Tensioning
Catenary wires are kept at a mechanical tension because the pantograph causes mechanical oscillations in the wire and the wave must travel faster than the train to avoid producing standing waves that would cause wire breakage. Tensioning the line makes waves travel faster.
For medium and high speeds, the wires are generally tensioned by means of weights or occasionally by hydraulic tensioners. Either method is known as auto-tensioning (AT), or constant tension and ensures that the tension in the equipment is virtually independent of temperature. Tensions are typically between 9 and 20 kN (2,000 and 4,500 lbf) per wire. Where weights are used, they slide up and down on a rod or tube attached to the mast, to prevent the weights from swaying.
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 (2,200 lbf). 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, that is, the distance between anchors. This leads to the concept of maximum tension length. For most 25 kV OHL equipment in the UK, the maximum tension length is 1970 m.
An additional issue with AT equipment is that, if balance weights are attached to both ends, the whole tension length will be free to move along track. To rectify this issue, a midpoint anchor (MPA), close to the centre of the tension length, restricts movement of the messenger wire by anchoring it; the contact wire and its suspension hangers can move only within the constraints of the MPA. MPAs are sometimes fixed to low bridges; otherwise, they are anchored to the typical vertical catenary poles or portal catenary supports. Therefore, a tension length can be seen as a fixed centre point, with the two half tension lengths expanding and contracting with temperature.
Most overhead systems include a brake to stop the wires from unravelling completely should a wire break or tension be lost for any other reason. German systems usually use a single large tensioning pulley with a toothed rim, mounted on an arm hinged to the mast. Normally the downward pull of the weights, and the reactive upward pull of the tensioned wires, lifts the pulley so its teeth are well clear of a stop on the mast. The pulley can turn freely while the weights move up or down as the wires contract or expand. If a wire breaks or tension is otherwise lost, the pulley falls back toward the mast, and one of its teeth will jam against the stop. This stops further rotation, limits the damage, and keeps the undamaged part of the wire intact until it can be repaired. Other systems use various other braking mechanisms, usually with multiple smaller pulleys in a block and tackle arrangement.
Read more about this topic: Overhead Line