Investigators combed through the burned-out remains of a power substation near London’s Heathrow Airport on Friday, seeking the cause of a spectacular blaze that shuttered Europe’s busiest travel hub for much of a day and raised broader questions about Britain’s energy infrastructure.
Officials and energy experts said a fault in a transformer with 275,000 volts running through it probably sparked a massive, oil-fueled fire that severed the airport and tens of thousands of nearby homes from the power grid. Systems designed to prevent such a fire apparently failed, and the size of the blaze appeared to keep a second, nearby transformer from restoring electricity.
But the mystery of what caused that fault in the first place remained far from solved by the end of the day on Friday, even as flights resumed at Heathrow.
The Metropolitan Police in London said that counterterrorism specialists had taken charge of the investigation, “given the location of the substation and the impact this incident has had on critical national infrastructure.” At the same time, political leaders and industry experts said it appeared most likely that the fire was an accident.
Both possibilities left residents of Britain and global travelers rattled.
If a malicious adversary can so dramatically disrupt worldwide travel by causing a fire in a neighborhood power station, it raises new concerns about the ability of open societies like Britain’s to guard against such nontraditional attacks.
And if the fire was the result of an undetected weakness in the basic infrastructure of Britain’s power grid, the scope of the chaos that was unleashed could undermine confidence in the nation’s ability to fix crumbling systems at a time when finances are already strained.
Prime Minister Keir Starmer and Sadiq Khan, the mayor of London, sought to reassure residents and travelers on Friday, with Mr. Khan telling Sky News in an interview that despite the involvement of counterterrorism officers in the investigation, there was “no reason at all for anyone to be concerned or alarmed.”
The London police later on Friday also played down the possibility of foul play, saying in a statement that after an initial assessment, they “are not treating this incident as suspicious, although inquiries do remain ongoing.”
Still, neither the prime minister nor the mayor offered answers to some of the urgent questions being asked by frustrated travelers, nervous neighbors of the airport and political officials across the country.
Why didn’t the airport have sufficient power backups? Did the electric utility not plan for the possibility of such a fire, either from sabotage or technical malfunction? Do major airports usually have backup systems that can power the entire operation, or do they rely on one main source of energy?
John McDonnell, a lawmaker who represents Hayes, the area where the fire broke out, said that any investigation undertaken in the days ahead would need to look at “why backup arrangements have not worked.”
“There are lessons that have got to be learned here,” he told reporters on Friday afternoon.
By early afternoon on Friday, Britain’s National Grid said the network of the North Hyde substation, where the fire happened, had been reconfigured to restore power to the airport and neighborhood, calling it an “interim solution” while repairs get underway. Officials with National Grid did not respond to an email requesting information about the incident.
That announcement paved the way for a partial opening of the airport, where the first flights began landing again by the evening.
“We will now work with the airlines on repatriating the passengers who were diverted to other airports in Europe,” airport officials said in a statement. “We hope to run a full operation tomorrow.”
But even as Heathrow attempts to return to normal operations, a sense of uncertainty remains.
Ed Miliband, Britain’s energy secretary, said in an interview early on Friday with Sky News that the fire at the electrical substation that crippled Heathrow Airport also took out at least one of the main backup systems designed to keep the power running.
“There was a backup generator, but that was also affected by the fire, which gives a sense of how unusual, unprecedented it was,” Mr. Miliband said.
Transformers convert current from one voltage to another, and are often filled with oil that acts as both an insulator and a coolant. The types of oil used can withstand high temperatures, but they can ignite if they get hot enough.
In the case of the transformer near Heathrow, experts said it would have been turning 275,000 volts into 66,000 volts when it apparently failed. Jonathan Smith, the deputy commissioner for the London Fire Brigade, said the blaze involved “a transformer comprising 25,000 liters of cooling oil that was fully alight” at the substation.
The failure of at least one backup system to quickly restore power after such a major outage is likely to be at the center of questions about the reliability of Britain’s infrastructure in the aftermath of the fire and airport closure.
Britain’s National Infrastructure Commission, which makes recommendations to the government on major infrastructure, said that the fire had underscored the need for better preparedness for shocks, and for operators to build resilience into their systems and conduct regular stress testing.
“We’ve been clear the U.K. needs national resilience standards for our transport, digital, energy and water infrastructure,” said the commission’s chair, John Armitt, in a statement. “These will give clarity to operators and users about what levels of service they should expect in the face of short- and long-term disruptions, and ensure regulators have a clear yardstick against which they can ensure sufficient investment in resilience is made.”
In a post on social media, Willie Walsh, the director general of the International Air Transport Association, a global trade association of airlines, wrote: “How is it that critical infrastructure — of national and global importance — is totally dependent on a single power source without an alternative.”
“If that is the case — as it seems,” he added, “then it is a clear planning failure by the airport.”
In a statement, Heathrow Airport said that the facility had “multiple sources of energy” but that there was no backup that would supply enough power to operate the entire airport, which it said “uses as much energy as a small city.”
The statement said that backup diesel generators and uninterruptible power supplies did kick in that would have allowed planes to land and passengers to disembark. But they would not have been enough to allow the airport to operate fully.
Simon Gallagher, a former senior executive at Britain’s largest power provider, said that he believed the substation near Heathrow had been designed so that if the first transformer had a problem, a second one could kick in quickly. “Basically, we designed things so that something can fail,” and the system can still continue working, he said.
But, he said, a number of things must have gone wrong, seemingly allowing the fire to rage through the prevention systems and damage both of the transformers.
That is highly unusual, said Mr. Gallagher, who is now the managing director at U.K. Networks Services, which advises clients on the resilience of their electricity networks.
Mr. Gallagher said the emergency generator systems mentioned in the Heathrow statement were designed to keep runway lights and control tower systems working even during an incident like the one that took place on Friday.
But he said that it would have been impossible to continue to land planes, barring an emergency, because there would have been no electricity to move luggage, light the terminals, operate doors and more. By his estimation, doing all of that would require at least 20 massive diesel generators the size of 40-foot shipping containers, each one capable of generating a megawatt of power.
Heathrow does not have such a system, which would have been capable of keeping the power to the entire airport flowing for about six hours before needing to be refueled, he said. But he added that other major power customers, such as data centers, had installed big backup generators to ensure power in the event of an emergency.
“I think things will change,” Mr. Gallagher said. “I think Heathrow and other airports will install backup generation.”