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
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
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