Electricity pylon

Transmission tower

A transmission tower
Type	structure, lattice tower and overhead power line
First production	20th century

A transmission tower or power tower (electricity pylon in the United Kingdom, Canada and parts of Europe) is a tall structure, usually a steel lattice tower, used to support an overhead power line.

They are used in high-voltage AC and DC systems, and come in a wide variety of shapes and sizes. Typical height ranges from 15 to 55 m (49 to 180 ft), though the tallest are the 370 m (1,214 ft) towers of a 2,700 m (8,858 ft) span of Zhoushan Island Overhead Powerline Tie. In addition to steel, other materials may be used, including concrete and wood.

There are four major categories of transmission towers:suspension, terminal, tension, and transposition. Some transmission towers combine these basic functions. Transmission towers and their overhead power lines are often considered to be a form of visual pollution. Methods to reduce the visual effect include undergrounding.

"Transmission tower" is the name for the structure used in the industry in the United States, and some other English-speaking countries. The term "pylon" comes from the basic shape of the structure, an obelisk-like structure which tapers toward the top, and is mostly used in the United Kingdom and parts of Europe in everyday colloquial speech. This term is used infrequently in the United States, as the word "pylon" is commonly used for many other things, mostly for traffic cones.

Three-phase electric power systems are used for high voltage (66- or 69-kV and above) and extra-high voltage (110- or 115-kV and above; most often 138- or 230-kV and above in contemporary systems) AC transmission lines. 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 Canada, Germany, and Scandinavia in some cases) and the insulators are either glass or porcelain discs or composite insulators using silicone rubber or EPDM rubber material assembled in strings or long rods whose lengths are dependent on the line voltage and environmental conditions.