingly popular choice in UK houses. In fact, combination boilers now account for over half of all the new domestic boilers installed in Britain.
In certain parts of South America as well as Costa Rica and Puerto Rico, a point of use style water heater commonly referred to as the "Electric Shower Head" is used in many residential and some commercial installations. As the name implies, an electric heating element is incorporated into such shower heads to heat the water. However, many of these units are often poorly installed, often with exposed wiring in wet locations.
Under current North American conditions, the most cost effective configuration from an operating viewpoint is usually to use a central tankless water heater for most of the house, and install a point of use tankless water heater at any distant faucets or bathrooms. However, this may vary according to how much electricity, gas and water costs in the area, the layout of the house, and how much hot water is used. Only electric tankless water heaters were available at first and they are still used for almost all point of use heaters, but natural gas and propane heaters are now common. When consumers are considering a whole house gas tankless unit, they are advised to look at how the unit functions when raising the water temperature by about 42°C (75–77°F). Thus, if they live in a cold weather climate, they are advised to look at the unit's capacity with 3-10°C (38–50°F) inlet water temperatures, and find a size that produces approximately 15 litres/minute (4 gpm) even in winter if they have a typical-sized house and desire what is called a 2-appliance heater. This same unit may produce 25-30 litres/minute (6.3–6.9 gpm) in summer with higher inlet temperatures, but there is greater interest in year round production and usability.
Since the water must be heated instantly, tankless water heaters use a very large amount of electricity or gas while they are on. If a storage water heater is being replaced with a tankless one, the size of the electrical wiring or gas pipeline may have to be increased to handle the load, adding significant expense to the installation. Gas units are efficient but require a large volume of gas when operating at high firing rates. Many tankless units have fully modulating gas valves that can range from as low as 10,000 to over 1,000,000 BTUs. For electrical installations, AWG 10 or 8 wire, corresponding to 10 or 6 mm², is required for most POU heaters at North American voltages. Larger whole house electric units may require up to AWG 2 wire. In gas appliances both pressure and volume requirements must be met for optimum operation.

Water heater safety

Water heaters potentially can explode and cause significant damage, injury, or death if certain safety devices are not installed. When the water temperature exceeds 100°C (212°F), the water will remain a liquid inside the tank, but when the pressure is released as the water comes out the tap the water will boil, potentially inflicting steam burns. Water above about 88°C (190°F) will cause third-degree burns on contact. A safety device called a temperature and pressure relief (T&P or TPR) valve, is normally fitted on the top of the water heater to dump water if the temperature or pressure becomes too high. Most plumbing codes require that a discharge pipe be connected to the valve to direct the flow of discharged hot water to a drain, typically a nearby floor drain, or outside the living space. Some building codes will allow for the discharge pipe to terminate in the garage.
If a water heater is installed in a garage, it is recommended, and many codes require, that it be elevated at least 18 inches (0.46 m) above the floor to reduce the potential for fire or explosion due to spillage or leakage of combustible liquids in the garage. Furthermore, some local codes mandate that tank-type heaters in new and retrofit installations be braced to an adjacent wall with a strap to prevent them from tipping over and breaking the water and gas pipes in the event of an earthquake.
For older houses where the water heater is part of the space heating boiler, and plumbing codes allow, some plumbers will install a "Watts 210" device in place of a TPR valve. When the device senses that the temperature reaches 99°C (210 °F), it will shut off the gas supply and prevent further heating. In addition, an expansion tank or exterior pressure relief valve must be installed to prevent pressure buildup in the plumbing from rupturing pipes, valves, or the water heater.
Scalding is a serious concern with any water heater. Human skin burns quickly at high temperature, e.g., only 60°C (140°F), but also at lower temperatures, e.g., 50°C (120°F), if the exposure times are sufficient. Older people and children often receive the most serious scalds due to disabilities or slow reaction times. In Australia and elsewhere it is common practice to put a tempering valve on the outlet of the water heater. A tempering valve mixes enough cold water with the hot from the heater to keep the outgoing water temperature fixed, often set to 50°C. Without a tempering valve, reduction of the water heater's setpoint temperature is the most direct way to reduce scalding. However, for sanitation, hot water is needed. Most residential dishwashing machines, for example, include an electric heating element for increasing the water temperature above that provided by water heaters.
There are two seemingly conflicting safety issues around water heater temperature — the risk of scalding from excessively hot water, and the risk of incubating bacteria colonies, particularly Legionella, in water that is not hot enough to kill them. Both risks are potentially life threatening and are balanced by setting the water heater's thermostat to 50°C (120°F). However, if there is a dishwasher without a booster heater, it may require a water temperature within a range of 57°C (130ºF) to 60°C (140ºF) for optimum cleaning,in which case tempering valves set to no more than 55°C can be applied to faucets to avoid scalding.

An electric water boiler, also sometimes called an electric dispensing pot, electric water heater, or electric water urn, is a consumer electronics small appliance used for boiling water and possibly maintaining it at a constant temperature. It is typically used to provide an immediate source of hot water for making tea, hot chocolate, ramen noodles, or baby formula, or any other household use where clean hot water is required.
An electric water boiler consists of a water reservoir with a heating element in the bottom. Some models offer multiple temperature settings. Other models are part of larger water systems that boil water and provide hot, cold, and lukewarm water. Water may be dispensed in various ways, e.g. by pouring, an electric pump or by pressing a large button that functions as a diaphragm pump.

What in Distribution board

2007-12-09T22:06:00.000-08:00

A distribution board divides the electrical mains feed into various circuits, providing a fuse or circuit breaker for each circuit. They usually include a main switch, and often 1 or more RCD or RCBOs.
Distribution boards are also known as:-

breaker panel

fuse box

circuit breaker panel

consumer unit, or CU

panelboard

load center

Fuseboxes & CUs

Domestic CUs in Britain usually fall into 2 categories:

Modern CU with mcbs & RCD

Older fusebox, usually containing no RCD.

Domestic fuseboxes/CUs normally have the breakers or fuses arranged in a single horizontal row. Twin vertical row boxes are seen in commercial premises where a greater number of ways is required.

Modern CU
A typical new domestic CU used as a main panel may have anything from 6 to 12 ways for mcbs. Secondary CUs used for outbuildings usually have 1 or 2 ways. Most CUs have one RCD built in, but not all. These boxes are frequently white or grey. Split CUs are now popular in new installs. Commercial CUs are larger with more ways.
New British distribution boards generally have the live parts enclosed to IP20, even when the cover has been removed for servicing.

Older fusebox
Older fuseboxes usually use rewirable fuses with no other protective device, and basic 4 ways boxes are very common. A lot of these boxes are made of brown-black bakelite, sometimes with a wooden base. Although their design is historic, these were standard equipment for new installs as recently as the 1980s, so are very common.
Users should be wary of these fuseboxes, as typically pulling a fuse carrier out with the power on results in fingers grasping live connections, and these boxes are widespread even in modern installations.
The popular 4 way fusebox usually has 2 lighting and 2 socket circuits, with heavy or sustained loads such as immersion heater and oven on a socket circuit. This arrangement is not recommended practice today, but it is common in existing installations. Larger boxes with more ways will have separate fuses for large loads such as immersion heater, oven and shower.

MCB Retrofits
Some of these fuseboxes have had their fuse carriers relaced with plug in MCBs. These retrofit mcbs are typically rated at 3kA breaking capacity, but many properties have PSCCs as high as 6kA. Hence these mcbs do not have sufficient breaking capacity for safe reliable operation in many premises.

Historic Fuseboxes
There is also a small number of pre-1950 fuseboxes still in service. Where encountered these should be treated with caution. Bare live parts are common on these boxes, and the installations they supply are frequently not EEBAD.
Pre-1950 installations in the UK should be retired from service, as UK wiring of this age is in most cases dangerously deteriorated, as well as lacking basic safety fatures. This contrasts with historic American systems, which are frequently still in serviceable condition.

Split CUs
A split CU is one CU divided electrically into 2 halves, each of which is switched independantly. These are popular in new installs in the UK. They allow power to remain on whille work is carried out, and usually one main switch is replaced by an RCD, giving both RCD and non-RCD circuits.

Mobile operation
Sometimes it is desired to have a portable breaker panel, for example, for special events. In this case, a breaker panel is mounted to a board, together with various sockets. These are common in the movie industry. The American one pictured at the right has a cord with an L21-30 plug to supply power. Power leaves the board through four three-phase circuits: three 15 ampere circuits; and one 20 A circuit. The 15 A circuits each go to a triplex-box. The 20 A circuit goes to an L21-20 receptacle, and one leg of it goes to a 20 A duplex receptacle shown at the upper left. The neon nightlights on the upper right triplex box are to show the phase sequence.

