A lightning strike and the sudden loss of two large electricity generators caused nearly a million people to lose power in England and Wales earlier this month, an interim report has found.
The National Grid outage affected homes, businesses and transport, and while power was restored quite quickly, disruption continued into the next day.
Regulator Ofgem has opened an investigation into National Grid and other companies involved.
That could involve a financial penalty.
Ofgem said its investigation “would try to establish what lessons can be drawn from the power cut to ensure that steps can be taken to further improve the resilience of Britain’s energy network”.
National Grid ESO, National Grid Electricity Transmission, 12 distribution network operators in England and Wales, as well as generators RWE Generation (Little Barford Power station) and Orsted (Hornsea) are all being investigated by Ofgem.
The distribution companies are part of six network groups: Electricity North West Limited, Northern Powergrid, Scottish and Southern Energy, ScottishPower Energy Networks, UK Power Networks and Western Power Distribution.
Ofgem will establish whether they breached their licence conditions.
Any punishment could involve a fine of 10% of the firms’ turnover, an order to pay money to charities to help less well-off consumers, or to put cash into a fund to compensate those who lost out because of the power cuts.
The National Grid has to publish a final, technical report by 6 September and any Ofgem enforcement action will follow after that.
Rare and unexpected event
The power cut, which happened just before 17:00 BST on Friday 10 August, caused blackouts across the Midlands, the South East, South West, North West and North East of England, and Wales.
Thousands of homes lost power, people were stranded on trains and traffic lights stopped working
Power was restored by 17:40 BST, but problems on the rail network carried on over the weekend.
Two almost simultaneous unexpected power losses at Hornsea and Little Barford occurred independently of one another – but each associated with the lightning strike.
The main reason why there were so many travel problems was that about 60 Govia Thameslink trains were affected by the outage.
About half were able to restart when power was restored, but the other 30 required engineers to be despatched to restart them, and the delay to them moving again caused a backlog.