The National Energy System Operator (NESO) published its final report on 2 July 2025 into the outage at North Hyde 275kV substation near Hayes in West London, which took place late on 20 March1. NESO was commissioned by the Department for Energy Security and Net Zero and Ofgem to review the incident, identify lessons and make recommendations for the prevention and management of future power disruption1. The final report follows an interim report published on 8 May 2025, which set out the timeline and sequence of events1. NESO says it reviewed nearly 900 pieces of evidence1.
At 23:21 on Thursday 20 March, one of three supergrid transformers at the substation, SGT3, and its connecting circuit tripped2. A catastrophic failure on one of the transformer's high voltage bushings caused a fire; this was most likely caused by moisture entering the bushing, causing an electrical fault1. An elevated moisture reading had been detected in oil samples taken in July 2018, but mitigating actions appropriate to its severity were not implemented1. Twenty-eight minutes after the loss of SGT3, the protection systems on the adjacent transformer SGT1 operated, switching it and the remaining transformer SGT2 out of service, and all supplies from North Hyde were lost from 23:492.
The outage led to 71,655 domestic and commercial customers losing power1. This comprised 66,919 customers of SSEN Distribution and an additional 4,736 customers of Independent Distribution Network Operators supplied via North Hyde 66kV and 22kV substations2. By 12:24 on 21 March all except two of the 66,919 SSEN Distribution customers had power restored following the re-energisation of North Hyde 66kV1. The Metropolitan Police Service declared a major incident at 00:422. Heathrow Airport Limited took the decision to close the airport at 01:11 because of disruption to operationally critical systems2; it closed for most of 21 March, reopened for some flights late that day and was fully operational from 22 March1. The review found that the design of Heathrow's private internal network meant the loss of one of its three supply points would cut power to some critical systems, and that reconfiguring the network was estimated to take 10 to 12 hours1. Residents living near the substation were evacuated, some needing alternative accommodation, and three data centres lost power but continued on backup generators1. The Metropolitan Police, supported by its Counter Terrorism Command, confirmed on 25 March that it had found no evidence the incident was suspicious2. London Fire Brigade issued its stop message at 17:13 on 26 March, five days and 18 hours after first arriving2.
The report makes 12 recommendations, covering asset management systems, maintenance actions, fire and asset risk assessments, site accessibility for emergency services, visibility of total site risk, the Electricity Safety, Quality and Continuity Regulations, incident management protocols, resilience of infrastructure with multiple supply points, and the energy resilience of Critical National Infrastructure1. It found that energy network operators are not generally aware whether customers on their networks are Critical National Infrastructure, that such facilities have no priority within the electricity legal or regulatory framework, and that government-led work is underway to identify cross-sector dependencies1.
"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."
Why it matters for households
The review describes an outage that affected more than 70,000 homes and businesses in West London, as well as road, rail and Hillingdon Hospital, and found that restoration for the directly supplied customers was completed within expected timeframes, with communication to domestic customers described as effective and timely1. It also identifies weaknesses that could affect households in a similar event: maintenance issues recorded but not acted on, fire controls at a site built before current standards, and no single process for assessing risk across a whole site1. The recommendations are addressed to industry, regulators and government departments rather than to households2. NESO states that all parties involved are working to deliver the recommendations and that much of the work is already underway1. National Grid Electricity Transmission has initiated an end-to-end review of its oil sampling process and is reviewing all recorded oil samples to ensure appropriate actions have been taken where required1. For anyone who does lose supply, the electricity industry's reporting line is 105, as set out in our guide to reporting a loss of electricity; our power cuts hub covers what happens during a supply interruption.
What happens next
The report sets out an implementation plan for its recommendations, requiring cooperation across industry, regulators and lead government departments2. NESO says work on the recommendations is already underway with its full support1. No further publication dates have been reported.
