Editing Overhead line (section)

== Construction ==
[[File:Pociąg sieciowy pod Łowiczem.jpeg|thumb|upright|[[Lineworker]]s on a [[maintenance of way]] vehicle repairing overhead lines (Poland)]]
[[File:Overhead frog for pantograph & trolley pole.jpg|thumb|Overhead over a switch in [[Toronto Transit Commission|Toronto]]: Two runners for pantographs flank the trolley pole frog.]]

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 ''{{visible anchor|messenger wire}}'' or ''catenary''. This wire approximates the natural path of a wire strung between two points, a [[Catenary|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 (tool)|clamp]]. The whole system is then subjected to mechanical [[Tension (physics)|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 [[zigzag]]ged 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 [[tram]]ways, 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:

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

Earlier dropper wires provided physical support of the contact wire without joining the catenary and contact wires electrically. Modern systems use current-carrying droppers, eliminating the need for separate wires.

The present transmission system originated about 100 years ago. A simpler system was proposed in the 1970s by the Pirelli Construction Company, consisting of a single wire embedded at each support for {{convert|2.5|m}} of its length in a clipped, extruded aluminum beam with the wire contact face exposed. A somewhat higher tension than used before clipping the beam yielded a deflected profile for the wire that could be easily handled at {{convert|400|km/h|abbr=on}} by a pneumatic servo pantograph with only 3&nbsp;[[Gravity of Earth|g]] acceleration.{{Citation needed|date=September 2017}}

=== Parallel overhead lines ===
[[File:Catenaryswitch.jpg|thumb|A switch in parallel overhead lines]]
[[File:Trolley3WireSwitch.jpg|thumb|left|[[Trolleybus#Wire switches|Trolleybus wire switch]]]]

An electrical circuit requires at least two conductors. Trams and railways use the overhead line as the positive terminal of the circuit and the steel rails as the negative terminal of the circuit. For a [[trolleybus]] or a [[trolleytruck]], no [[Rail profile|rails]] are available for the return current, as the vehicles use rubber tyres on the road surface. Trolleybuses use a second parallel overhead line for the return, and two [[trolley pole]]s, one contacting each overhead wire. ([[Pantograph (transport)|Pantographs]] are generally incompatible with parallel overhead lines.) The circuit is completed by using both wires. Parallel overhead wires are also used on the rare railways with [[three-phase AC railway electrification]].