National Energy System Operator publishes final report from the review into the North Hyde Substation outage

2 July
Press release - 01 July 2025
Houses

Key facts
Today, National Energy System Operator (NESO) publishes its final report from the review into the North Hyde Substation outage, having reviewed nearly 900 pieces of evidence.
This follows NESO’s interim report published on 8 May which focussed on establishing the timeline and sequence of events of the outage.
This review has sought to undertake root cause analysis of the incident, to understand both how it unfolded at an asset and site level, and the impact it had on domestic and commercial customers, including Heathrow.
This review makes 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 Great Britain's energy resilience.

Today (02 July 2025) National Energy System Operator (NESO) publishes its final report from the review into the North Hyde Substation outage which took place late on 20 March. The consequence was the loss of all supplies from North Hyde 275kV substation, impacting thousands of customers, including Heathrow Airport.

The impacts beyond Heathrow were significant, affecting essential services including road, rail and Hillingdon Hospital, as well as thousands of homes and businesses. Residents living near the substation had to be evacuated with some needing alternative accommodation. Three data centres lost power but were able to continue operations through the use of backup generators.   

Customer restoration occurred within expected timeframes: by 12:24 all except two of the 66,919 domestic and commercial customers directly supplied by SSEN Distribution had power restored following the re-energisation of North Hyde 66KV. Information seen by this review suggests communication between SSEN Distribution and its domestic customers was effective and timely.

Using forensic analysis from both National Grid Electricity Transmission and London Fire Brigade, this review has seen evidence that a catastrophic failure on one of the transformer’s high voltage bushings [1] at National Grid Electricity Transmission’s 275kV substation 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. 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. In addition, they are undertaking a review and assurance exercise of all recorded oil samples to ensure all appropriate actions have been taken where required.

This 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 have a plan to deal with this which includes reconfiguring their 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 20 March caused a loss of supply to one of Heathrow Airport’s supply points.  This resulted in the airport closing for most of 21 March 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 21 March and was fully operational from 22 March.

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 analyse cross-sector CNI interdependencies.

This review makes 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 Great Britain and its energy resilience. The 12 recommendations set out in the report correspond to the following key 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
  • Energy resilience of CNI

Fintan Slye, Chief Executive Officer, National Energy System Operator 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.”
 

[1] A transformer bushing is a structure which provides an insulated pathway for current to flow safely in and out of a transformer. They are typically made of materials like porcelain, resin, or oil-impregnated paper, chosen for their insulating and mechanical properties.

ENDS

About NESO

National Energy System Operator, NESO, is an independent, public corporation at the centre of the energy system, taking a whole system view to create a world where everyone has access to reliable, clean, and affordable energy.

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Notes to editors:

  • NESO’s final report from the review into the North Hyde Substation outage is published today
  • NESO was commissioned by DESNZ and Ofgem to review the incident at North Hyde, identify lessons and recommendations for the prevention and management of future power disruption events, and lessons for Great Britain’s energy resilience more broadly.
  • The Terms of Reference were set by DESNZ and Ofgem and set out the remit of the review, including timescales for submission and publication.
  • NESO published its initial findings on 8 May 2025
  • The outage led to 71,655 domestic and commercial customers losing power, and the complete loss of supplies to part of Heathrow Airport’s private internal electrical distribution network. 
    Further information explaining the functioning of the electricity system and of electricity system assets is available in section 4 - ‘Electricity Explainer’. A glossary is also provided at the end of the report.