Air conditioning

2007-11-23T22:15:00.000-08:00

The term air conditioning most commonly refers to the cooling and dehumidification of indoor air for thermal comfort. In a broader sense, the term can refer to any form of cooling, heating, ventilation or disinfection that modifies the condition of air.An air conditioner (AC or A/C in North American English, aircon in British and Australian English) is an appliance, system, or mechanism designed to stabilise the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time) , typically using a refrigeration cycle but sometimes using evaporation, most commonly for comfort cooling in buildings and transportation vehicles.
The concept of air conditioning is known to have been applied in Ancient Rome, where aqueduct water was circulated through the walls of certain houses to cool them. Similar techniques in medieval Persia involved the use of cisterns and wind towers to cool buildings during the hot season. Modern air conditioning emerged from advances in chemistry during the 19th Century, and the first large-scale electrical air conditioning was invented and used in 1902 by Willis Haviland Carrier.Comfort applications aim to provide a building indoor environment that remains relatively constant in a range preferred by humans despite changes in external weather conditions or in internal heat loads.
The highest performance for tasks performed by people seated in an office is expected to occur at 72 °F (22 °C) Performance is expected to degrade about 1% for every 2 °F change in room temperature.The highest performance for tasks performed while standing is expected to occur at slightly lower temperatures. The highest performance for tasks performed by larger people is expected to occur at slightly lower temperatures. The highest performance for tasks performed by smaller people is expected to occur at slightly higher temperatures. Although generally accepted, some dispute that thermal comfort enhances worker productivity, as is described in the Hawthorne effect.
Comfort air conditioning makes deep plan buildings feasible. Without air conditioning, buildings must be built narrower or with light wells so that inner spaces receive sufficient outdoor air via natural ventilation. Air conditioning also allows buildings to be taller since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings. Comfort applications for various building types are quite different and may be categorized as
Low-Rise Residential buildings, including single family houses, duplexes, and small apartment buildings
High-Rise Residential buildings, such as tall dormitories and apartment blocks
Commercial buildings, which are built for commerce, including offices, malls, shopping centers, restaurants, etc.
Institutional buildings, which includes hospitals, governmental, academic, and so on.
Industrial spaces where thermal comfort of workers is desired.
In addition to buildings, air conditioning can be used for comfort in a wide variety of transportation including land vehicles, trains, ships, aircraft, and spacecraft.
Process applications aim to provide a suitable environment for a process being carried out, regardless of internal heat and humidity loads and external weather conditions. Although often in the comfort range, it is the needs of the process that determine conditions, not human preference. Process applications include these:
Hospital operating theatres, in which air is filtered to high levels to reduce infection risk and the humidity controlled to limit patient dehydration. Although temperatures are often in the comfort range, some specialist procedures such as open heart surgery require low temperatures (about 18 °C, 64 °F) and others such as neonatal relatively high temperatures (about 28 °C, 82 °F).
Cleanrooms for the production of integrated circuits, pharmaceuticals, and the like, in which very high levels of air cleanliness and control of temperature and humidity are required for the success of the process.
Facilities for breeding laboratory animals. Since many animals normally only reproduce in spring, holding them in rooms at which conditions mirror spring all year can cause them to reproduce year round.
Aircraft air conditioning. Although nominally aimed at providing comfort for passengers and cooling of equipment, aircraft air conditioning presents a special process because of the low air pressure outside the aircraft.
Data processing centers
Textile factories
Physical testing facilities
Plants and farm growing areas
Nuclear facilities
Chemical and biological laboratories
Mines
Industrial environments
Food cooking and processing areas
In both comfort and process applications the objective may be to not only control temperature, but also humidity, air quality, air motion, and air movement from space to space.Refrigeration air conditioning equipment usually reduces the humidity of the air processed by the system. The relatively cold (below the dewpoint) evaporator coil condenses water vapor from the processed air, (much like an ice cold drink will condense water on the outside of a glass), sending the water to a drain and removing water vapor from the cooled space and lowering the relative humidity. Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space. In food retailing establishments large open chiller cabinets act as highly effective air dehumidifying units.
Some air conditioning units dry the air without cooling it, and are better classified as dehumidifiers. They work like a normal air conditioner, except that a heat exchanger is placed between the intake and exhaust. In combination with convection fans they achieve a similar level of comfort as an air cooler in humid tropical climates, but only consume about a third of the electricity. They are also preferred by those who find the draft created by air coolers discomforting.It should be noted that in a thermodynamically closed system, any energy input into the system that is being maintained at a set temperature (which is a standard mode of operation for modern air conditioners) requires that the energy removal rate from the air conditioner increase . This increase has the effect that for each unit of energy input into the system (say to power a light bulb in the closed system) requires the air conditioner to remove that energy. In order to do that the air conditioner must increase its consumption by the inverse of its efficiency times the input unit of energy. As an example presume that inside the closed system a 100 watt light bulb is activated, and the air conditioner has an efficiency of 200%. The air conditioner's energy consumption will increase by 50 watts to compensate for this, thus making the 100 W light bulb utilise a total of 150 W of energy.
Note that it is typical for air conditioners to operate at "efficiencies" of significantly greater than 100%, see Coefficient of performance.

What is Chiller

2007-11-23T22:00:00.000-08:00

A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. Most often water is chilled, but this water may also contain ~20% glycol and corrosion inhibitors; other fluids such as thin oils can be chilled as well.
Chilled water is used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional (CII) facilities. Most chillers are designed for indoor operation, but a few are weather-resistant. Chillers are precision machines that are very expensive to purchase and operate, so great care is needed in their selection and maintenance. Engineers are normally retained to evaluate applications' cooling needs, and to specify the optimal machines.In air conditioning systems, chilled water is distributed to heat exchangers, or coils, in air handling units, and used water is returned to the chiller. These cooling coils transfer sensible heat and latent heat from the air to the chilled water, thus cooling and usually dehumidifying the air stream. A typical chiller for air conditioning applications is rated between 15 to 1500 tons (180,000 to 18,000,000 BTU/h or 53 to 5,300 kW) in cooling capacity.In their industrial application, cooled water or other liquid from the chiller is pumped through process or laboratory equipment. Industrial chillers are used for controlled cooling of products, mechanisms and factory machinery in a wide range of industries. They are often used in the plastic industry in injection and blow molding, metal working cutting oils, welding equipment, die-casting and machine tooling, chemical processing, pharmaceutical formulation, food and beverage processing, vacuum systems, X-ray diffraction, power supplies and power generation stations, analytical equipment, semiconductors, compressed air and gas cooling. They are also used to cool high-heat specialized items such as MRI machines and lasers.
The chillers for industrial applications can be centralized, where multiple chillers serve multiple cooling needs, or decentralized where each application or machine has its own chiller. Each approach has its advantages. It is also possible to have a combination of both central and decentral chillers, especially if the cooling requirements are the same for some applications or points of use, but not all.
Decentral chillers are usually small in size (cooling capacity), usually from 0.2 tons to 10 tons. Central chillers generally have capacities ranging from ten tons to hundreds or thousands of tons.A vapor-compression chiller uses a refrigerant internally as its working fluid. Many refrigerants options are available; when selecting a chiller, the application cooling temperature requirements and refrigerant's cooling characteristics need to be matched. Important parameters to consider are the operating temperatures and pressures.
There are several environmental factors that concern refrigerants, and also affect the future availability for chiller applications. This is a key consideration in intermittent applications where a large chiller may last for 25 years or more. Ozone depletion potential (ODP) and global warming potential (GWP) of the refrigerant need to be considered. ODP and GWP data for some of the more common vapor-compression refrigerants; R-134a ODP = 0 and GWP = 1300; R-123 ODP = 0.012 and GWP = 120; R-22 ODP = 0.05, GWP = 1700.Important specifications to consider when searching for industrial chillers include the power source, chiller IP rating, chiller cooling capacity, evaporator capacity, evaporator material, evaporator type, condenser material, condenser capacity, ambient temperature, motor fan type, noise level, internal piping materials, number of compressors, type of compressor, number of fridge circuits, coolant requirements, fluid discharge temperature, and COP (the ratio between the cooling capacity in KW to the energy consumed by the whole chiller in KW). For medium to large chillers this should range from 3.5-4.8 with higher values meaning higher efficiency. Chiller efficiency is often specified in kilowatts per refrigeration ton (kW/RT).
Process pump specifications that are important to consider include the process flow, process pressure, pump material, elastomer and mechanical shaft seal material, motor voltage, motor electrical class, motor IP rating and pump rating. If the cold water temperature is lower than -5ºC, then a special pump needs to be used to be able to pump the high concentrations of ethylene glycol. Other important specifications include the internal water tank size and materials and full load amperage.
Control panel features that should be considered when selecting between industrial chillers include the local control panel, remote control panel, fault indicators, temperature indicators, and pressure indicators.
Additional features include emergency alarms, hot gas bypass, city water switchover, and casters.

Electrical Short circuit

2007-11-23T21:56:00.000-08:00

A short circuit (sometimes abbreviated to short or s/c) allows a charge to flow along a different path from the one intended. The electrical opposite of a short circuit is an open circuit, which is infinite resistance between two nodes. It is common to misuse "short circuit" to describe any electrical malfunction, regardless of the actual problem.
A short circuit is an accidental low-resistance connection between two nodes of an electrical circuit that are meant to be at different voltages. This results in an excessive electric current limited only by the Thevenin equivalent resistance of the rest of the network and potentially causes circuit damage, overheating, fire or explosion. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of voltage-sensing crowbar circuit protectors.
In circuit analysis, the term short circuit is used by analogy to designate a zero-impedance connection between two nodes. This forces the two nodes to be at the same voltage. In an ideal short circuit, this means there is no resistance and no voltage drop across the short. In simple circuit analysis, wires are considered to be shorts. In real circuits, the result is a connection of nearly zero impedance, and almost no resistance. In such a case, the current drawn is limited by the rest of the circuit.Examples,short circuit is to connect the positive and negative terminals of a battery together with a low-resistance conductor, like a wire. With low resistance in the connection, a high current flows, causing the cell to deliver a large amount of energy in a short time.
In electrical devices, unintentional short circuits are usually caused when a wire's insulation breaks down, or when another conducting material is introduced, allowing charge to flow along a different path than the one intended.
A large current through a battery (also called a cell) can cause the rapid buildup of heat, potentially resulting in an explosion or the release of hydrogen gas and electrolyte, which can burn tissue and may be either an acid or a base. Overloaded wires can also overheat, sometimes causing damage to the wire's insulation, or a fire. High current conditions may also occur with electric motor loads under stalled conditions, such as when the impeller of an electrically driven pump is jammed by debris.
In mains circuits, short circuits are most likely to occur between two phases, between a phase and neutral or between a phase and earth (ground). Such short circuits are likely to result in a very high current flowing and therefore quickly trigger an overcurrent protection device. However, it is possible for short circuits to arise between neutral and earth conductors, and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a large current flowing and are therefore less likely to be detected. Possible effects include unexpected energisation of a circuit presumed to be isolated. To help reduce the negative effects of short circuits, power distribution transformers are deliberately designed to have a certain amount of leakage reactance. The leakage reactance (usually about 5 to 10% of the full load impedance) helps limit both the magnitude and rate of rise of the fault current.Damage from short circuits can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, which disconnect the power in reaction to excessive current. Overload protection must be chosen according to the maximum prospective short circuit current in a circuit. For example, large home appliances (such as clothes dryers) typically draw 10 to 20 amperes, so it is common for them to be protected by 20 - 30 ampere circuit breakers, whereas lighting circuits typically draw less than 10 amperes and are protected by 10 - 15 ampere breakers. Wire sizes are specified in building and electrical codes, and must be carefully chosen for their specific application to ensure safe operation in conjunction with the overload protection.

