The National Energy System Operator (NESO) has published its final report from the review into the North Hyde Substation outage which caused the closure of Heathrow Airport earlier this year.
On 20 March 2025, the North Hyde electrical substation in Hayes, operated by National Grid Electricity Transmission (NGET), caught on fire at around 11:23PM GMT, destroying or severely damaging not only the main transformers but also the backup generator. As a result, around 16,000 homes lost power around the Hayes region and Heathrow Airport was forced to close, disrupting 200,000 passengers and cancelling or disrupting around 1,000 flights.
In May, NESO released an interim report which indicated that fire-damaged transformers were 57 years old and were on the same circuit as the intended backup – likely triggering a total system failure.
In its final report into the incident, released today, NESO finds that the fire was likely caused by moisture in the bushings – first detected in 2018 with National Grid Electricity Transmission’s (NGET) internal report at the time concluding that the moisture levels indicated risk of an “imminent failure” and should be replaced. No mitigating action appropriate to the severity of this fault was implemented for seven years from 2018 – 2025. The NESO report finds that there were multiple opportunities where this could have been addressed and NGET failed to do so (Sections 6.13 – 6.16)
The NESO report finds that basic maintenance on the North Hyde substation was originally scheduled for 2021 but that NGET missed this maintenance and then cancelled it each year subsequently (2022, 2023 and 2024). It was subsequently scheduled for October 2025. (Sections 6.24 – 6.25)
While NGET’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. (Section 2.1)
The NESO report finds that a NGET review in 2022 indicated that the fire suppression system 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 NGET report at the time concluded that if a fire started “it would not be suitably supressed”. There was no planned replacement until October 2025. (Sections 7.17 – 7.18)
The NESO report confirms that it took over six hours from the NGET Engineer arriving onsite to issuing the permit for the London Fire Brigade to enter the site and begin tackling the fire and confirmed that the fire at North Hyde continued burning for nearly six days after it started. (Section 5.22-5.25)
Responding to the final report, Andrew Dakers, CEO of West London Business, said: “Today’s report is damning. It highlights basic failings in stewardship by National Grid of the country’s high voltage transmission network. National Grid appear to have had a failure of governance and internal controls extending over 7 years.
“It is right that Ofgem, the energy watchdog, has opened an official enforcement investigation.
“National Grid’s new CEO is clearly needed now, rather than in November. She will need to engage with stakeholders and explain how in future the information will flow effectively within National Grid on essential maintenance needs and how the board make sound investment decisions.
“West London desperately needs accelerated investment to boost supply. A lack of energy capacity is slowing investment and growth in the West of the capital. Ofgem will now need to ensure the new National Grid business plan addresses growth needs, but also basic maintenance and corporate governance.”
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 21st 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 underway to implement all 28 recommendations.”
Fintan Slye, NESO Chief Executive Officer, said: “NESO’s final report into the North Hyde Substation outage sets out the root cause and a clear set of recommendations to further improve the resilience of Great Britain’s energy system, and the resilience of its critical national infrastructure.
“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.
“All parties involved are focussed on working together to deliver these important recommendations and much of this work is already underway with NESO’s full support.
“I would like to thank all the organisations who have provided evidence to the review for their cooperation.”
In response to the findings, 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 re-testing 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 cooperate closely with Ofgem’s investigation.
“There are important lessons to be learnt about cross-sector resilience and the need for increased coordination, and we look forward to working with government, regulators and industry partners to take these recommendations forward.”
