Incident at North Hyde Substation caused widespread disruption in March
The National Energy System Operator (NESO) has published its final report on the North Hyde Substation fire, confirming that a transformer fault at the 275kV site led to a major outage on 20 March.
According to NESO, the failure resulted in the loss of all electricity supply from the substation, directly affecting 66,919 customers and causing operational disruptions at Heathrow Airport and other critical services.
The outage also impacted road and rail networks, Hillingdon Hospital, three data centres and thousands of homes and businesses across the area.
Residents living near the site were evacuated, with some requiring alternative accommodation during emergency response efforts.
NESO stated that power restoration for affected domestic and commercial customers supplied by SSEN Distribution occurred within expected timeframes.
Forensic analysis identified cause of fire at 275kV site
NESO reported that a forensic investigation by National Grid Electricity Transmission and the London Fire Brigade identified a transformer fire as the source of the incident.
According to the report, the fire originated from a high voltage bushing failure, which was most likely caused by moisture ingress leading to an electrical fault.
Elevated moisture levels had been recorded in the bushing oil sample in July 2018, but NESO noted that mitigating measures were not implemented in response to the finding.
National Grid Electricity Transmission has since committed to a full review of its oil sampling procedures and is reviewing all historical oil sample records for appropriate follow-up actions.
The organisation stated that it aims to ensure the robustness of its asset condition monitoring practices.
Heathrow Airport’s internal network limitations extended power loss
NESO’s report concluded that the configuration of Heathrow Airport’s internal electrical network contributed to extended disruption at the site following the loss of one of its three supply points.
Heathrow Airport Limited’s contingency plan involves switching to the two other operational supply points, a process estimated to take 10 to 12 hours.
This plan was not widely known outside of the airport’s technical team and had not been communicated to energy providers.
The airport was closed for most of 21 March to complete the reconfiguration and safety checks.
It reopened later that day for limited flights and resumed full operations on 22 March.
Energy sector lacks visibility of critical infrastructure links
The review found that energy network operators are generally unaware of whether customers on their systems qualify as Critical National Infrastructure (CNI).
NESO reported that there is no requirement in the current regulatory framework for CNI operators to confirm continuity arrangements in the event of a power failure.
It also stated that CNI customers do not receive priority status under existing electricity regulations.
The government is leading a programme of work to map and analyse CNI interdependencies across sectors.
NESO said the findings point to a need for improved collaboration and data sharing between infrastructure operators and electricity networks.
Twelve recommendations issued to support future energy resilience
NESO set out 12 recommendations aimed at reducing the risk and impact of similar incidents in future.
The recommendations cover a range of themes including asset management systems, maintenance action protocols and fire risk assessment processes.
Other areas include accessibility for emergency responders, resilience of sites with multiple power sources, and updates to Electricity Safety, Quality and Continuity Regulations (ESQCR).
Additional recommendations address incident management, visibility of site-wide risk, and the resilience of Critical National Infrastructure.
NESO Chief Executive Officer Fintan Slye 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.”
North Hyde Substation fire triggers national energy resilience review: Summary
NESO published its final report on the North Hyde Substation outage on 7 July 2025.
The incident occurred on 20 March 2025 and resulted in the loss of power from the 275kV substation.
Heathrow Airport and other critical services were affected.
A total of 66,919 customers were impacted, according to SSEN Distribution.
Residents near the site were evacuated during the response.
Forensic analysis attributed the fire to a transformer bushing failure.
Moisture ingress caused an electrical fault that triggered the fire.
Elevated moisture levels were previously detected in 2018.
National Grid Electricity Transmission has launched a review of its sampling process.
NESO found limitations in Heathrow Airport’s internal power network design.
The airport was closed for most of 21 March to reconfigure its power supply.
NESO highlighted a lack of awareness around CNI status among energy providers.
Twelve recommendations have been made to support future energy resilience.