ElectrificationSee also: Railway electrification in Great Britain
The Underground is one of the few networks in the world that uses a four-rail system. The additional rail carries the electrical return that on third-rail and overhead networks is provided by the running rails. A top-contact third rail beside the track is energised at +420 V DC, and a top-contact fourth rail centrally between the running rails is at -210 V DC, combining to provide the traction voltage of 630 V DC.
Most tube lines run in cast-iron tunnels, with some of the more recent constructions using concrete lining. Using a third-rail scheme necessitates that the return current is conducted through an earthed running rail. Such current is just as easily able to travel through the cast-iron tunnel lining, and unless the joints between the sections are electrically sound, the current will arc across the sections causing considerable damage, or corrode the tunnel segments via electrolysis. There are also many cast-iron gas and water mains in the vicinity of the tube tunnels, and the return current would travel along these just as easily. Some of these mains date back to the 19th century and the joints between separate sections would not have been designed to be electrically sound, as deep-level electric tube trains were some way off.
Another advantage of the fourth rail system is that the two running rails are available exclusively for track circuits, of which there are many.
The surface sections of the lines are constructed using fourth rail to permit through running with the tube lines, there being no technical reason to do so.
The traction current has no direct earth point, but there are two resistors connected across the traction supply. The centre tap of the resistors is earthed, establishing the reference point between the positive and negative rails by voltage division. The resistors are large enough to prevent large currents flowing through the earthed infrastructure. The positive resistor is twice as large as the negative resistor, since the positive rail carries twice the voltage of the negative rail.
Some above-ground sections are shared with National Rail trains that use the three-rail system. On these sections the fourth rail is bonded to the running rails to keep it at earth potential, and the third rail is at +630 volt DC.
Read more about this topic: London Underground Infrastructure
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