Causes
Initial reports were that there appeared to be a problem with a cable feed from the national grid in the Wimbledon area of south London. A spokesman for National Grid said it was "an unusual occurrence", but "not even vaguely on the scale of what happened in the U.S.", adding that there was a fault in the 275 kV system. Later, Chief Operating Officer Mark Fairbairn said the problem was caused by the extremely rare coincidence of two faults happening only seven seconds apart.
Later National Grid's report revealed that the second fault, and the ultimate cause of the blackout, was the fitting of a wrongly rated part in a backup system - similar to fitting a 1 ampere fuse instead of a 5 ampere fuse. The first fault was a problem with a transformer. The impression was given that this fault was in a sense routine - about 13 such faults happen in a year. Normally it would be possible to switch the transformer out of the system and continue with power uninterrupted, but in this case the switch over exposed the second problem.
On 17 October it emerged that the transformer fault was due to an oil leak, spotted some weeks before the blackout. The oil had been topped up but the leak had not been cured. National Grid's Director Of Transmission admitted to a "small backlog" of maintenance checks. On 21 October it became clear that the leak had been known of since March. National Grid had started a regular programme of topping-up until the transformer could conveniently be serviced. Evidently this was ultimately not enough to keep up with the leak; "it got bigger" said a spokesman.
Read more about this topic: 2003 London Blackout