At about 17:00 BST on Friday, 9 August the UK suffered power blackouts for several hours across Wales, and the midlands, the south east, south west, north west and north east of England. National Grid power was restored by 17:40 BST on Friday but some train services continued to be disrupted on Saturday.
The system was compromised after a gas-fired power station at Little Barford, Bedfordshire, failed at 4:58 pm followed, two minutes later, by the Hornsea offshore wind farm disconnecting from the grid.
Speaking on BBC radio, Natinal Grid ESO director of operations Duncan Burt acknowledged the "immense disruption" the blackout had caused.
He said the near-simultaneous loss of two generators was more than the grid was routinely prepared for, prompting automatic safety systems to shut off power to some places. He emphasised that National Grid ESO systems were not to blame, but it will "learn the lessons" after nearly one million people across England and Wales lost power for several hours.
He stated that its systems "worked well" after the "incredibly rare event" of two power stations disconnecting., and said he did not believe that a cyber-attack or unpredictable wind power generation was to blame.
The government has announced an investigation into what happened. Business secretary Andrea Leadsom said she is asking the Energy and Emergencies Executive Committee to carry out the investigation, which will look at whether National Grid's procedures are "fit for purpose". Regulator Ofgem has demanded an "urgent detailed report" into what went wrong.
It said it could take enforcement action, including a fine, after train passengers were stranded, traffic lights failed to work and thousands of homes lost power during the blackout.