What is the Wave power

2007-11-23T21:43:00.000-08:00

Wave power refers to the energy of ocean surface waves and the capture of that energy to do useful work - including electricity generation, desalination, and the pumping of water (into reservoirs). Wavey power is a form of renewable energy. Though often co-mingled, wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents. Wave power generation is not a widely employed technology, and no commercial wave farm has yet been established. Plans to install three 750 kW Pelamis devices at the europe in 2006 have been delayed and no installation had taken place by August 2007. Other plans for wave farms include a 3MW array of four 750 kW Pelamis devices in the Orkneys, off northern europe, and the 20MW Wave hub development off the europe.
The north and south temperate zonss have the best sites for capturing wave power. The prevailing westerlies in these zones blow strongest in winter.Physical concepts are Waves generated by wind passing over the sea: organized waves form from disorganized turbulence because wind pressure pushes down wave troughs and lifts up wave crests, the later due to Bernoulli's principle. See Ocean surface wave.
In general, large waves are more powerful. Specifically, wave power is determined by wave height, wave speed, wavelength, and water density.
Wave size is determined by wind speed and fetch (the distance over which the wind excites the waves) and by the depth and topography of the seafloor (which can focus or disperse the energy of the waves). A given wind speed has a matching practical limit over which time or distance will not produce larger waves. This limit is called a "fully developed sea."
Wave motion is highest at the surface and diminishes exponentially with depth; however, wave energy is also present as pressure waves in deeper water.
The potential energy of a set of waves is proportional to wave height squared times wave period (the time between wave crests). Longer period waves have relatively longer wavelengths and move faster. The potential energy is equal to the kinetic energy (that can be expended). Wave power is expressed in kilowatts per meter (at a location such as a shoreline).
The formula below shows how wave power can be calculated. Excluding waves created by major storms, the largest waves are about 15 meters high and have a period of about 15 seconds. According to the formula, such waves carry about 1700 kilowatts of potential power across each meter of wavefront. A good wave power location will have an average flux much less than this: perhaps about 50 kW/m.
Formula: Power (in kW/m) = k H² T ~ 0.5 H² T,
where k = constant, H = wave height (crest to trough) in meters, and T = wave period (crest to crest) in seconds.Good wave power locations have a flux of about 50 kilowatts per meter of shoreline. Capturing 20 percent of this, or 10 kilowatts per meter, is plausible. Assuming very large scale deployment of (and investment in) wave power technology, coverage of 5000 kilometers of shoreline (worldwide) is plausible. Therefore, the potential for shoreline-based wave power is about 50 gigawatts. Deep water wave power resources are truly enormous, but perhaps impractical to capture.

What is Electricity pylon

2007-11-23T21:29:00.000-08:00

An electricity pylon or transmission tower is a tall, usually steel lattice structure used to support overhead electricity conductors for electric power transmission.Electricity transmision towers have been used since at least the 1910s.For High voltage AC transmission towers,Three-phase electric power systems are used for high and extra-high voltage AC transmission lines (50 kV and above). The towers must be designed to carry three (or multiples of three) conductors. The towers are usually steel lattices or trusses (wooden structures are used in few europe country in exceptional cases) and the insulators are either glass or porcelain discs assembled in strings, whose length is dependent on the line voltage and environmental conditions. One or two earth conductors (alternative term: Ground conductors) for lightning protection are often mounted at the top of each tower.
In some countries, towers for high and extra-high voltage are usually designed to carry two or more electric circuits. For double circuit lines in Germany, the "Danube" towers or more rarely, the "fir tree" towers, are usually used. If a line is constructed using towers designed to carry several circuits, it is not necessary to install all the circuits at the time of construction.
Some high voltage circuits are often erected on the same tower as 110 kV lines. Paralleling circuits of 380 kV, 220 kV and 110 kV-lines on the same towers is common. Sometimes, especially with 110 kV circuits, a parallel circuit carries traction lines for railway electrification.High voltage DC transmission pylons are transmission lines either monopolar or bipolar systems. With bipolar systems a conductor arrangement with one conductor on each side of the tower is used. For single-pole HVDC transmission with ground return, towers with only one conductor can be used. In many cases, however, the towers are designed for later conversion to a two-pole system. In these cases, conductors are installed on both sides of the tower for mechanical reasons. Until the second pole is needed, it is either grounded, or joined in parallel with the pole in use. In the latter case the line from the converter station to the earthing (grounding) electrode is built as underground cable.Railway traction line pylons,Towers used for single phase AC railway traction lines are similar in construction to those towers used for 110 kV-three phase lines. Steel tube or concrete poles are also often used for these lines. However, railway traction current systems are two-pole AC systems, so traction lines are designed for two conductors (or multiples of two, usually four, eight, or twelve). As a rule, the towers of railway traction lines carry two electric circuits, so they have four conductors. These are usually arranged on one level, whereby each circuit occupies one half of the crossarm. For four traction circuits the arrangement of the conductors is in two-levels and for six electric circuits the arrangement of the conductors is in three levels.
With limited space conditions, it is possible to arrange the conductors of one traction circuit in two levels. Running a traction power line parallel to a high voltage transmission lines for three-phase AC on a separate crossarm of the same tower is possible. If traction lines are led parallel to 380 kV-lines, the insulation must be designed for 220 kV, because in the event of a fault, dangerous overvoltages to the three-phase alternating current line can occur. Traction lines are usually equipped with one earth conductor. In Austria, on some traction circuits, two earth conductors are used.There are tower testing stations for testing the mechanical properties of towers.Lattice towers can be assembled horizontally on the ground and erected by push-pull cable, but this method is rarely used because of the large assembly area needed. Lattice towers are more usually erected using a crane or, in inaccessible areas, a helicopter.Alternatives to pylons and the cables that they support are generally regarded to be unattractive. An alternative to pylons is underground cables. This is a more expensive solution than cables that are supported by pylons but have aesthetic advantages. There are schemes in various countries to improve the appearance of the environment by removing the pylons and undergrounding the cables

Wi-Fi

2007-11-19T22:51:00.000-08:00

Wi-Fi is a wireless technology brand owned by the Wi-Fi Alliance intended to improve the interoperability of wireless local area network products based on the IEEE 802.11 standards. "Wi-Fi" was thought to be derived from "Wireless Fidelity", as the Wi-Fi Alliance used this term in some of its early press releases.However, the Wi-Fi Alliance's recent official position is, that the term has no meaning.
Common applications for Wi-Fi include Internet and VoIP phone access, gaming, and network connectivity for consumer electronics such as televisions, DVD players, and digital cameras.A Wi-Fi enabled device such as a PC, game console, cell phone, MP3 player or PDA can connect to the Internet when within range of a wireless network connected to the Internet. The area covered by one or more interconnected access points is called a hotspot. Hotspots can cover as little as a single room with wireless-opaque walls or as much as many square miles covered by overlapping access points. Wi-Fi has been used to create a mesh networks, for example, in the Europe.Both architectures are used in community networks.
Wi-Fi also allows connectivity in peer-to-peer (wireless ad-hoc network) mode, which enables devices to connect directly with each other. This connectivity mode is useful in consumer electronics and gaming applications.
When the technology was first commercialized there were many problems because consumers could not be sure that products from different vendors would work together. The Wi-Fi Alliance began as a community to solve this issue so as to address the needs of the end user and allow the technology to mature. The Alliance created the branding Wi-Fi CERTIFIED to show consumers that products are interoperable with other products displaying the same branding.
Many consumer devices use Wi-Fi. Amongst others, personal computers can network to each other and connect to the Internet, mobile computers can connect to the Internet from any Wi-Fi hotspot, and digital cameras can transfer images wirelessly.
Routers which incorporate a DSL or cable modem and a Wi-Fi access point are often used in homes and other premises, and provide Internet access and internetworking to all devices connected wirelessly or by cable into them. Devices supporting Wi-Fi can also be connected in ad-hoc mode for client-to-client connections without a router.
Business and industrial Wi-Fi is widespread as of 2007. In business environments, increasing the number of Wi-Fi access points provides redundancy, support for fast roaming and increased overall network capacity by using more channels or creating smaller cells. Wi-Fi enables wireless voice applications (VoWLAN or WVOIP). Over the years, Wi-Fi implementations have moved toward 'thin' access points, with more of the network intelligence housed in a centralized network appliance, relegating individual Access Points to be simply 'dumb' radios. Outdoor applications may utilize true mesh topologies. As of 2007 Wi-Fi installations can provide a secure computer networking gateway, firewall, DHCP server, intrusion detection system, and other functions.
In addition to restricted use in homes and offices, Wi-Fi is publicly available at Wi-Fi hotspots provided either free of charge or to subscribers to various providers. Free hotspots are often provided by businesses such as hotels, restaurants, and airports who offer the service to attract or assist clients. Sometimes free Wi-Fi is provided by enthusiasts, or by organizations or authorities who wish to promote business in their area. Metropolitan-wide WiFi (Mu-Fi) already has more than 300 projects in process.In the Europe, a portion of the 2.4 GHz Wi-Fi radio spectrum is also allocated to amateur radio users. In the U.S., FCC Part 15 rules govern non-licensed operators (i.e. most Wi-Fi equipment users). Under Part 15 rules, non-licensed users must "accept" (i.e. endure) interference from licensed users and not cause harmful interference to licensed users. Amateur radio operators are licensed users, and retain what the FCC terms "primary status" on the band, under a distinct set of rules (Part 97). Under Part 97, licensed amateur operators may construct their own equipment, use very high-gain antennas, and boost output power to 100 watts on frequencies covered by Wi-Fi channels 2-6. However, Part 97 rules mandate using only the minimum power necessary for communications, forbid obscuring the data, and require station identification every 10 minutes. Therefore, output power control is required to meet regulations, and the transmission of any encrypted data (for example https) is questionable.
In practice, microwave power amplifiers are expensive. On the other hand, the short wavelength at 2.4 GHz allows for simple construction of very high gain directional antennas. Although Part 15 rules forbid any modification of commercially constructed systems, amateur radio operators may modify commercial systems for optimized construction of long links, for example. Using only 200 mW link radios and high gain directional antennas, a very narrow beam may be used to construct reliable links with minimal radio frequency interference to other users.

