Overhead line

An overhead line or overhead wire is an electrical cable that is used to transmit electrical energy to electric locomotives, electric multiple units, trolleybuses or trams. The generic term used by the International Union of Railways for the technology is overhead line. It is known variously as overhead catenary, overhead contact line (OCL), overhead contact system (OCS), overhead equipment (OHE), overhead line equipment (OLE or OHLE), overhead lines (OHL), overhead wiring (OHW), traction wire, and trolley wire.

An overhead line consists of one or more wires (or rails, particularly in tunnels) situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regularly spaced intervals along the track. 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.

Vehicles like buses that have rubber tyres cannot provide a return path for the current through their wheels, and must instead use a pair of overhead wires to provide both the current and its return path.

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 or catenary. 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 pantograph moves along under the contact wire, the carbon insert on top of the pantograph becomes worn with time. 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 insert wears evenly, thus preventing any notches. On curves, the "straight" wire between the supports causes the contact point to cross over the surface of the pantograph as the train travels around the curve. The movement of the contact wire across the head of the pantograph is called the "sweep".

The zigzagging of the overhead line is not required for trolley poles. For tramways, a contact wire without a messenger wire is used.

Depot areas tend to have only a single wire and are known as "simple equipment" or "trolley wire". When overhead line systems were first conceived, good current collection was possible only at low speeds, using a single wire. To enable higher speeds, two additional types of equipment were developed: