LONDON, March 24, 2025 (BSS/AFP) - The head of the UK's national grid has said there was "enough power" to keep Heathrow Airport running on Friday, after an electrical substation fire shuttered Europe's busiest air hub.

The shutdown -- which lasted most of the day -- affected thousands of passengers around the world and raised questions about the reliability of one of the UK's key pieces of infrastructure.

National Grid chief executive John Pettigrew told the Financial Times on Sunday that power had remained available to the airport in west London through two other substations.

"There was no lack of capacity from the substations," he said. "Each substation individually can provide enough power to Heathrow."

"Losing a substation is a unique event -- but there were two others available."

Airport officials said the closure was due to the time it took to switch to the other substations and make safety checks.

"Hundreds of critical systems across the airport were required to be safely powered down and then safely and systematically rebooted," a Heathrow Airport spokesperson said.

"Given Heathrow's size and operational complexity, safely restarting operations after a disruption of this magnitude was a significant challenge."

Heathrow chief executive Thomas Woldbye had said a back-up transformer failed, meaning systems had to be closed so power supplies could be restructured from the two other substations.

The government has ordered a six-week investigation into the shutdown.

About 1,350 flights were affected by Friday's closure, according to the Flightradar24 tracking website. Around 120 Heathrow-bound planes were in the air when the closure was announced and had to be diverted.

Fire officials have said the blaze, which broke out on Thursday night, was "believed to be non-suspicious" and that enquiries would "focus on the electrical distribution equipment".

Asked on Monday about the way airport bosses handled the events, transport secretary Heidi Alexander said: "I'm not going to justify decisions that Heathrow leadership did or didn't take."