XHTML(Extensible HyperText Markup Language)

2007-11-18T02:23:00.000-08:00

The Extensible HyperText Markup Language, or XHTML, is a markup language that has the same depth of expression as HTML(HTML, an initialism of Hypertext Markup Language, is the predominant markup language for web pages. It provides a means to describe the structure of text-based information in a document — by denoting certain text as headings, paragraphs, lists, and so on — and to supplement that text with interactive forms, embedded images, and other objects. HTML is written in the form of labels (known as tags), surrounded by angle brackets. HTML can also describe, to some degree, the appearance and semantics of a document, and can include embedded scripting language code which can affect the behavior of web browsers and other HTML processors.
HTML is also often used to refer to content of the MIME type text/html or even more broadly as a generic term for HTML whether in its XML-descended form (such as XHTML 1.0 and later) or its form descended directly from (such as HTML 4.01 and earlier) SGML)., but also conforms to XML syntax.
Whereas HTML is an application of Standard Generalized Markup Language (SGML), a very flexible markup language, XHTML is an application of XML, a more restrictive subset of SGML. Because they need to be well-formed, true XHTML documents allow for automated processing to be performed using standard XML tools—unlike HTML, which requires a relatively complex, lenient, and generally custom parser. XHTML can be thought of as the intersection of HTML and XML in many respects, since it is a reformulation of HTML in XML. XHTML 1.0 became a World Wide Web Consortium (W3C) Recommendation on January 26, 2000. XHTML 1.1 became a W3C recommendation on May 31, 2001.XHTML is the successor to HTML. As such, many consider XHTML as the current or latest version of HTML [attribution needed]. However, XHTML is a separate recommendation; the W3C continues to recommend the use of XHTML 1.1, XHTML 1.0, and HTML 4.01 for web publishing, and HTML 5 is currently being actively developed.

Universal Mobile Telecommunications System(UMTS)

2007-11-18T02:17:00.001-08:00

What is the UTMS?Universal Mobile Telecommunications System (UMTS) is one of the third-generation (3G) cell phone technologies. Currently, the most common form uses W-CDMA as the underlying air interface, is standardized by the 3GPP, and is the European answer to the ITU IMT-2000 requirements for 3G cellular radio systems.
To differentiate UMTS from competing network technologies, UMTS is sometimes marketed as 3GSM, emphasizing the combination of the 3G nature of the technology and the GSM standard which it was designed to succeed.UMTS combines the W-CDMA, TD-CDMA, or TD-SCDMA air interfaces, GSM's Mobile Application Part (MAP) core, and the GSM family of speech codecs. In the most popular cellular mobile telephone variant of UMTS, W-CDMA is currently used. Note that other wireless standards use W-CDMA as their air interface, including FOMA.
UMTS over W-CDMA uses a pair of 5 MHz channels. In contrast, the competing CDMA2000 system uses one or more arbitrary 1.25 MHz channels for each direction of communication. UMTS and other W-CDMA systems are widely criticized for their large spectrum usage, which has delayed deployment in countries that acted relatively slowly in allocating new frequencies specifically for 3G services (such as the United States).
The specific frequency bands originally defined by the UMTS standard are 1885-2025 MHz for the mobile-to-base (uplink) and 2110-2200 MHz for the base-to-mobile (downlink). In the US, 1710-1755 MHz and 2110-2155 MHz will be used instead, as the 1900 MHz band was already utlized.Additionally, in some countries UMTS operators use the 850 MHz and 1900 MHz bands (independently, meaning uplink and downlink are within the same band), notably in the US by AT&T Mobility.
For existing GSM operators, it is a simple but costly migration path to UMTS: much of the infrastructure is shared with GSM, but the cost of obtaining new spectrum licenses and overlaying UMTS at existing towers can be prohibitively expensive.
A major difference of UMTS compared to GSM is the air interface forming Generic Radio Access Network (GeRAN). It can be connected to various backbone networks like the Internet, ISDN, GSM or to a UMTS network. GeRAN includes the three lowest layers of OSI model. The network layer (OSI 3) protocols form the Radio Resource Management protocol (RRM). They manage the bearer channels between the mobile terminals and the fixed network including the handovers.

Enhanced Data Rates for GSM Evolution(EDGE)

2007-11-18T02:11:00.000-08:00

Enhanced Data rates for GSM Evolution (EDGE) or Enhanced GPRS (EGPRS), is a digital mobile phone technology that allows increased data transmission rates and improved data transmission reliability. Although technically a 3G network technology, it is generally classified as the unofficial standard 2.75G, due to its slower network speed. EDGE has been introduced into GSM networks around the world since 2003, initially in North America.
EDGE can be used for any packet switched application, such as an Internet connection. High-speed data applications such as video services and other multimedia benefit from EGPRS' increased data capacity. EDGE Circuit Switched is a possible future development.
EDGE Evolution continues in Release 7 of the 3GPP standard providing doubled performance e.g. to complement High-Speed Packet Access (HSPA).Technology,EDGE/EGPRS is implemented as a bolt-on enhancement to 2G and 2.5G GSM and GPRS networks, making it easier for existing GSM carriers to upgrade to it. EDGE/EGPRS is a superset to GPRS and can function on any network with GPRS deployed on it, provided the carrier implements the necessary upgrade.
Although EDGE requires no hardware or software changes to be made in GSM core networks, base stations must be modified. EDGE compatible transceiver units must be installed and the base station subsystem (BSS) needs to be upgraded to support EDGE. New mobile terminal hardware and software is also required to decode/encode the new modulation and coding schemes and carry the higher user data rates to implement new services.Transmission techniques,In addition to Gaussian minimum-shift keying (GMSK), EDGE uses higher-order PSK/8 phase shift keying (8PSK) for the upper five of its nine modulation and coding schemes. EDGE produces a 3-bit word for every change in carrier phase. This effectively triples the gross data rate offered by GSM. EDGE, like GPRS, uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. It introduces a new technology not found in GPRS, Incremental Redundancy, which, instead of retransmitting disturbed packets, sends more redundancy information to be combined in the receiver. This increases the probability of correct decoding.
EDGE can carry data speeds up to 236.8 kbit/s for 4 timeslots (theoretical maximum is 473.6 kbit/s for 8 timeslots) in packet mode and will therefore meet the International Telecommunications Union's requirement for a 3G network, and has been accepted by the ITU as part of the IMT-2000 family of 3G standards. It also enhances the circuit data mode called HSCSD, increasing the data rate of this service.

GPRS(General Packet Radio Service)

2007-11-18T02:02:00.000-08:00

General Packet Radio Service (GPRS) is a Mobile Data Service available to users of Global System for Mobile Communications (GSM) and IS-136 mobile phones. GPRS data transfer is typically charged per kilobyte of transferred data, while data communication via traditional circuit switching is billed per minute of connection time, independent of whether the user has actually transferred data or has been in an idle state. GPRS can be used for services such as Wireless Application Protocol (WAP) access, Short Message Service, Multimedia Messaging Service, and for Internet communication services such as email and World Wide Web access.
2G cellular systems combined with GPRS is often described as "2.5G", that is, a technology between the second (2G) and third (3G) generations of mobile telephony. It provides moderate speed data transfer, by using unused Time division multiple access (TDMA) channels in, for example, the GSM system. Originally there was some thought to extend GPRS to cover other standards, but instead those networks are being converted to use the GSM standard, so that GSM is the only kind of network where GPRS is in use. GPRS is integrated into GSM Release 97 and newer releases. It was originally standardized by European Telecommunications Standard Institute (ETSI), but now by the 3rd Generation Partnership Project.
Capability classes
Class A
Can be connected to GPRS service and GSM service (voice, SMS), using both at the same time. Such devices are known to be available today.
Class B
Can be connected to GPRS service and GSM service (voice, SMS), but using only one or the other at a given time. During GSM service (voice call or SMS), GPRS service is suspended, and then resumed automatically after the GSM service (voice call or SMS) has concluded. Most GPRS mobile devices are Class B.
Class C
Are connected to either GPRS service or GSM service (voice, SMS). Must be switched manually between one or the other service.
A true Class A device may be required to transmit on two different frequencies at the same time, and thus will need two radios. To get around this expensive requirement, a GPRS mobile may implement the dual transfer mode (DTM) feature. A DTM-capable mobile may use simultaneous voice and packet data, with the network coordinating to ensure that it is not required to transmit on two different frequencies at the same time. Such mobiles are considered pseudo-Class A, sometimes referred to as "simple class A". Some networks are expected to support DTM in 2007.
Services and hardware
GPRS upgrades GSM data services providing:
Multimedia Messaging Service (MMS)
Push to talk over Cellular PoC / PTT
Instant Messaging and Presence -- Wireless Village
Internet Applications for Smart Devices through Wireless Application Protocol (WAP)
Point-to-point (PTP) service: internetworking with the Internet (IP protocols)
Short Message Service (SMS)
Future enhancements: flexible to add new functions, such as more capacity, more users, new accesses, new protocols, new radio networks.
USB GPRS modem
USB GPRS modems use a terminal-like interface USB 2.0 and later, data formats V.42bis, and RFC 1144 and external antennas. Modems can be add in cards (for laptop) or external USB devices which are similar in shape and size to a computer mouse.
GPRS can be used as the bearer of SMS. If SMS over GPRS is used, an SMS transmission speed of about 30 SMS messages per minute may be achieved. This is much faster than using the ordinary SMS over GSM, whose SMS transmission speed is about 6 to 10 SMS messages per minute

