North Hyde Substation outage causes loss of supplies and Heathrow closure

A fire broke out at North Hyde Electricity Substation in Hayes, London, late on the evening of Thursday 20 March 2025, cutting supplies from the 275kV substation and disrupting power to homes, businesses and Heathrow Airport¹. The National Energy System Operator (NESO) says the outage led to 71,655 domestic and commercial customers losing power¹.

NESO's final report, published on 2 July 2025, found that a catastrophic failure on one of the transformer's high voltage bushings at National Grid Electricity Transmission's substation caused the transformer to catch fire¹. NESO says this was most likely caused by moisture entering the bushing, causing an electrical fault, and that an elevated moisture reading had been detected in oil samples taken in July 2018 but mitigating actions appropriate to its severity were not implemented¹. National Grid Electricity Transmission has since begun an end-to-end review of its oil sampling process and a review and assurance exercise of all recorded oil samples¹. The review drew on nearly 900 pieces of evidence and followed an interim report published on 8 May¹.

Restoration was quick for most customers. National Grid said that as of 0600 on 21 March power had been restored to 62,000 customers, with 4,900 still without power³. NESO reports that 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 66kV¹. The incident also affected road, rail and Hillingdon Hospital; residents living near the substation had to be evacuated, with some needing alternative accommodation, and three data centres lost power but continued operations using backup generators¹.

At Heathrow, NESO found that the design and configuration of the 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 some operationally critical systems¹. Heathrow Airport Limited's plan to reconfigure the network to take power from the other two supply points is estimated to take 10-12 hours¹. This resulted in the airport closing for most of 21 March; it reopened for some flights late that day and was fully operational from 22 March¹. The terms of reference for the review, set by the Energy Secretary and Ofgem, state that it would not investigate knock-on impacts to the transport sector, which were to be covered in the Ruth Kelly review².

The review made 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 energy resilience of critical national infrastructure¹. NESO also found that energy network operators are not generally aware whether customers connected to their networks are Critical National Infrastructure, and that such facilities have no priority within the electricity legal or regulatory framework¹.

Why it matters for households

The outage shows how a single fault on transmission equipment can remove supply from tens of thousands of homes and businesses at once, and how restoration can run through the night: National Grid reported 4,900 customers still off supply at 0600 the morning after the fire³. NESO's report records that communication between SSEN Distribution and its domestic customers was effective and timely¹. Households can report a loss of electricity through the arrangements set out in our guide to reporting a loss of electricity: ring 105, and the common causes of supply faults are set out in why the electricity goes off.

The review also found that operators of critical national infrastructure have no priority within the electricity legal or regulatory framework, and that network operators are not generally aware which of their customers are such facilities¹. For households, the practical effect described in the report is that continuity of supply to essential sites depends on their own arrangements, such as the backup generators that kept three data centres running¹.

What happens next

The government response of 18 November 2025 sets out an implementation plan against NESO's recommendations, developed with the Energy Resilience Group, a partnership between government, the regulator and industry, and commits to improving GB energy resilience through an Energy Resilience Strategy⁴. NESO's chief executive said much of the work is already underway¹. The government response applies to England, Scotland and Wales⁴. No dates for completing individual recommendations are given in the published response summary⁴. Further background on supply interruptions is in our power cuts hub.