The report of current leakage on the Croydon tram network (NCE 11 November 1999) fails to mention one of the more serious consequences - the electrolytic attack of steel structures, including water and gas pipelines and reinforcement in the vicinity of the leak. This is a phenomenon typically associated with DC electrified railways (where the rails are used as the return conductor). AC electrified railways do not suffer from the same problem.
The Hong Kong MTR Corporation, whose railway operates at 1,500V DC, controls stray currents through the use of insulating trackforms and a 'stray-current collection' system. The latter typically incorporates a layer of mesh reinforcement in the track supporting structure between the rails and the main reinforcement, from which it is electrically insulated.
The stray current mesh is connected back to the traction substation to complete the electrical circuit. All this is achieved in a dedicated trackway and in a strictly controlled operating and maintenance environment.
The much more variable surface conditions of a street tramway system (particularly one which may share road space with other traffic), affected by water, oil, salt and possibly other electrolytes, will continue to present a considerable challenge.
David Sorton (M), DSORTON@mtrcorp.com