Heathrow Airport power outage cause identified 7 years before substation fire

A report into what caused a substation fire in March 2025 that left London-Heathrow Airport (LHR) virtually powerless and over 270,000 passengers facing huge disruptions has placed the likely blame on a problem first identified in 2018.  

In its final report looking into the power outage, the National Energy System Operator (NESO) said that a catastrophic failure on one of the transformer’s high voltage bushings at a substation that Heathrow Airport primarily relied upon caused the transformer to catch fire. 

“This was most likely caused by moisture entering the bushing, causing an electrical fault. An elevated moisture reading in the bushing had been detected in oil samples taken in July 2018, but mitigating actions appropriate to its severity were not implemented,” the NESO report said.  

The power outage at the North Hyde substation late on March 20, 2025, had far reaching consequences for Heathrow and the wider aviation community. Around 10 fire engines and 70 firefighters responded to the emergency.    

London Fire BrigadeThe incident resulted in the airport closing for most of the day on March 21, 2025, so that its internal network could be reconfigured to take power from the two other operational supply points and to perform safety checks. 

“The power outage and closure of Heathrow airport were hugely disruptive and our report seeks to improve the way parties plan for and respond to these incidents, building on the underlying resilience of our energy system,” said CEO of NESO, Fintan Slye. 

According to NESO, Heathrow said that reconfiguration of its power supply to the other supply points is estimated to take 10-12 hours. 

“The review also found that energy network operators are not generally aware whether customers connected to their networks are Critical National Infrastructure,” said NESO in its report. 

Heathrow reaction to report

In response to the report, Heathrow said on July 2, 2025, that it welcomed the findings which “shed light on the external power supply failure”. 

“A combination of outdated regulation, inadequate safety mechanisms, and National Grid’s failure to maintain its infrastructure led to this catastrophic power outage.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,” a Heathrow spokesperson said. 

@BBCLondonNews There’s a huge fire in Hayes – London, looks like North Hyde Electricity sub station pic.twitter.com/PJNpbHjZxu— Vil (@fire_at_Vill) March 20, 2025 Directed at Heathrow, NESO said that the airport had identified power disruption could “greatly impact operations”.  

“However, Heathrow assessed the total loss of power to one of its three supply points as a high-impact, low probability event,” said the NESO report 

The report recommended that Heathrow should diversify its configuration such that the loss of one supply point does not impact the entire airport. 

Ofgem response 

The Office of Gas and Electricity Markets (Ofgem) announced on July 2, 2025, that the regulator was launching an official enforcement investigation into National Grid Electricity Transmission (NGET). 

“In the NESO final report, it has been established that the root cause of the fire was a preventable, technical fault – Ofgem will review whether NGET complied with the relevant legislation and licence conditions relating to the development and maintenance of its electricity system at North Hyde,” the regulator said.   

London Fire BrigadeOfgem also intends to discuss with the Civil Aviation Authority (CAA) what lessons can be learnt from the North Hyde incident for the wider resilience of the transport system. 

In response to the report, the National Grid said it had “taken further action since the fire” including an “end-to-end review of our oil sampling process and results”. 

“We fully support the recommendations in the report and are committed to working with NESO and others to implement them. We will also cooperate closely with Ofgem’s investigation,” a spokesperson for the National Grid said. 

Heathrow’s own review into the outage, led by former Cabinet Minister Ruth Kelly, identified key areas for improvement. 

Following that review Heathrow said that work was “already underway to implement all 28 recommendations”. 

Ruth Kelly’s report that the CEO of Heathrow, Thomas Woldbye, could not be contacted when the incident first unfolded as his mobile phone was on silent.   

Other key findings from NESO final report: 

While National Grid Electricity Transmission’s current standards for fire controls at substation sites prioritise distance and physical barriers between oil-filled equipment, neither of these measures were in place at North Hyde substation site as it was built prior to the application of these standards. 

A National Grid Electricity Transmission review in 2022 indicated that the fire suppression system on two supergrid transformers at North Hyde substation was inoperable. A further fire risk assessment report conducted in July 2024 indicated that the fire suppression system at North Hyde substation was still out of service. 

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. 

The National Grid Electricity Transmission standby engineer had arrived at site by 23:50. By 06:00, permits were in place confirming electrical safety, which allowed London Fire Brigade to enter the site. This was in line with the National Fire Chief Council’s National Operational Guidance for London Fire Brigade. 

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