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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 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.
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 wire ( trams generally use pantographs as well but sometimes use bow collector s, or trolleypole s). The pantograph, bow collector, or trolleypole is electrically conductive, and allows current to flow to earth. This path takes the current through the traction motors of the train/tram, and back to the feeder station via the steel wheels and one or both track running rails. Diesel trains may pass along these tracks without affecting the overhead line, although clearance may be an issue.
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 & Canada). This wire is allowed to follow the natural path of a wire strung between two points, which is known as a catenary shape, 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. In this way the contact wire is effectively supported at numerous points. 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.
The dropper wires usually only provide physical support of the contact wire, and do not join the catenary and contact wires electrically. Separate wires are provided for this function.
Another reason to use an auxiliary wire is that such a wire could be constructed of a more conductive but less wear-resistant metal, increasing the efficiency of power transmission.
For street tramways there often is just a simple conact wire and no message wire.
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 20 kN per wire.
For low speeds and in tunnels where temperatures are constant, 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 10 kN. 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 for 25kV OHL 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. MPA's are often fixed to low bridges.
Therefore a tension length can be seen as a fixed centre point with the two half tension lengths expanding and contracting with temperature.
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