Neither a cyber-attack nor an unpredictable wind power generation was behind a power outage that left nearly a million people in England and Wales in the dark hours of Friday.
The stopwatch stranded trains, disabled traffic signals and left thousands of homes across large parts of England and Wales without electricity, including large parts of London.
National Grid said two generators, one gas-powered and one wind-powered, were run out Friday afternoon. Automatic safety systems turned off the juice to other places after the generators went down.
Ofgem, the power regulator in the United Kingdom, demanded an urgent investigation and said that enforcement measures could come when the cause was determined.