Earth leakage problems and solutions on Tyne and Wear Metro

The Tyne and Wear Metro has nine traction substations to rectify the 11 kV supplies to feed the overhead line at 1500 V DC, at an average spacing of 6.5 km. Traction current return is through the running rails, which embody insulated block joints and impedance bonds for signalling track circuitry. Where the track is not welded, continuity is provided by thermit type rail-head bonds or studs and cables, with cross-bonding provided where practicable. The track is not deliberately earthed at any point, and all bonded structures, such as overhead line masts and signal posts, are insulated from earth. Re-sealing spark gaps are provided to allow clearance of faults to earthed metalwork. A Corrosion Working Party, involving local utilities, and chaired by the Railway Inspectorate, was established at the design stage. During operation of the Test Track and the early days of Metro testing and operation, a variety of leakage problems occurred. The author describes the work of the Working Party to alleviate these problems and their effects