SGT3, one of three supergrid transformers at North Hyde 275kV substation near Hayes in West London, tripped at 23:21 on Thursday 20 March 2025 along with the circuit connecting it to the wider transmission system, and was later confirmed to have caught fire1. The National Energy System Operator (NESO) review, commissioned by the Secretary of State for Energy Security and Net Zero and Ofgem and published in its final form on 30 June 2025, found that a catastrophic failure on one of SGT3's high voltage bushings caused the fire, most likely because moisture entered the bushing and caused a short circuit1.
At 23:49, 28 minutes after the loss of SGT3, SGT1's protection systems operated and switched SGT1, and its circuit, out of service1. Heat and smoke from the fire had damaged SGT1's high voltage bushings and marshalling kiosk, and the review says it was not possible to keep the 275kV or 66kV sites operational because of the effects of heat and smoke and the need to fight the fire safely1. The loss of all supplies at North Hyde disconnected 66,919 of SSEN Distribution's domestic and commercial customers and a further 4,736 customers of Independent Distribution Network Operators supplied via North Hyde 66kV and 22kV substations, a total of 71,655, including a loss of supply to Heathrow Substation A1.
The review records that the Metropolitan Police Service declared a major incident at 00:42, and that at 01:11 Heathrow Airport Limited took the decision to close Heathrow Airport because of disruption to operationally critical systems1. The airport's private internal network was configured so that losing one of its three independent supply points would cut power to critical systems; reconfiguration was estimated at 10 to 12 hours, and NESO, National Grid Electricity Transmission and SSEN Distribution told the review they had not previously been aware of that potential impact1.
The review found that an elevated moisture reading in one of SGT3's bushings had been detected in oil samples in July 2018, and that National Grid Electricity Transmission's guidance indicated such a reading meant an imminent fault and that the bushing should be replaced; the reading was recorded but the appropriate mitigations were not actioned1. North Hyde was built in 1968 with limited physical distance and no fire barrier between SGT1 and SGT3, and fire suppression was added to both in 20021. A National Grid Electricity Transmission review in 2022 indicated the suppression system on SGT1 and SGT3 was inoperable, and a fire risk assessment report in July 2024 indicated it was still out of service1.
"The controls in place were not effective and failed to identify subsequently that action had not been taken in relation to the elevated moisture reading."
| Time and date | Event |
|---|---|
| 23:21, Thu 20 March 2025 | SGT3 and its circuit trip; fire later confirmed |
| 23:49, Thu 20 March | SGT1 protection operates; all supplies at North Hyde lost |
| 00:42, Fri 21 March | Metropolitan Police Service declares a major incident |
| 01:11, Fri 21 March | Heathrow Airport Limited decides to close the airport |
| 09:49, Fri 21 March | North Hyde 66kV busbars re-energised |
| 00:55, Sat 22 March | SGT2 energised and connected to the transmission system |
| Tue 25 March | Police confirm no evidence the incident was suspicious and close the investigation |
| 17:13, Wed 26 March | London Fire Brigade issues stop message, five days and 18 hours after first arriving |
Why it matters for households
The outage affected homes and businesses supplied from North Hyde in West London from 23:21 on 20 March 2025, and the review says customer restoration occurred within expected timeframes1. It also sets out wider points about how essential services are protected: energy network operators are not currently aware whether customers on their networks are Critical National Infrastructure, there is no cross-sector requirement for such operators to ensure continuity of operations after a power disruption, and they have no priority within the electricity legal or regulatory framework1. Work led by government is under way to identify and analyse cross-sector dependencies1. The review's recommendations cover asset management controls, maintenance, fire and asset risk assessments, and site accessibility for emergency services1. Household guidance on preparing for and coping with supply interruptions is set out in the guide to power cuts.
What happens next
The review published an interim report on 6 May 2025 and its final report on 30 June 2025, setting out recommendations that it says will require close cooperation across industry, regulators and lead government departments1. A forum chaired by NESO's Director of System Operations first met on Monday 24 March 20251. The final report includes an implementation plan for its recommendations1.
Sources (1)
- download, neso.energy
