An investigation is underway to find the cause of electricity blackout in London that left the city in chaos during Thursday evening鈥檚 rush hour.
Power was restored within 40 minutes, but the failure left more than 250,000 London Underground users stranded as 60 per cent of the network ground to a halt. Many trains were stranded in tunnels between stations for as long as 90 minutes.
The lack of power to signals also prevented many mainline trains from running. And on the streets there was further turmoil as 270 sets of traffic lights blanked out, meaning buses were caught in long traffic jams. National Grid Transco (NTC), which runs the UK鈥檚 national electricity grid, apologised for the disruption.
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The outage began at 1810 BST, when an alarm indicated a fault with a transformer at a substation in Hurst, Kent. Ten minutes later, the transformer was switched off, a standard procedure that should be imperceptible to electricity users.
However, within seven seconds, a second fault occurred stopping electricity from flowing down a 275,000 volt underground cable between two substations in New Cross and Wimbledon.
鈥淎t this point, we do not know the reason why the equipment failed,鈥� said NTC in a statement. Back-up systems are in place in case this sort of thing happens. 鈥淚n this case, the back-up also failed.鈥�
Independent or related
The crucial question now is whether the two failures were related, says Daniel Kirschen, an expert in power systems security at the University of Manchester Institute of Science and Technology, in the UK.
鈥淚f they were truly independent events then that鈥檚 within guidelines and just a case of plain old bad luck,鈥� he says. 鈥淏ut if one caused the other, then there鈥檚 a problem with the way the system is designed.鈥�
These kinds of 鈥渉idden鈥� faults are almost impossible to predict when designing a system, says Kirschen, but represent a serious problem. NTC described the second fault as 鈥渢otally exceptional鈥� and said that its management systems behaved exactly as they should have.
Until 2002, London Underground had its own independent power station. But it was shut down as part of a controversial deal to hand over the tube鈥檚 power supply to a private consortium, which took its power from the national grid.
According to NTC, no parallels can be drawn between this power failure and the massive blackout that hit North America on 14 August, leaving as many as 60 million people in the dark. 鈥淭here is no similarity in cause, scale or duration,鈥� says the company.