The National Energy System Operator (NESO) has published its final report into the North Hyde Substation outage, which disrupted power supply to over 71,000 customers, including Heathrow Airport.
The report, based on analysis of nearly 900 pieces of evidence, identifies the root causes of the outage and sets out 12 recommendations aimed at improving the resilience of Great Britain’s energy system and its Critical National Infrastructure (CNI).
The incident occurred late on 20 March when a catastrophic failure of a high-voltage transformer bushing at National Grid Electricity Transmission’s (NGET) North Hyde 275kV substation led to a transformer fire. Forensic evidence from NGET and the London Fire Brigade indicates the failure was likely due to moisture ingress into the bushing, with prior signs detected as early as 2018. According to NESO, the necessary remedial actions were not taken at that time.
Among the most affected customers was Heathrow Airport, which was forced to close for much of 21 March. The outage disabled one of the airport’s three independent supply points, triggering a shutdown of key operational systems. Although Heathrow has an internal procedure to switch to alternate supply points, the process took approximately 12 hours. Some repatriation flights resumed late on 21 March, with full operations restored the following day.
Beyond aviation, the outage impacted road and rail infrastructure, Hillingdon Hospital, data centres, and thousands of homes and businesses. Nearby residents were evacuated due to the substation fire, and some required temporary housing.
NESO’s report highlights several systemic issues, including limited visibility among energy operators and a lack of regulatory prioritisation for such sites in the event of disruption. It also notes that energy network operators are often unaware of the internal configurations of customer infrastructure, limiting their ability to coordinate responses effectively.
The 12 recommendations in the report focus on areas such as asset maintenance, risk assessments, emergency access to sites, and enhanced protocols for managing incidents that affect infrastructure with multiple supply points. Of particular note is the call for clearer cross-sector communication and planning around energy resilience for CNI operators.
Fintan Slye, Chief Executive of NESO, said: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.
The review was commissioned by the Department for Energy Security and Net Zero (DESNZ) and Ofgem, and follows NESO’s interim report published in May. The final document serves not only as an account of the March incident but also as a foundation for wider improvements to national energy security.
The full report, including a glossary and an ‘Electricity Explainer’ section, is available via NESO’s website.