WHAT IS GSM

2007-11-18T01:57:00.000-08:00

Global System for Mobile communications (GSM) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 82% of the global mobile market uses the standard. GSM is used by over 2 billion people across more than 212 countries and territories.Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital call quality, and so is considered a second generation (2G) mobile phone system. This has also meant that data communication were built into the system using the 3rd Generation Partnership Project (3GPP).The key advantage of GSM systems to consumers has been better voice quality and low-cost alternatives to making calls, such as the Short message service (SMS, also called "text messaging"). The advantage for network operators has been the ease of deploying equipment from any vendors that implement the standard.Like other cellular standards, GSM allows network operators to offer roaming services so that subscribers can use their phones on GSM networks all over the world.
Newer versions of the standard were backward-compatible with the original GSM phones. For example, Release '97 of the standard added packet data capabilities, by means of General Packet Radio Service (GPRS). Release '99 introduced higher speed data transmission using Enhanced Data Rates for GSM Evolution(EDGE).Technical details,GSM is a cellular network, which means that mobile phones connect to it by searching for cells in the immediate vicinity. GSM networks operate in four different frequency ranges. Most GSM networks operate in the 900 MHz or 1800 MHz bands. Some countries in the Americas (including Canada and the United States) use the 850 MHz and 1900 MHz bands because the 900 and 1800 MHz frequency bands were already allocated.
The rarer 400 and 450 MHz frequency bands are assigned in some countries, notably Scandinavia, where these frequencies were previously used for first-generation systems.
In the 900 MHz band the uplink frequency band is 890–915 MHz, and the downlink frequency band is 935–960 MHz. This 25 MHz bandwidth is subdivided into 124 carrier frequency channels, each spaced 200 kHz apart. Time division multiplexing is used to allow eight full-rate or sixteen half-rate speech channels per radio frequency channel. There are eight radio timeslots (giving eight burst periods) grouped into what is called a TDMA frame. Half rate channels use alternate frames in the same timeslot. The channel data rate is 270.833 kbit/s, and the frame duration is 4.615 ms.
The transmission power in the handset is limited to a maximum of 2 watts in GSM850/900 and 1 watt in GSM1800/1900.
GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 5.6 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (5.6 kbit/s) and Full Rate (13 kbit/s). These used a system based upon linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.
GSM was further enhanced in 1997 with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full rate channels, and less robust but still relatively high quality when used in good radio conditions on half-rate channels.
There are four different cell sizes in a GSM network—macro, micro, pico and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base station antenna is installed on a mast or a building above average roof top level. Micro cells are cells whose antenna height is under average roof top level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen meters; they are mainly used indoors. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.
Cell horizontal radius varies depending on antenna height, antenna gain and propagation conditions from a couple of hundred meters to several tens of kilometers. The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept of an extended cell, where the cell radius could be double or even more, depending on the antenna system, the type of terrain and the timing advance.
Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when a lot of call capacity is needed indoors, for example in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from nearby cells.
The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a kind of continuous-phase frequency shift keying. In GMSK, the signal to be modulated onto the carrier is first smoothed with a Gaussian low-pass filter prior to being fed to a frequency modulator, which greatly reduces the interference to neighboring channels (adjacent channel interference).

tHE Electronic money

2007-11-12T02:54:00.000-08:00

Electronic money (electronic cash, electronic currency, digital money, digital cash or digital currency) refers to money or scrip which is exchanged only electronically. Typically, this involves use of computer networks, the internet and digital stored value systems. Electroni Funds Transfer (EFT) and direct deposit are examples of electronic money. Also, it is a collective term for financial cryptography and technologies enabling it.
While electronic money has been an interesting problem for cryptography, to date, use of digital cash has been relatively low-scale.which started as a transit payment system and has grown into a widely used electronic cash system. Another success is EUROPE Interac network, which in 2000 at retail surpassed cash as a payment method. Asean also has an electronic money implementation for its public transportation system (commuter trains, bus, etc), which is very.Octopus card and based on the same type of card (FeliCa).Alternative systems,Technically electronic or digital money is a representation, or a system of debits and credits, used (but not limited to this) to exchange value, within another system, or itself as a stand alone system, online or offline. Also sometimes the term electronic money is used to refer to the provider itself. A private currency may use gold to provide extra security, such as digital gold currency. An e-currency system may be fully backed by gold, non-gold backed, or both gold and non-gold backed.
Many systems will sell their electronic currency directly to the end user, such as Paypal and WebMoney, but other systems, such as gold service, sell only through third party digital currency exchangers.
In the case of Octopus Card in some place, deposits work similarly to banks'. After Octopus Card Limited receives money for deposit from users, the money is deposited into banks, which is similar to debit-card-issuing banks redepositing money at central banks.
Some community currencies, like some LETS systems, work with electronic transactions. Cyclos Software allows creation of electronic community currencies.
Ripple monetary system is a project to develop a distributed system of electronic money independent of local currency.Virtual debit cards,Various companies now sell VISA, Mastercard or Maestro debit cards, which can be recharged via electronic money systems. This system has the advantage of greater privacy if a card provider is located offshore, and greater security since the client can never be debited more than the value on the prepaid card. Such debit cards are also useful for people who do not have a bank account. Generally cards can be recharged with either, or via a wire transfer

testing of electrical instolation

2007-11-10T22:12:00.000-08:00

1.0 AM
pengujian adalah perkara penting yg perlu dilakukan kpd setiap pemasangan elektrik sebelum bekalan boleh diberikan.bekalan elektrik tidak boleh disambung sebelum ujian-ujian tertentu dilaksanakan dan di sahkan tiada kecacatan pada pemasangan tersebut.
tujuan utama pengujian ialah utuk memastikan bahawa sesuatu pemasangan telah disiapkan dengan sempurna dan selamat digunakan selaras dng kehendak akta dan peraturan

Akta Bekalan Elektrik 1990
Peraturan-peraturan elektrik 1994
Peraturan IEE semasa.

ujian-ujian hendaklah dilakukan mengikut peraturan semasa dan disahkan oleh orang yang kompeten yg berdaftar dgn jabatan bekalan elektrik dan masih releven.sbg contoh,pemegang sijil PW2 hanya layak utk mengendos borang H,pengujian dan pemasangan fasa tunggal.manakala pemegang PW4 pula layak mengendos borang H,pengujian dan pemasangan fasa tunggal dan tiga fasa.

Circuit breakers(High-voltage circuit breakers)

2007-11-06T23:45:00.000-08:00

Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of "high voltage" varies but in power transmission work is usually thought to be 72,500 V or over, according to a recent definition by the International Electrotechnical Commission. High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protection relay scheme can be complex, protecting equipment and busses from various types of overload or ground/earth fault.
High-voltage breakers are broadly classified by the medium used to extinguish the arc.
Oil-filled (dead tank and live tank)
Oil-filled, min oil volume
Air blast
Sulfur hexafluoride
High voltage breakers are routinely available up to 765 kV AC.
Live tank circuit breakers are where the enclosure that contains the breaking mechanism is at line potential, that is, "Live". Dead tank circuit breaker enclosures are at earth potential.

Interrupting principles for high-voltage circuit-breakers

High-voltage circuit-breakers have greatly changed since they were first introduced about more 40 years ago, and several interrupting principles have been developed that have contributed successively to a large reduction of the operating energy.
Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as sulfur hexafluoride (SF6), having excellent dielectrical and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.
Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. Sulfur hexafluoride is generally used in present high-voltage circuit-breakers (of rated voltage higher than 52 kV).
In the 1980s and 1990s, the pressure necessary to blast the arc was generated mostly by gas heating using arc energy. It is now possible to use low energy spring-loaded mechanisms to drive high-voltage circuit-breakers up to 800 kV.
Brief history
The first industrial application of SF6 for current interruption dates back to 1953. High-voltage 15 kV to 161 kV load switches were developed with a breaking capacity of 600 A. The first high-voltage SF6 circuit-breaker built in 1956 by Westinghouse, could interrupt 5 kA under 115 kV, but it had 6 interrupting chambers in series per pole. In 1957, the puffer-type technique was introduced for SF6 circuit breakers where the relative movement of a piston and a cylinder linked to the moving part is used to generate the pressure rise necessary to blast the arc via a nozzle made of insulating material (figure 1). In this technique, the pressure rise is obtained mainly by gas compression. The first high-voltage SF6 circuit-breaker with a high short-circuit current capability was produced by Westinghouse in 1959. This dead tank circuit-breaker could interrupt 41.8 kA under 138 kV (10,000 MV·A) and 37.6 kA under 230 kV (15,000 MV·A). This performance were already significant, but the three chambers per pole and the high pressure source needed for the blast (1.35 MPa) was a constraint that had to be avoided in subsequent developments. The excellent properties of SF6 lead to the fast extension of this technique in the 1970s and to its use for the development of circuit breakers with high interrupting capability, up to 800 kV.

