TRAINS - POWERCUT

A one amp fuse was blamed for the power cut that plunged London into chaos, and cost the country £3million. Human error was the root cause of the failure, because the fuse was too weak to cope. The shoebox-sized component, known as a protection relay, should have been the five amp version. It was the biggest electricity supply failure for ten years. Thousands of commuters were stranded on stations and underground trains, and 410,000 homes and businesses were left without power. National Grid chief executive Roger Urwin said South London was usually supplied by four power lines.

But on that night two of the lines were closed for maintenance. A third line was then shut when an alarm sounded, requiring urgent inspection by engineers. That left all power coming through a single line, and that was the one with the wrong fuse. The one amp fuse meant the relay was too sensitive, and shut the system down when it should not have done. Mr Urwin said the fuse had been fitted two years previously by a specialist installer who was still doing work for National Grid.

The faulty equipment has been replaced, and National Grid has checked 9,000 of the 43,000 protection relays on its network. A protection relay, which costs about £2,000, is an electronic microprocessor which measures voltage and current and monitors the health of transmission equipment. It sends signals to circuit-breakers which can identify and pinpoint faults and prevent them from spreading to other parts of the network. Watchdog Ofgem is investigating whether National Grid or other firms breached their legal duty to maintain secure electricity networks.