In order to increase the electrical supply to the West Coast Mainline, Network Rail (formely Railtrack) have installed an autotransformer supply to power the 25kV rail overhead caternary from which the locomotive collects its high voltage supply.  The locomotives employ the running rail as the low voltage electrode. Whilst the rail is connected to the centre tap of the autotransformer, it is also connected to the mass of earth and to return current cables which return a portion of the return current to a busbar (return current busbar) in the Network Rail track feeder substation.

The autotransformer requires a 25kV two pole anti-phase supply and a neutral. The 25kV poles are designated catenary pole (C pole), auxilliary pole (A pole) and neutral (N). This is supplied  by National Grid transformers, switchgear and cables via two independant circuits to a Network Rail 25kV track feeder substation.


CCI have provided training for National Grid Senior Authorised Persons.

As the neutral is connected to the mass of earth, via the rail and to earth mats, return current may flow along any conductor connected between the National Grid and Network Rail earth mats. These return current flowing in National Grid assets is described as alternative conductor return (ACR) current.

CCI assisted National Grid in producing safety documentation for the control of ACR current on the many substation and supply arrangements existing on railway supplies.