Overhead lines or overhead wires are used to transmit electrical energy to trams, trolleybuses or trains at a distance from the energy supply point. Overhead line is designed on the principle of one or more overhead wires situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regular intervals. The feeder stations are usually fed from a high-voltage electrical grid.
Electric trains that collect their current from overhead line system use a device such as pantograph, bow collector, or trolley pole. The current collection device presses against the underside of the lowest wire of an overhead line system, which is called a contact wire. The current collectors are electrically conductive, and allow current to flow through to the transformer of the train or tram, and back to the feeder station via the steel wheels and one or both running rails of the track. Diesel trains may pass along these tracks without affecting the overhead line, although overhead clearance may be an issue.
Reference: Wikipedia (Overhead lines)
Nagareyama-ōtakanomori. January 10th, 2008
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