A power outage at North Hyde 275kV substation near Hayes in West London on the night of Thursday 20 March 2025 led to 71,655 domestic and commercial customers losing power, including a loss of supply to Heathrow Substation A1. The figure comprises 66,919 customers of SSEN Distribution and an additional 4,736 customers of Independent Distribution Network Operators who were being supplied via North Hyde 66kV and 22kV substations1. The National Energy System Operator (NESO) published its final review report on 30 June 20251.
At 23:21, SGT3 at the substation and the circuit connecting it to the wider transmission system tripped1. At 23:49, 28 minutes after the loss of SGT3, SGT1's protection systems operated, automatically switching SGT1, and the circuit connecting it to the wider transmission system, out of service1. This resulted in the loss of all supplies at North Hyde 275kV and 66kV substations, and all customers fed from North Hyde were disconnected from 23:491. The review found that a catastrophic failure on one of SGT3's high voltage bushings caused a fire to ignite on the supergrid transformer, most likely caused by moisture entering the bushing causing a short circuit1. The protection systems operated as expected, isolating the power supply in approximately 110 milliseconds1.
At 00:42 the Metropolitan Police Service declared a major incident1. At 01:11, Heathrow Airport Limited's Chief Operating Officer took the decision to close Heathrow Airport due to the disruption caused to operationally critical systems following the power outage1. At 09:49 on Friday 21 March, North Hyde 66kV busbars were re-energised, and at 00:55 on Saturday 22 March, North Hyde 275kV substation's SGT2 was energised and connected to the wider transmission system1. London Fire Brigade issued the stop message, confirming cessation of firefighting, at 17:13 on Wednesday 26 March, five days and 18 hours after it first arrived on site1. The Metropolitan Police Service, supported by the Metropolitan Police Counter Terrorism Command, confirmed on Tuesday 25 March that it had "found no evidence to suggest that the incident was suspicious in nature" and closed its investigation into the fire1.
The review traced the failure to an elevated moisture reading in one of SGT3's bushings, detected in oil samples taken in July 2018, which National Grid Electricity Transmission's guidance indicated meant the bushing should be replaced; the reading was recorded but the mitigations appropriate to its severity were not actioned1. A National Grid Electricity Transmission review in 2022 indicated that the fire suppression system on SGT1 and SGT3 was inoperable, and a further fire risk assessment report conducted in July 2024 indicated that the fire suppression system at North Hyde substation was still out of service1. North Hyde 275kV substation was built in 1968, with limited physical distance and no fire barrier in place between SGT1 and SGT31.
"This resulted in 66,919 of SSEN Distribution's domestic and commercial customers, and an additional 4,736 customers of Independent Distribution Network Operators who were being supplied via North Hyde 66kV and 22kV substations losing power, including a loss of supply to Heathrow Substation A."
Why it matters for households
The review found that the incident's effect on the wider electricity transmission system beyond North Hyde was limited, and the electricity system operated within the applicable standards1. It records that customer restoration occurred within expected timeframes, and that the wider impact to the local area, including road, rail and the environment, was relatively contained1. Relevant parties communicated effectively with the public and domestic customers, although some commercial customers told the review that the communications they received were less coordinated1.
The review also sets out a wider point about infrastructure that households depend on. Energy network operators are not currently aware whether customers connected to their networks are Critical National Infrastructure, there is no cross-sector requirement for such operators to ensure continuity of operations in response to power disruption, and they have no priority within the electricity legal or regulatory framework1. Work is underway, led by government, to identify and analyse cross-sector dependencies1. The review found that the design and configuration of Heathrow Airport's private internal electrical distribution network meant the loss of one of its three independent supply points would result in the loss of power to operationally critical systems, and that NESO, National Grid Electricity Transmission and SSEN Distribution stated they were not previously aware of the potential impact to Heathrow Airport of the loss of a supply point1.
For households that lose supply, the site's guide to reporting a loss of electricity sets out how to contact the network operator. Households preparing for storm related disruption can read how storms affect the electricity network, and the power cuts hub collects the related guidance.
What happens next
The review published an interim report on Tuesday 6 May 2025 establishing the timeline and sequence of events, the roles and responsibilities of the key stakeholders, and areas of further investigation1. The final report presents the root cause and makes recommendations for improving the resilience of essential services, including Critical National Infrastructure, to energy disruption1. Its recommendations cover asset management systems, maintenance actions, fire and asset risk assessments, and site accessibility for emergency services1. The review states that these recommendations will require close cooperation across industry, regulators and lead government departments to ensure they are effectively implemented1. A first meeting of a forum connected to the review was held on Monday 24 March, chaired by NESO's Director of System Operations1.
Sources (1)
- download, neso.energy
Storms and the Electricity Grid
Reporting a Power Cut
