British energy watchdog Ofgem has launched an investigation after a report found a fire that caused the shutdown of Heathrow Airport was due to a preventable technical fault.
The report by the National Energy System Operator (Neso) said an “elevated moisture reading” had been found in oil samples at the North Hyde substation in west London in July 2018, but action was not taken to replace electrical insulators known as bushings.
In 2022, a decision was made to “defer basic maintenance” at the substation, meaning the issue went “unaddressed”, the final report said.
On Wednesday, Ofgem said it had opened an official enforcement investigation into National Grid Electricity Transmission (NGET) based on the Neso review’s findings.
The fire on March 20 this year was caused by a “catastrophic failure” in one of the transformers, “most likely caused by moisture entering the bushing causing a short circuit”, which ignited the oil, the Neso report said.
A review by the National Grid in 2022 found the fire suppression system at the North Hyde site was “inoperable”, while another assessment in July 2024 indicated the system was still “out of service”, the report said.
No flights operated at the west London airport until about 6pm on March 21 because of the blaze which started late the previous night.
More than 270,000 air passenger journeys were disrupted by the closure.
The electricity outage led to 71,655 domestic and commercial customers losing power and the “complete loss of supplies” to part of Heathrow Airport’s internal electrical distribution network, the report said.
Akshay Kaul, director general for infrastructure at Ofgem, said: “The North Hyde substation fire resulted in global disruption, impacted thousands of local customers, and highlighted the importance of investment in our energy infrastructure.
“As a result of the report’s findings, we have opened an investigation into National Grid Electricity Transmission.
“We have also commissioned an independent audit of their most critical assets.
“Ofgem will also further examine the incident and its causes and take further action as appropriate.
“We expect energy companies to properly maintain their equipment and networks to prevent events like this happening.
“Where there is evidence that they have not, we will take action and hold companies fully to account.
“Britain has one of most reliable energy systems in the world and thankfully incidents like this are rare. We must continue to invest in the system to maintain that resilience.”
Outcomes from Ofgem’s investigation could include a finding of no breach, or compliance action to bring companies into compliance, along with potential contributions to the voluntary redress fund, penalties and licence revocation.
The Neso report, which was published on Wednesday and commissioned by Energy Secretary Ed Miliband, read: “An elevated moisture reading in one of SGT3’s (the transformer’s) bushings had been detected in oil samples taken in July 2018.
“According to National Grid Electricity Transmission’s relevant guidance, such readings indicate ‘an imminent fault and that the bushing should be replaced’.
“While the reading was recorded in National Grid Electricity Transmission’s online system, the mitigations appropriate to its severity were not actioned.
“The controls in place were not effective and failed to identify subsequently that action had not been taken in relation to the elevated moisture reading.
“This includes an opportunity in 2022 when a decision was taken to defer basic maintenance on SGT3. The issue therefore went unaddressed.”
The report continued: “The July 2024 fire risk assessment stated that if a fire started on any of the three supergrid transformers, ‘it would not be suitably suppressed’.
“As a result, a high priority action was created on the pump of the water mist system to be appropriately serviced and maintained.
“That action remained outstanding at the time of the incident, in March 2025.
“National Grid Electricity Transmission stated that it was planning a combination of replacements and refurbishments of the fire suppression system by October 2025.”
The Neso review also found that the design and configuration of Heathrow’s internal power network meant the loss of just one of its three supply points would “result in the loss of power to operationally critical systems, leading to a suspension of operations for a significant period”.
Plans were in place to respond to such an event, including the reconfiguration of the network which would take approximately 10 to 12 hours, but the loss of a supply point was “not assessed to be a likely scenario by Heathrow Airport Limited, due to its expectation of the resilience of the wider network”, the report said.
“This meant that its internal electrical distribution network was not designed or configured to take advantage of having multiple supply points to provide quick recovery following such a loss and relied on manual switching.”
Mr Miliband said: “The report is deeply concerning because known risks were not addressed by the National Grid Electricity Transmission, and Ofgem has now opened an official enforcement investigation to consider any possible licence breaches relating to the development and maintenance of its electricity system at North Hyde.
“There are wider lessons to be learned from this incident. My department, working across Government, will urgently consider the findings and recommendations set out by Neso and publish a response to the report in due course.”
A Heathrow spokesperson said: “Heathrow welcomes this report, which sheds further light on the external power supply failure that forced the airport’s closure on 21 March.
“A combination of outdated regulation, inadequate safety mechanisms and National Grid’s failure to maintain its infrastructure led to this catastrophic power outage.
“We expect National Grid to be carefully considering what steps they can take to ensure this isn’t repeated.
“Our own review, led by former cabinet minister Ruth Kelly, identified key areas for improvement and work is already under way to implement all 28 recommendations.”
Heathrow is Europe’s largest airport with more than 83.9 million passengers travelling through its terminals in 2024.
A National Grid spokesperson said: “As Neso’s report sets out, in Great Britain we have one of the most reliable networks in the world, and events of this nature are rare.
“National Grid has a comprehensive asset inspection and maintenance programme in place, and we have taken further action since the fire. This includes an end-to-end review of our oil-sampling process and results, further enhancement of fire risk assessments at all operational sites, and retesting the resilience of substations that serve strategic infrastructure.
“We fully support the recommendations in the report and are committed to working with Neso and others to implement them. We will also co-operate closely with Ofgem’s investigation.
“There are important lessons to be learnt about cross-sector resilience and the need for increased co-ordination, and we look forward to working with government, regulators and industry partners to take these recommendations forward.”