The achievement around 1983 of the first single-break 245 kV and the corresponding 420kV to 550 kV and 800 kV, with respectively 2, 3, and 4 chambers per pole, lead to the dominance of SF6 circuit breakers in the complete range of high voltages.
Several characteristics of SF6 circuit breakers can explain their success:
Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber;
Autonomy provided by the puffer technique;
The possibility to obtain the highest performance, up to 63 kA, with a reduced number of interrupting chambers;
Short break time of 2 to 2.5 cycles;
High electrical endurance, allowing at least 25 years of operation without reconditioning;
Possible compact solutions when used for GIS or hybrid switchgear;
Integrated closing resistors or synchronised operations to reduce switching overvoltages;
Reliability and availability;
Low noise levels.
The reduction in the number of interrupting chambers per pole has led to a considerable simplification of circuit breakers as well as the number of parts and seals required. As a direct consequence, the reliability of circuit breakers improved, as verified later on by CIGRE surveys. Thermal blast chambers
New types of SF6 breaking chambers, which implement innovative interrupting principles, have been developed over the past more 15 years, with the objective of reducing the operating energy of the circuit-breaker. One aim of this evolution was to further increase the reliability by reducing the dynamic forces in the pole. Developments since 1996 have seen the use of the self-blast technique of interruption for SF6 interrupting chambers.
These developments have been facilitated by the progress made in digital simulations that were widely used to optimize the geometry of the interrupting chamber and the linkage between the poles and the mechanism.
This technique has proved to be very efficient and has been widely applied for high voltage circuit breakers up to 550 kV. It has allowed the development of new ranges of circuit breakers operated by low energy spring-operated mechanisms.

The reduction of operating energy was mainly achieved by the lowering energy used for gas compression and by making increased use of arc energy to produce the pressure necessary to quench the arc and obtain current interruption. Low current interruption, up to about 30% of rated short-circuit current, is obtained by a puffer blast.

Self-blast chambers

Further development in the thermal blast technique was made by the introduction of a valve between the expansion and compression volumes. When interrupting low currents the valve opens under the effect of the overpressure generated in the compression volume. The blow-out of the arc is made as in a puffer circuit breaker thanks to the compression of the gas obtained by the piston action. In the case of high currents interruption, the arc energy produces a high overpressure in the expansion volume, which leads to the closure of the valve and thus isolating the expansion volume from the compression volume. The overpressure necessary for breaking is obtained by the optimal use of the thermal effect and of the nozzle clogging effect produced whenever the cross-section of the arc significantly reduces the exhaust of gas in the nozzle. In order to avoid excessive energy consumption by gas compression, a valve is fitted on the piston in order to limit the overpressure in the compression to a value necessary for the interruption of low short circuit currents.

This technique, known as “self-blast” has now been used extensively since 1996 for the development of many types of interrupting chambers. The increased understanding of arc interruption obtained by digital simulations and validation through breaking tests, contribute to a higher reliability of these self-blast circuit breakers. In addition the reduction in operating energy, allowed by the self blast technique, leads to longer service life.

Double motion of contacts

An important decrease in operating energy can also be obtained by reducing the kinetic energy consumed during the tripping operation. One way is to displace the two arcing contacts in opposite directions so that the arc speed is half that of a conventional layout with a single mobile contact.

The thermal and self blast principles have enabled the use of low energy spring mechanisms for the operation of high voltage circuit breakers. They progressively replaced the puffer technique in the 1980s; first in 72.5 kV breakers, and then from 145 kV to 800 kV.

Comparison of single motion and double motion techniques

The double motion technique halves the tripping speed of the moving part. In principle, the kinetic energy could be quartered if the total moving mass was not increased. However, as the total moving mass is increased, the practical reduction in kinetic energy is closer to 60%. The total tripping energy also includes the compression energy, which is almost the same for both techniques. Thus, the reduction of the total tripping energy is lower, about 30%, although the exact value depends on the application and the operating mechanism. Depending on the specific case, either the double motion or the single motion technique can be cheaper. Other considerations, such as rationalization of the circuit-breaker range, can also influence the cost.
Thermal blast chamber with arc-assisted opening
In this interruption principle arc energy is used, on the one hand to generate the blast by thermal expansion and, on the other hand, to accelerate the moving part of the circuit breaker when interrupting high currents. The overpressure produced by the arc energy downstream of the interruption zone is applied on an auxiliary piston linked with the moving part. The resulting force accelerates the moving part, thus increasing the energy available for tripping.
With this interrupting principle it is possible, during high-current interruptions, to increase by about 30% the tripping energy delivered by the operating mechanism and to maintain the opening speed independently of the current. It is obviously better suited to circuit-breakers with high breaking currents such as Generator circuit-breakers.
Generator circuit-breakers
Generator circuit-breakers are connected between a generator and the step-up voltage transformer. They are generally used at the outlet of high power generators (100 MVA to 1800 MVA) in order to protect them in a reliable, fast and economic manner. Such circuit breakers must be able to allow the passage of high permanent currents under continuous service (6.3 kA to 40 kA), and have a high breaking capacity (63 kA to 275 kA). They belong to the medium voltage range, but the TRV withstand capability required by ANSI/IEEE Standard C37.013 is such that the interrupting principles developed for the high-voltage range must be used. A particular embodiment of the thermal blast technique has been developed and applied to generator circuit-breakers. The self-blast technique described above is also widely used in SF6 generator circuit breakers, in which the contact system is driven by a low-energy, spring-operated mechanism. An example of such a device is shown in the figure below; this circuit breaker is rated for 17.5 kV and 63 kA.Evolution of tripping energy
The operating energy has been reduced by 5 to 7 times during this period of 27 years. This illustrates well the great progress made in this field of interrupting techniques for high-voltage circuit-breakers. Future perspectives
In the near future, present interrupting technologies can be applied to circuit-breakers with the higher rated breaking currents (63 kA to 80 kA) required in some networks with increasing power generation.
Self blast or thermal blast circuit breakers are nowadays accepted world wide[citation needed] and they have been in service for high voltage applications for about 15 years[citation needed], starting with the voltage level of 72.5 kV. Today this technique is also available for the voltage levels 420/550/800 kV.

Circuit breaker(Types of circuit breaker)

2007-11-06T23:42:00.000-08:00

There are many different technologies used in circuit breakers and they do not always fall into distinct categories. Types that are common in domestic, commercial and light industrial applications at low voltage (less than 1000 V) include:
MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.
MCCB (Moulded Case Circuit Breaker)—rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable.
Electric power systems require the breaking of higher currents at higher voltages. Examples of high-voltage AC circuit breakers are:
Vacuum circuit breaker—With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These can only be practically applied for voltages up to about 35,000 V, which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.
Air circuit breaker—Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

Circuit breaker(Common trip breakers)

2007-11-06T23:36:00.000-08:00

When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two pole common trip breakers are common on 120/240 volt systems where 240 volt loads (including major appliances or further distribution boards) span the two live wires. Three pole common trip breakers are typically used to supply three phase power to large motors or further distribution boards.

Voice over IP

2007-11-05T21:28:00.000-08:00

Voice over Internet Protocol, also called VoIP (pronounced voyp), IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of voice conversations over the Internet or through any other IP-based network.
Companies providing VoIP service are commonly referred to as providers, and protocols which are used to carry voice signals over the IP network are commonly referred to as Voice over IP or VoIP protocols. They may be viewed as commercial realizations of the experimental Network Voice Protocol (1973) invented for the ARPANET providers. Some cost savings are due to utilizing a single network to carry voice and data, especially where users have existing underutilized network capacity that can carry VoIP at no additional cost. VoIP to VoIP phone calls are sometimes free, while VoIP to public switched telephone networks, PSTN, may have a cost that is borne by the VoIP user.
Voice over IP protocols carry telephony signals as digital audio, typically reduced in data rate using speech data compression techniques, encapsulated in a data packet stream over IP.
There are two types of PSTN to VoIP services: Direct Inward Dialing (DID) and access numbers. DID will connect the caller directly to the VoIP user while access numbers require the caller to input the extension number of the VoIP user.

Functionality

VoIP can facilitate tasks that may be more difficult to achieve using traditional networks that have been typically used historically:
Ability to transmit more than one telephone call down the same broadband-connected telephone line. This can make VoIP a simple way to add an extra telephone line to a home or office.
Many VoIP packages include PSTN features that most telcos (telecommunication companies) normally charge extra for, or may be unavailable from your local telco,such as 3-way calling, call forwarding, automatic redial, and caller ID.
VoIP can be secured with existing off-the-shelf protocols such as Secure Real-time Transport Protocol. Most of the difficulties of creating a secure phone over traditional phone lines, like digitizing and digital transmission are already in place with VoIP. It is only necessary to encrypt and authenticate the existing data stream.
VoIP is location independent, only an internet connection is needed to get a connection to a VoIP provider; for instance call center agents using VoIP phones can work from anywhere with a sufficiently fast and stable Internet connection.
VoIP phones can integrate with other services available over the Internet, including video conversation, message or data file exchange in parallel with the conversation, audio conferencing, managing address books and passing information about whether others (e.g. friends or colleagues) are available online to interested parties.

