There was enough power for Heathrow to remain open during the entire period it was shut down on Friday, the head of National Grid has said.

Speaking for the first time since a fire forced North Hyde substation to close, the National Grid chief executive, John Pettigrew, said two other substations that serve Heathrow were working and could have supplied the airport with all the power it needed to remain open.

“There was no lack of capacity from the substations,” he told the Financial Times. “Each substation individually can provide enough power to Heathrow.”

NEW Asked about Pettigrew’s comments, Heidi Alexander, the transport secretary, said on Monday that the issue was wider than just the amount of power available, and that the need to temporarily close the airport was set out to her by its chief executive.

“He explained to me that there were significant problems in terminals two and four, and given that they had to turn everything off and reboot all the systems, that was their judgement as to why they said that they would close the airport until midnight on Friday night,” she told Sky News. “In the event, they were able to get power back up, and they did have planes landing from about 4pm on Friday.

There were also backup generators, she said, but these were “designed to protect the critical systems within airport, and not to power the entire airport”, she added. ENDS NEW

The government has ordered an investigation into the closure of the airport, which returned to normal business on Sunday. The shutdown is estimated to have cost the airline industry £60m to £70m and disrupted the journeys of more than 200,000 passengers around the world.

Ed Miliband, the energy secretary, said the government was “determined to properly understand what happened and what lessons need to be learned”, in regard to the incident at Heathrow and the UK’s “energy resilience for critical national infrastructure”.

But Pettigrew said two substations were “always available for the distribution network companies and Heathrow to take power”. He added it was a “question for Heathrow” as to why it took the action it did.

“Losing a substation is a unique event – but there were two others available,” he said. “So that is a level of resilience.”

In response to Pettigrew’s comments, a Heathrow spokesperson said: “As the National Grid’s chief executive noted, he has never seen a transformer failure like this in his 30 years in the industry.

“His view confirms that this was an unprecedented incident and that it would not have been possible for Heathrow to operate uninterrupted.

“Hundreds of critical systems across the airport were required to be safely powered down and then safely and systematically rebooted. Given Heathrow’s size and operational complexity, safely restarting operations after a disruption of this magnitude was a significant challenge.”

On the day of the fire Heathrow’s chief executive, Thomas Woldbye, said the airport’s backup systems “worked the way they should” and told reporters outside the airport that the power supply of Heathrow is a “bit of a weak point”.

The next day he told the BBC that although the airport has other substations “to switch to them takes time”, and he was proud of how Heathrow workers had responded to the fire. “The situation was not created at Heathrow airport and we had to deal with the consequences,” he said.

“I don’t know of an airport that has backup supply that can switch on in minutes to the magnitude of what we experienced yesterday. The same would happen in other airports.”

Pettigrew said all three transformers at the North Hyde substation were damaged by the fire, which firefighters have said burned 25,000 litres of cooling oil. This included one transformer that is a backup and is located further away from the other two.

He added that the cause of the fire was not yet known, as the site is still too hot for full forensic work, and third-party involvement is not ruled out. “I can’t remember a transformer failing like this in my 30-plus years in industry,” Pettigrew said.

Counter-terrorism officers from the Metropolitan police initially led the investigation, but the force said the fire was not believed to be suspicious. London fire brigade is now leading the inquiry, which will focus on the electrical distribution equipment.

Asked why the North Hayes site, built in the 1960s, was designed in a way that meant a single fire could knock out all three transformers, he said: “You have to build substations in the space available, and obviously you do all you can to mitigate the risks.

“But that is something that clearly I think an investigation will look into a bit further.”