Also known as:

Overhead contact system, Overhead line equipment, Catenary

Overhead track equipment is designed with one or more overhead wires directly over railway tracks. These lines are raised to a high electrical potential by connecting to mini-substations at regular intervals, these mini-substations are usually fed from a high voltage electrical grid.

Electric trains use a device called a trolley pole or pantograph. This device presses against the underside of the lowest wire of the overhead track equipment; this wire is called the contact wire. These current collectors are electrically conductive, and allow electricity to flow through to the train and back to the mini-substation through the steel wheels and one or both running rails.

Construction

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 above by a second wire known as the messenger wire. This wire is allowed to follow 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. The messenger wire is supported regularly at structures, by a pulley, link, or clamp. The whole system is then subjected to a mechanical tension.

As the contact wire makes contact with the pantograph, the carbon surface of the insert on top of the pantograph is worn down. Going around a curve, the “straight” wire between supports will cause the contact wire to cross over the whole surface of the pantograph as the train travels around the curve, causing an even wear and avoiding any notches. On straight track, the contact wire is zigzagged slightly to the left and right of centre at each successive support so that the pantograph wears evenly.

The zigzagging of the overhead line is not required for trolley-based trams or trolleybuses.

Depot areas tend to have only a single wire and are known as simple equipment. 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:

• Stitched equipment uses an additional wire at each support structure, terminated on either side of the messenger wire.
• Compound equipment uses a second support wire, known as the auxiliary, between the messenger wire and the contact wire. Droppers support the auxiliary from the messenger wire, and additional droppers support the contact wire from the auxiliary. The auxiliary wire can be constructed of a more conductive but less wear-resistant metal, increasing the efficiency of power transmission.

Dropper wires traditionally only provide physical support of the contact wire, and do not join the catenary and contact wires electrically. Contemporary systems use current-carrying droppers, which eliminate the need for separate wires.

For tramways there is often just a simple contact wire and no messenger wire.