Key telephone system

2007-11-05T01:49:00.000-08:00

A key system or key telephone system is a multiline telephone system typically used in small office environments.
Key systems are noted for their expandability and having individual line selection buttons for each connected phone line, however some features of a private branch exchange such as dialable intercoms may also commonly be present.
Key systems can be built using three principal architectures:
Electromechanical shared-control
Electronic shared-control
Independent keysets
Electromechanical shared-control key systems
Before the advent of large-scale integrated circuits, key systems were typically built out of the same electromechanical components (relays) as larger telephone switching systems. The system marketed in North America as the 1A2 Key System was entirely typical and sold for many decades.
Electronic shared-control systems
With the advent of LSI ICs, the same architecture could be implemented much less expensively than was possible using relays. In addition, it was possible to eliminate the many-wire cabling and replace it with much simpler cable similar to (or even identical to) that used by non-key systems. One of the most recognized such systems is the AT&T Merlin.
Additionally, these more-modern systems allowed vastly more features including:
Answering machine functions
Remote supervision of the entire system
Automatic call accounting
Speed dialing
Caller ID
Etc.
Features could be added or modified simply using software, allowing easy customization of these systems.
Independent keysetsLSI also allowed smaller systems to distribute the control (and features) into individual telephone sets that don't require any single shared control unit. Generally, these systems are used with a relatively few telephone sets and it is often more difficult to keep the feature set (such as speed-dialing numbers) in synchrony between the various sets.
PBX systems
The line between the largest key systems and full PBX systems is blurred. In the 1A2 days, the line was clear: 1A2 systems did not allow the sharing of anonymous "trunk" lines and PBX systems did. Modern key systems blur this distinction by often allowing this feature.
Hybrid keyphone systems
Into the 21st century, the distinction between key systems and PBX has become increasingly confusing. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations. The modern key system now supports ISDN, analogue handsets (in addition to its own dedicated handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems. The fact that they support both analogue and digital signalling types gives rise to the "Hybrid" designation.
The modern key system is usually fully digital (although analogue variants persist) and some systems embrace VOIP. Indeed, key systems now can be considered to have left their humble roots and become small PBXes. Effectively, the aspects that separate a PBX from a key system are the amount, scope and complexity of the features and facilities offered.

Hydroelectricity

2007-10-31T00:40:00.000-07:00

Hydroelectricity is electricity produced by hydropower. Hydroelectricity now supplies about 715,000 MWe or 19% of world electricity (16% in 2003), accounting for over 63% of the total electricity from renewables in 2005.
Although large hydroelectric installations generate most of the world's hydroelectricity, small hydro schemes are particularly popular in China, which has over 50% of world small hydro capacity.

Engine generator

2007-10-29T04:51:00.000-07:00

An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a gen-set. In many contexts, the engine is taken for granted and the combined unit is simply called a generator.
In addition to the engine and generator, engine-generators generally include a fuel tank, an engine speed regulator and a generator voltage regulator. Many units are equipped with a battery and electric starter. Standby power generating units often include an automatic starting system and a transfer switch to disconnect the load from the utility power source and connect it to the generator.
Engine-generators are often used to supply electrical power in places where utility power is not available and in situations where power is needed only temporarily. Small generators are sometimes used to supply power tools at construction sites. Trailer-mounted generators supply power for temporary installations of lighting, sound ampliification systems, amusement rides etc.
Standby power generators are permanently installed and kept ready to supply power to critical loads during temporary interruptions of the utility power supply. Hospitals, communications service installations, sewage pumping stations and many other important facilities are equipped with standby power generators.
Small and medium generators are especially popular in third world countries to supplement grid power, which is often unreliable. Trailer-mounted generators can be towed to disaster areas where grid power has been temporarily disrupted.
The generator can also be driven by the human muscle power (for instance, in the field radio station equipment).
The generator voltage (volts), frequency (Hz) and power (watts) ratings are selected to suit the load that will be connected.
Engine-generators are available in a wide range of power ratings. These include small, hand-portable units that can supply several hundred watts of power, hand-cart mounted units, as pictured above, that can supply several thousand watts and stationary or trailer-mounted units that can supply over a million watts. The smaller units tend to use gasoline (petrol) as a fuel, and the larger ones have various fuel types, including diesel, natural gas and propane (liquid or gas).
There are only a few portable three-phase generator models available in the US. Most of the portable units available are single phase power only and most of the three-phase generators manufactured are large industrial type generators.
Portable engine-generators may require an external power conditioner to safely operate some types of electronic equipment.

what is Direct on line starter

2007-10-28T23:35:00.001-07:00

Definition
A motor starter is an electrical/electronic circuit composed of electro-mechanical and electronic devices which are employed to start and stop an electric motor. Regardless of the motor type (AC or DC), the types of starters differ depending on the method of starting the motor. A DOL starter connects the motor terminals directly to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, high starting current flows through the motor. This type of starting is suitable for small motors below 5 hp (3.75 kW). Reduced-voltage starters are employed with motors above 5 hp.

Starter Circuits
The motor starter is composed of push-button switches, relays, isolators, circuit breakers, transformers and interlock switches. Its circuit is split into two circuits; control and power. The power circuit is the higher-voltage circuit to which the motor terminals connect with the power supply. The control circuit is the lower-voltage circuit and it is electrically isolated from the power circuit. The starter is energized by the control circuit using electro-mechanical relays and push button switches. Modern starters employ solid state relays (electronic relays).

DOL Reversing Starter
Most motors are reversible or, in other words, they can be run clockwise and anti-clockwise. A reversing starter is an electrical or electronic circuit that reverses the speed of a motor automatically. Logically, the circuit is composed of two DOL circuits; one for clockwise operation and the other for anti-clockwise operation.

Example of Motor Starters

Control and power circuits of 3-phase cage motor DOL starter.
A very well-known motor starter is the DOL Starter of a 3-Phase Squirrel-Cage Motor. This starter is sometimes used to start water pumps, compressors, fans and conveyor belts. With a 400V, 50 Hz, 3-phase supply, the power circuit connects the motor to 400V. Consequently, the starting current may reach 3-8 times the normal current. The control circuit is typically run at 24V with the aid of a 400V/24V transformer. An animation of the circuits of this starter is shown here.

Motor direction reversal
Changing the direction of a 3-Phase Squirrel-Cage Motor requires swapping any two phases. This could be achieved by a contactor KM1 swapping phase L2 and L3 between the supply and the motor.

Motor controller(Direct on line starter)

2007-10-28T23:26:00.000-07:00

A direct on line starter, often abbreviated DOL starter, is a widely-used starting method of electric motors. The term is used in electrical engineering and associated with electric motors. There are many types of motor starters, the simplest of which is the DOL starter.

switch gear for substation

2007-10-24T03:28:00.000-07:00

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.

Locations
Switchgears are located anywhere that isolation and protection may be required. These locations include generators, motors, transformers, and substations.
Substations
Typically switchgear in substations is located on both the high voltage and the low voltage side of large power transformers. The switchgear located on the low voltage side of the transformers in distribution type substations, now are typically located in what is called a Power Distribution Center (PDC). Inside this building are typically smaller, medium-voltage (~15kV) circuit breakers feeding the distribution system. Also contained inside these Power Control Centers are various relays, meters, and other communication equipment allowing for intelligent control of the substation.
For industrial applications, a transformer and switchgear line-up may be combined in one housing, called a unit substation.
Housing
Switchgear for low voltages may be entirely enclosed within a building. For transmission levels of voltage (high voltages over 66 kV), often switchgear will be mounted outdoors and insulated by air, though this requires a large amount of space. Gas-insulated switchgear used for transmission-level voltages saves space, although it has a higher equipment cost.
At small substations, switches may be manually operated, but at important switching stations on the transmission network all devices have motor operators to allow for remote control.
Types
A piece of switchgear may be a simple open air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is "gas insulated switchgear" (GIS), where the conductors and contacts are insulated by pressurized (SF6) sulfur hexafluoride gas. Another common type is oil insulated switchgear.
Circuit breakers are a special type of switchgear that are able to interrupt fault currents. Their construction allows them to interrupt fault currents of many hundreds or thousands of amps. The quenching of the arc when the contacts open requires careful design, and falls into four types:
Oil circuit breakers rely upon vaporisation of some of the oil to blast a jet of oil through the arc.
Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.
Vacuum circuit breakers have minimal arcing (as there is nothing to ionise other than the contact material), so the arc quenches when it is stretched a very small amount (<2-3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts. Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc. Circuit breakers are usually able to terminate all current flow very quickly: typically between 30mS and 150mS depending upon the age and construction of the device. Several different classifications of switchgear can be made:
By size of current that they may safely switch:
Circuit breakers can open and close on fault currents
Load-break/Load-make switches can switch normal system load currents
Isolators may only be operated while the circuit is dead, or the load current is very small.
By voltage class:
Low voltage (less than 1000 volts AC)
Medium voltage (1000-35,000 volts AC)
High voltage (more than 35,000 volts AC)
By insulating medium:
Air
Gas (SF6 or mixtures)
Oil
Vacuum
By construction type:
Indoor
Outdoor
Industrial
Utility
Marine
Draw-out elements (removable without many tools)
Fixed elements (bolted fasteners)
Live-front
Dead-front
Open
Metal-enclosed
Metal-clad
Arc-resistant
By IEC degree of internal separation
No Separation (Form 1)
Busbars separated from functional units (Form 2a, 2b, 3a, 3b, 4a, 4b)
Terminals for external conductors separated from busbars (Form 2b, 3b, 4a, 4b)
Terminals for external conductors separated from functional units but not from each other (Form 3a, 3b)
Functional units separated from each other (Form 3a, 3b, 4a, 4b)
Terminals for external conductors separated from each other (Form 4a, 4b)
Terminals for external conductors separate from their associated functional unit (Form 4b)
By interrupting device:
Fuses
Air Blast Circuit Breaker
Minimum Oil Circuit Breaker
Oil Circuit Breaker
Vacuum Circuit Breaker
Gas (SF6) Circuit breaker
By operating method:
Manually-operated
Motor-operated
Solenoid/stored energy operated
By type of current:
Alternating current
Direct current
By interrupting rating (maximum short circuit current that the device can safely interrupt)
By application:
Transmission system
Distribution.
A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles.
Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.
Functions
One of the main basic functions of switchgear is protection: discrimination between circuit breakers enhances availability, that is to say continuity of service. The overall approach is termed coordination: the standards provide a framework for discrimination and cascading that protects the integrity of the power system and minimizes the scope of downstream outages.
Safety
To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. James Harry Castell invented this technique in 1922. For example, if only one of two sources of supply are permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.

