Catenary, or an overhead-wire electrical system, is the most common current-supplying system today. Catenary refers to the cables and wires that hang over electrified rail-lines. 'Catenary' is a mathematical word that describes the curve created when two masts support a cable; if the cable is not taut, the cable hangs in a 'valley-like' curve. This is a catenary.
Though, in the rail transport sense, catenaries are what supply pantographs, of electric locomotives and electric multiple units (EMUs), with electricity. This is usually alternating current between 20–25 kV.
On an overhead electrified rail-line there are gantries over the line at intervals — of a maximum of 240 ft (73 meters) on double-track. Each gantry is made up of two masts, these support struts, tubes and cables — one such cable is the catenary cable which is strung from gantry to gantry. These, in turn, support the contact wire. The catenary cable holds 'droppers', which are supporting cables that keep the contact wire at the right height and position, between gantries, for pantographs to use it. On four-track lines and wider, the contact wire is supported by span-cables. These cables run across the line from masts at the line-side, and connected via droppers, another cable carries arms onto which the contact cable is attached. Therefore, in this design, the catenary is at a right-angle to the contact wire.
The contact wire is strung over lengths of 6000 ft (1.8 km), and overlapped to provide constant power to pantographs. Isolator switches, at ground-level, are sometimes used between overlaps to allow minor maintenance work. To avoid uneven wear on pantographs the contact wire is 'zig-zagged' from one side of the line to the other. The contact wire is attached to moveable mounts, called 'steady-arms' that slide along 'registration tubes', that provides the two modes, "pull-on" and "push-off", that are alternately used when on a straight stretch of track.
The supply of power to the catenary is provided, through sub-stations (power boosters), at distances of 20–30 miles (32–48 kilometers). To separate each sub-station's grid from another, a neutral piece of contact wire is attached between insulator 'beads'. Circuit-breakers in the locomotive, or EMU, open automatically on approach, the pantograph slides over the 'beads' and neutral wire, and the circuit-breakers close automatically once back on the live catenary. Half-way between neutral sections there are isolator switches, housed in line-side boxes, that are controlled by operators in headquarters. These operators can re-close open switches, because faults can sometimes self-cure, or send engineers to remedy the problem. The operators are also responsible for keeping the traffic moving if the isolation switches cannot be closed.
Electric locomotives and EMUs run return current through the rails. This can become problematic for phone-lines. To solve this, a return wire is run from the rails to the back of the gantry's masts at intervals — transformers are responsible for pulling the current out of the rails.
With catenary, the struts, tubes and cables that make up overhead systems are live — including the actual catenary cable, and droppers — usually carrying 25 kV. People not understanding this has lead to fatal accidents.
- Book: The Railway Data File published by Silverdale Books. ISBN: 1-85605-499-3.