Overhead Lines

Overhead Lines are used to transmit electrical energy to a train at a distance from the energy supply point. These overhead lines are known variously as OCS (Overhead Contact System - US & Europe), OLE (Overhead Line Equipment - UK) or Catenary (generic term). For the purposes of this article the generic term Overhead Line has been used.

Overhead Line is designed on the principle of one or more overhead wires situated over a track, 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.

As an electric train passes under the lowest wire, known as the Contact Wire, a device on the train roof called the Pantograph makes contact with the Contact Wire. The pantograph is electrically conductive, and allows current to flow to earth. This path takes the current through the traction motors of the train, and back to the feeder station via the train wheels and one or both track running rails. Diesel trains may pass along these tracks without affecting the Overhead Line.

To achieve good high speed current collection, it is necessary to keep the contact wire nominally level throughout the length of the Overhead Line. This is usually achieved by supporting the contact wire from above by means of a second wire, known variously as the Messenger Wire (US & Europe) or catenary (UK). This wire is attached to the contact wire at regular intervals by vertical wires known as Droppers. The Messenger Wire is supported regularly at structures, either by means of a pulley, link or clamp. The whole system is then subjected to a mechanical tension. Such a system, with a single supporting wire, is known as Simple Equipment.

When Overhead Line systems were first conceived, good current collection was not possible at high speed using a single supporting wire. Two additional types of equipment were developed to combat this problem. Stitched Equipment used an additional wire at each support structure, which was terminated either side to the Messenger Wire. Compound Equipment used a second support wire, known as the Auxiliary, running the whole length of the Overhead Line between the Messenger Wire and the Contact wire. Droppers are provided to support the Auxiliary from the Messenger Wire, and additional droppers support the Contact Wire from the Auxiliary.

For medium and high speeds the wires are generally tensioned by means of weights, or occasionally, by hydraulic tensioners. This is known as auto-tensioning (AT), and ensures that the tension in the equipment is virtually independent of temperature. Tensions are typically between 9 and 20kN per wire.

For low speeds, fixed termination (FT) equipment may be used, with the wires terminated directly on structures at each end of the Overhead Line. Here the tension is generally about 10kN. This type of equipment will sag on hot days and hog on cold days.

Where AT is used, there is a limit to the continuous length of Overhead Line which may be installed. This is due to the change in the position of the weights with temperature as the Overhead Line expands and contracts. This movement is proportional to the Tension Length, i.e. the distance between anchors. This leads to the concept of maximum Tension Length. For most equipment in the UK the maximum Tension Length is 1970 metres.

An additional issue with AT equipment is that, if balance weights are attached to each end, the whole tension length will be free to move along track. Therefore, a Mid Point Anchor (MPA) is introduced close to the centre of the Tension Length to restrict movement.

Therefore a Tension Length can be seen as a fixed centre point with the two half tension lengths expanding and contracting with temperature.