The incident happened on 20 March 2025 at 23:21, when one of three supergrid transformers (“SGT3”) at the North Hyde 275 kV substation and its associated circuit, connecting it to the wider transmission system, tripped. SGT3 was later confirmed to have caught fire. Shortly after, the adjacent transformer (“SGT1”) also tripped, resulting in the simultaneous loss of connection to the remaining transformer (“SGT2”). The consequence was the loss of all supplies from North Hyde 275 kV substation.
The resulting 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. Heathrow Airport Limited took the decision to close the airport due to the impact of the outage on some of its operationally critical systems.
Using forensic analysis from both National Grid Electricity Transmission and London Fire Brigade, the NESO review says it has seen evidence that a catastrophic failure of one of SGT3’s high voltage bushings caused the fire.
This was most likely caused by moisture entering the bushing causing a short circuit. This led to arcing, which, combined with air and heat, ignited the oil, resulting in the fire.
An elevated moisture reading in one of SGT3’s bushings had been detected in oil samples taken in July 2018. According to National Grid Electricity Transmission’s relevant guidance, such readings indicate ‘an imminent fault and that the bushing should be replaced’. While the reading was recorded in National Grid Electricity Transmission’s online system, the mitigations appropriate to its severity were not actioned.
The controls in place were not effective and failed to identify subsequently that action had not been taken in relation to the moisture reading.
This includes an opportunity in 2022 when a decision was taken to defer basic maintenance on SGT3. The issue therefore went unaddressed.
The catastrophic failure and fire also caused damage to SGT1’s high voltage bushings and its marshalling kiosk, which then caused its protection systems to operate, 28 minutes after SGT3 tripped. This automatically switched SGT1 and SGT2 out of service, resulting in the loss of all supplies at the North Hyde 275kV substation.
It was not possible to keep either the North Hyde 275 kV or 66 kV substation sites operational because of the effects of heat and smoke from the fire, and the need to fight the fire safely.
While National Grid Electricity Transmission’s current standards for fire controls at substation sites prioritise distance and physical barriers between oil-filled equipment, neither of these measures were in place at the North Hyde 275 kV substation site as it was built prior to the application of these standards.
Risks at the North Hyde site were managed by National Grid Electricity Transmission using multiple separate processes, meaning there was no single structured process to provide a cumulative assessment of the risk to the whole site.
Considering the impact of the incident on Heathrow Airport, the NESO 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 operationally critical systems, leading to a suspension of operations for a significant period.
This situation was understood by Heathrow Airport Limited and there were plans in place to respond to such an event. These plans included the reconfiguration of the network, with an estimated timescale of 10-12 hours.
NESO, National Grid Electricity Transmission and Scottish and Southern Electricity Networks Distribution stated to NESO that they were not previously aware of the potential impact to Heathrow Airport of the loss of a supply point.
The loss of supplies from a supply point was not assessed to be a likely scenario by Heathrow Airport Limited, due to its expectation of the resilience of the wider network. This meant that its internal electrical distribution network was not designed or configured to take advantage of having multiple supply points to provide quick recovery following such a loss and relied on manual switching.
Source of text and images: NESO, North Hyde Review, Final Report
