The power supply to the north-east sector of London Underground Ltd's Central Line, and therefore the train service, was out of service for the majority of the time between the 24th and the 29th November 1993. During this period the power system faults were investigated, identified and corrected. This incident was the most extensive power fault on London Underground's power supply system in its recent history. The key cause was identified as an earth fault on a neutral cable of an alternator, which was located several kilometres from the part of the network that became inoperative. London Underground's power 22 kV generation and distribution system is described, and the feed arrangements historically made to minimise the possibility of such an extensive outage are explained. The events leading to the loss of supply, subsequent identification and correction of the underlying main fault are described, and a model of the 150 Hz harmonic earth-return current, which was the immediate cause of tripping owing to a series resonant circuit condition, is included. The effect of the neutral cable fault was to increase the 150 Hz current to the point where it caused simultaneous multiple feeder breaker trips from earth-fault relays for the feeders supplying the north-east sector of the Central Line, resulting in loss of power to that sector. A comparison is made between the model and practically measured results.