One wrong fuse caused worst power blackout in a decade

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The installation of a single incorrect fuse caused last month's power blackout in London, which plunged a large part of the capital into darkness and left thousands of commuters trapped on the Tube, an investigation concluded yesterday.

National Grid said that an automatic protection relay - a sophisticated £2,000 device the size of a shoebox - "tripped" after it misinterpreted a switch in power supply as a fault on the system. This knocked out three of the four substations supplying south London, cutting off electricity to 410,000 customers, including Network Rail and London Underground.

The blackout on 28 August was Britain's worst in a decade and highlighted grave deficiencies in communications between National Grid, police and the operators of the country's rail systems.

The relay, put in place in June 2001, should have been designed to cope with a current of more than 5,000 amps. Instead, a more sensitive 1,000-amp relay was installed by mistake. Despite the fact that the wrong fuse, installed by an outside contractor, had lain undetected for two years, National Grid said there would not be any disciplinary action against any member of staff. Nor is it seeking damages from the contractor.

Roger Urwin, chief executive of National Grid, said the power failure was an isolated incident caused by human error and was not the result of a lack of investment or inadequate maintenance and operating procedures.

He also maintained that National Grid was not liable to pay compensation to customers for negligence. It hadnot breached its operating licence - an offence that could lead to a £100m fine from the energy regulator, Ofgem.

Mr Urwin also denied that National Grid had become "accident prone" even though a second power failure just a week after the London blackout had cut off supplies to 200,000 homes in the West Midlands. The Department of Trade and Industry and Ofgem announced they are to investigate the two incidents.

The investigations are likely to focus on why EDF Energy, the French-owned electricity supplier for London, did not reconnect London Underground immediately. The Grid report says that EDF had made contingency arrangements to do just this in the event of a power failure at Wimbledon.

But only one of the three engineers in the EDF control room that evening is understood to have had the authority to switch supplies. Once London Underground had taken the decision to close the Tube, the power supply had to remain switched off for hours because trains were being evacuated on to the tracks.

The 43-page report by the Grid depicts a "sod's law" sequence of events that led to the 41-minute blackout. On the evening in question one of the two transmission circuits linking the four south London substations was out of service for routine maintenance.

A "Buchholz alarm" - of which there are only about 13 a year - then sounded at the Grid's national control centre at 6.20pm, warning that a transformer at one of the substations, Hurst, was in danger of failing.

The transformer was immediately disconnected from the system and the power flow switched so that Hurst could be supplied from the Wimbledon substation via another substation at New Cross. But the automatic relay protector between Wimbledon and New Cross with the wrong fuse interpreted the power surge as a fault, shutting the three substations and causing the blackout.

The Grid reconnected all of the substations to the national transmission system at 6.57pm. Four minutes later, EDF Energy reconnected all its affected customers.