An undersized fuse which lay undetected for two years caused the blackout that paralysed rush-hour London last month, the electricity network operator has revealed.
National Grid Transco, under pressure to find some fast answers, today provided an update into the power cut on August 28th that trapped thousands of commuters underground and cut power to 400,000 customers across the south of the capital.
Chief executive Mr Roger Urwin said today the incident was a "one off" caused by a single faulty installation which was not picked up on until an unlucky chain of events put it between Londoners and their electricity supply.
"This is a serious incident for us," Mr Urwin said, promising to review operating procedures. "But as far as we can tell at the moment this is a one-off."
Four main power lines serve the southern part of the city, including the input for London's Underground railway.
Two of these lines were out of action at the same time for maintenance.
Just after 6 p.m. an alarm - of a type that goes off on average once a month around the network - sounded on one of the remaining two, causing engineers to shut it down.
This left all power for a swathe of the southern part of the city dependent on a single line.
Two years previously, engineers had installed on that last remaining line a shoebox-sized item of safety equipment designed to act like a normal domestic fuse, isolating parts of the network in the event of an excessive power load.
They had installed by mistake a one amp version instead of a five amp version. Engineers said the two look very alike.
When the power surged to carry the extra load, the undersized box incorrectly activated protection systems, plunging parts of the capital into darkness, and prompting scenes reminiscent of the blackout that hit eastern North America earlier in August.
National Grid said the equipment which caused the outage - installed during a 2001 overhaul - had now been replaced and the company was undertaking a survey to ensure all similar equipment is sound.
The firm said 20 percent of 43,000 similar systems on the National Grid had been surveyed so far and no like cases found. The remaining equipment will be surveyed within four weeks.