Transformer

2007-10-22T20:26:00.000-07:00

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled wires. A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary). By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.
The secondary induced voltage VS is scaled from the primary VP by a factor ideally equal to the ratio of the number of turns of wire in their respective windings:

By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up — by making NS more than NP — or stepped down, by making it less.
A key application of transformers is to reduce the current before transmitting electrical energy over long distances through wires. Most wires have resistance and so dissipate electrical energy at a rate proportional to the square of the current through the wire. By transforming electrical power to a high-voltage, and therefore low-current form for transmission and back again afterwards, transformers enable the economic transmission of power over long distances. Consequently, transformers have shaped the electricity supply industry, permitting generation to be located remotely from points of demand.[1] All but a fraction of the world's electrical power has passed through a series of transformers by the time it reaches the consumer.[2]
Transformers are some of the most efficient electrical 'machines',[3] with some large units able to transfer 99.75% of their input power to their output.[4] Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigavolt-ampere-rated units used to interconnect portions of national power grids. All operate with the same basic principles, though a variety of designs exist to perform specialized roles throughout home and industry.

Electrical insulation

2007-10-06T21:51:00.000-07:00

Electrical insulator is a material or object that resists the flow of electric current. When a voltage is placed across an insulator, very little current flows. An object intended to support or separate electrical conductors without passing current through itself is called an insulator. An insulator is a material with atoms that have tightly bonded valence electrons and resist the flow of electrical current.
The term electrical insulation has the same meaning as the term dielectric.
Some materials such as silicon dioxide or teflon are very good electrical insulators. A much larger class of materials, for example rubber-like polymers and most plastics are still "good enough" to insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).

Physics of conduction in solids

Electrical insulation is the absence of electrical conduction. Electronic band theory (a branch of physics) predicts that a charge will flow whenever there are states available into which the electrons in a material can be excited. This allows them to gain energy and thereby move through the conductor (usually a metal). If no such states are available, the material is an insulator.
Most (though not all, see Mott insulator) insulators are characterized by having a large band gap. This occurs because the "valence" band containing the highest energy electrons is full, and a large energy gap separates this band from the next band above it. There is always some voltage (called the breakdown voltage) that will give the electrons enough energy to be excited into this band. Once this voltage is exceeded, the material ceases being an insulator, and charge will begin to pass through it. However, dielectric breakdown is usually accompanied by physical or chemical changes that permanently degrade the material's insulating properties.
Materials which lack electron conduction must also lack other mobile charges as well. For example, if a liquid or gas contains ions, then the ions can be made to flow as an electric current, and the material is a conductor. Electrolytes and plasmas contain ions and will act as conductors whether or not electron flow is involved.

Telegraph and power transmission insulators

Suspended wires for electric power transmission are bare, except when connecting to houses, and are insulated by the surrounding air and where connected to towers, as detailed below.

Material:-High-voltage insulators used for high-voltage power transmission are made from glass, porcelain, or composite polymer materials. Porcelain insulators are made from clay, quartz or alumina and feldspar, and are covered with a smooth glaze to shed dirt. The design of insulators often includes deep grooves, or sheds, that provides increased arc-lengths. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Glass insulators were (and in some places still are) used to suspend electrical power lines. Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to various ceramic and, more recently, composite materials.
Recently, some electric utilities have begun converting to polymer composite materials for some types of insulators which consist of a central rod made of fibre reinforced plastic and an outer weathershed made of silicone rubber or EPDM. Composite insulators are less costly, lighter in weight, and have excellent hydrophobic capability. This combination makes them ideal for service in polluted areas. However, these materials do not yet have the long-term proven service life of glass and porcelain.

History:-The first electrical systems to make use of insulators were telegraph lines; direct attachment of wires to wooden poles was found to give very poor results, especially during damp weather.
The first glass insulators used en masse had an unthreaded pinhole. These pieces of glass were positioned on a tapered wooden pin, vertically extending upwards from the pole's crossarm (commonly only two insulators to a pole and maybe one on top of the pole itself). Natural contraction and expansion of the wires tied to these "threadless insulators" resulted in insulators unseating from their pins, requiring manual reseating.
Amongst the first to produce ceramic insulators were companies in the United Kingdom, with Stiff and Doulton using stoneware from the mid 1840s, Joseph Bourne (later renamed Denby) producing them from around 1860 and Bullers from 1868. Utility patent number 48,906 was granted to Louis A. Cauvet on July 25, 1865 for a process to produce insulators with a threaded pinhole. To this day, pin-type insulators still have threaded pinholes.
The invention of suspension-type insulators made high-voltage power transmission possible. Pin-type insulators were unsatisfactory over about 60,000 volts.

SUBstation of electrical

2007-09-27T23:52:00.000-07:00

A 115 kV to 41.6/12.47 kV 5000 kVA 60 Hz substation with circuit switcher, regulators, reclosers and control building,An electrical substation is a subsidiary station of an electricity generation, transmission and distribution system where voltage is transformed from high to low or the reverse using transformers.

Explanation
Transformation may take place in several stages in sequence, starting at the generating plant where the voltage is increased for transmission purposes and is then progressively reduced to the voltage required for household or industrial use. The range of voltages in a power system varies from 110 V up to 765 kV depending on the country.
A substation that has a step-up transformer increases the voltage while decreasing the current, while a step-down transformer decreases the voltage while increasing the current for domestic and commercial distribution. The word substation comes from the days before the distribution system became a grid. The first substations were connected to only one power station where the generator was housed, and were subsidiaries of that power station.
Substations generally contain one or more transformers, and have switching, protection and control equipment. In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. Smaller distribution stations may use recloser circuit breakers or fuses for protection of branch circuits. Substations do not (usually) have generators, although a power plant may have a substation nearby. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, and metering. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation.
Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may have indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.
Where a substation has a fence, it must be properly grounded (UK: earthed) to protect people from high voltages that may occur during a fault in the transmission system. Earth faults at a substation can cause Earth Potential Rise at the fault location.

Transmission substation
A transmission substation is one whose main purpose is to connect together various transmission lines.
The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected together or isolated for maintenance.
Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, and a large amount of protection and control equipment (capacitors, relays, switches, breakers, voltage and current transformers).

Distribution substation

A distribution substation is one whose main purpose is to transfer power from the transmission system to the distribution system of some area. It is uneconomical to directly connect electricity consumers to the main transmission network (unless they use large amounts of energy); so the distribution station reduces voltage to a value suitable for connection to local loads.
The input for a distribution substation is typically at least two transmission or subtransmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility.
The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises.
Besides transforming the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.
Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

Design

The main issues facing a power engineer are reliability and cost. A good design attempts to strike a balance between these two, to achieve sufficient reliability without excessive cost. The design should also allow easy expansion of the station, if required.

Layout

In the largest stations, incoming lines will almost always have a disconnect switch and a circuit breaker. In some cases, the lines will not have both; with either a switch or a circuit breaker being all that is considered necessary. These devices are used as isolation and protection devices. A disconnect switch is almost always used solely to provide isolation, due to it not being rated for breaking a loaded circuit, whilst a circuit breaker is often used both as an isolation element as well as a protection device.[clarify] Where a large fault current flows through the circuit break this may be detected through the use of current transformers. The magnitude of the current transformer outputs may be used to 'trip' the circuit breaker resulting in a disconnection of the load supplied by the circuit break from the feeding point. This seeks to isolate the fault point from the rest of the system, and allow the system to continue operating with minimal impact.
Once past the switching components, the lines of a given voltage all tie in to a common bus. This is a number of thick metal bus bars, in most cases there are three bars, since three-phase electrical power distribution is largely universal around the world.
Substations that require additional reliability often have a double bus or even a double ring bus, in which the bus system is actually duplicated, with each feeder (and all or most outgoers) having a connection to each separate bus. Most substations will not have this, as it is mainly for reliability in substations whose failure would bring down a substantial part of the system, or whose load is of vital importance. Other compromises between a single and double bus can be found; for example, the breaker-and-a-half setup.
Once having established buses for the various voltage levels, transformers may be connected between the voltage levels. These will again have a circuit breaker, much like transmission lines, in case a transformer has a fault (commonly called a 'short circuit').
Along with this, a substation always has control circuitry needed to command the various breakers to open in case of the failure of some component.

Switching function

An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned".
A transmission line or other component may need to be deenergized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer.
To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.
Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The function of the substation is to isolate the faulted portion of the system in the shortest possible time.
There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.

Electric power supply

2007-09-27T23:05:00.000-07:00

Acme Electric is a full line manufacturer of low voltage (600 V and below) dry type distribution transformers using both copper and aluminum conductor, offering an array of products between 0.250-1000 KVA. Acme's product offering covers the full spectrum of applications, from commercial general power distribution and high harmonic conditions to specific industrial motor drive/factory automation systems, to low voltage landscape lighting applications. All Acme products are designed, constructed and rated to meet or exceed the standards established by UL, CSA, CE, NEMA, ANSI and IEEE.