The National Energy System Operator (NESO) has published its final report from the review into the North Hyde Substation outage and fire that closed Heathrow Airport in March of this year.
NESO was commissioned by the Department for Energy Security & Net Zero (DESNZ) and Ofgem to review the incident at North Hyde, and to identify lessons and recommendations for the prevention and management of future power disruption events, as well as lessons for the UK’s energy resilience more broadly.
Forensic findings
Using forensic analysis from both National Grid Electricity Transmission and London Fire Brigade, the review found evidence that a catastrophic failure on one of the transformer’s high-voltage bushings at National Grid Electricity Transmission’s 275kV substation caused the transformer to catch fire.
This was most likely caused by moisture entering the bushing, the report says, 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. National Grid Electricity Transmission has since initiated an end-to-end review of its oil sampling process, with a view to ensuring that it is robust. The organization is also undertaking a review and assurance exercise of all recorded oil samples to ensure all appropriate actions have been taken where required.
The review found that the design and configuration of Heathrow Airport’s private internal electrical distribution network meant that the loss of one of its three independent supply points would result in the loss of power to some of the airport’s operationally critical systems. Heathrow Airport Limited has a plan to deal with this, which includes reconfiguring its internal electrical distribution network to take power from the other two supply points. The process to enact this is estimated to take 10-12 hours, according to Heathrow Airport Limited’s plans. This was less well-known by those outside the technical team within Heathrow Airport Limited, and it was not known to the energy companies.
The incident at North Hyde on March 20, 2025, caused a loss of supply to one of Heathrow Airport’s supply points. This resulted in the airport closing for most of 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 before reopening for some repositioning and repatriation flights. Heathrow reopened for some flights late on March 21, 2025, and was fully operational from March 22, 2025.
The review also found that energy network operators are not generally aware whether customers connected to their networks are critical national infrastructure (CNI), and there is currently no explicit cross-sector requirement on CNI operators to ensure appropriate continuity of operations in response to power disruption. CNI facilities also have no priority within the electricity legal or regulatory framework. Work is underway, led by government, to identify and analyze cross-sector CNI interdependencies.
Future recommendations
The review has made a number of recommendations to reduce the likelihood and impact of a similar event in the future and serves as a starting point for an opportunity to make cross sector improvements for the benefit of the UK and its energy resilience.
The 12 recommendations set out in the report correspond to the following themes: asset management systems; maintenance actions; fire and asset risk assessments; site accessibility for emergency services; visibility of total site risk; electricity safety, quality and continuity regulations (ESQCR); incident management protocols; resilience of infrastructure with multiple supply points; and energy resilience of CNI.
Fintan Slye, CEO of NESO, 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 focused 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 organizations who have provided evidence to the review for their cooperation.”
In related news, earlier this year Heathrow announced the largest private investment program in its history and confirmed the airport is developing proposals for a third runway to share with the government by the summer of 2025. Click here to read the full story