Overhead line

An overhead line or overhead wire is used to transmit electrical energy to trams, trolleybuses or trains. It is known variously as: An overhead line or overhead wire is used to transmit electrical energy to trams, trolleybuses or trains. It is known variously as: In this article, the generic term overhead line is used, as used by the International Union of Railways. An overhead line is designed on the principle of one or more overhead wires (or rails, particularly in tunnels) 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 lines use a device such as a pantograph, bow collector or trolley pole. It presses against the underside of the lowest overhead wire, the contact wire. Current collectors are electrically conductive and allow current to flow through to the train or tram and back to the feeder station through the steel wheels on one or both running rails. Non-electric locomotives (such as diesels) may pass along these tracks without affecting the overhead line, although there may be difficulties with overhead clearance. Alternative electrical power transmission schemes for trains include third rail, ground-level power supply, batteries and electromagnetic induction. To achieve good high-speed current collection, it is necessary to keep the contact wire geometry within defined limits. This is usually achieved by supporting the contact wire from a second wire known as the messenger wire (in the US & Canada) or catenary (in the UK). This wire approximates the natural path of a wire strung between two points, a catenary curve, thus the use of 'catenary' to describe this wire or sometimes the whole system. This wire is attached to the contact wire at regular intervals by vertical wires known as 'droppers' or 'drop wires'. It is supported regularly at structures, by a pulley, link or clamp. The whole system is then subjected to mechanical tension. As the contact wire makes contact with the pantograph, the carbon insert on top of the pantograph is worn down. On curves, the 'straight' wire between the supports cause the contact wire to cross over the whole surface of the pantograph as the train travels around the curve, causing uniform wear and avoiding any notches. On straight track, the contact wire is zigzagged slightly to the left and right of the centre from each support to the next so that the pantograph wears evenly. The movement of the contact wire across the head of the pantograph is called the 'sweep'.