Heathrow fire caused by ‘years-old fault at power substation’
July 2, 2025
A fire that shut down London Heathrow Airport in March and triggered widespread passenger disruption was caused by a long-ignored fault at a critical power substation, an official investigation has revealed.
The report, published this week, confirmed the blaze originated from a catastrophic failure at the North Hyde 275kV substation, by an issue that had first been flagged seven years earlier.
The incident, which unfolded late on 20 March, began when one of the three supergrid transformers (SGT3) at the site tripped and caught fire, leading to a chain reaction that knocked out the two remaining transformers.
Heathrow fire shut down operations
The resulting power outage left 71,655 homes and businesses without electricity and cut off supplies to Heathrow’s internal electrical network, forcing the airport to shut down its operations.
Heathrow Airport Limited confirmed it had taken the decision to close the airport after the outage impacted critical systems, with flights grounded and thousands of travellers stranded. The company now says it is considering legal action against the National Grid.
The report states that the fire was likely caused by a high-voltage bushing on SGT3 failing due to moisture ingress, which caused a short circuit and electrical arcing that ignited the insulating oil.
Elevated moisture levels in the bushing had been detected as far back as July 2018, but no corrective action was taken despite internal guidance recommending immediate replacement due to the risk of imminent failure.
Damaged transformers
The failure to contain the fire – due in part to older fire safety standards at the site – meant that damage quickly spread to neighbouring transformers.
With no physical fire barriers, firefighters were unable to prevent the loss of all power at the site.
The findings raise questions about infrastructure governance and have prompted energy regulator Ofgem to open a formal investigation into National Grid Electricity Transmission’s handling of the site.
The regulator said it would examine whether the operator fulfilled its obligations and assess potential enforcement measures.
