LONDON — Britain’s energy regulator on Saturday demanded answers about the country’s biggest electrical blackout in more than a decade, which affected about a million homes and left commuters stranded on packed rush-hour trains in unlit tunnels.
In a statement, the regulator, Ofgem, said it had “asked for an urgent detailed report from National Grid,” the company that operates Britain’s electricity supply network. The regulator, which can fine energy companies if it believes they have been in breach of their license to supply energy or have violated consumer protection law, said it might begin enforcement action.
The director of National Grid, meanwhile, said the company was certain the power failure had not been the result of “malicious” action or a cyberattack.
Power cut off in scattered locations around the country shortly before 5 p.m. Friday, and the blackout lasted for less than an hour. At least one airport, in Newcastle, was affected, along with several rail lines, including one of the country’s two main north-south routes and Thameslink commuter services in London.
The transport authorities said some traffic lights also ceased to function in the British capital.
According to National Grid, the cutoff resulted when a gas power station and an offshore wind farm disconnected from the network almost simultaneously. Although other plants responded by increasing their output, a protection system imposed short blackouts on areas across Britain as a routine security measure to protect the network, the company said in a statement on Saturday.
“We think the system worked really well,” Duncan Burt, director of operations at National Grid, told BBC Radio on Saturday. “We’re already very confident that there was no malicious intent or cyberattack.”
Mr. Burt added that National Grid would seek to learn the lessons of Friday’s power failure. The company would look at whose power had been disconnected by the automatic protection systems, he said and whether there was a way to minimize the impact of similar events in the future.
According to EnAppSys, a company that monitors power networks in Britain, the gas-generating site involved was Little Barford, in Cambridgeshire, a power plant that opened in 1996 and that is operated by the German multinational energy company RWE. The wind farm was Hornsey One, in the North Sea off the coast of Yorkshire. It opened in June this year and is described by its operator, the Danish company Orsted, as the largest offshore wind farm in the world.
Large-scale power cutoffs in Britain have been extremely rare since the 1970s, when miners’ strikes forced the government to ration coal supplies, leading to frequent blackouts.
According to Mr. Burt, Britain’s last electricity failure on a comparable scale occurred in 2008. That, too, was a result of two power plants going offline: Homes in London and the southeast of England lost power, and rail passengers were affected.
Although it took less than an hour for power to be restored on Friday, hundreds of passengers were stuck on stalled trains, found themselves in darkened tunnels and endured long hours waiting in crowded stations. The delays on the rail network continued into Saturday morning, as trains and staff members, displaced overnight, were unavailable in the morning.
Adding to the delays, trains faced speed restrictions across the southeast of England because of expected high winds.
The Times of London ran a large picture of the chancellor, Sajid Javid, who visited National Grid on Friday, hours before the power cut, with the caption “Don’t touch that dial, chancellor.”