A newly published final report by the National Energy System Operator (NESO) has confirmed that the fire and subsequent shutdown at Heathrow Airport on March 21 was triggered by a neglected moisture fault in a 275kV transformer bushing, originally detected in 2018.

The failure caused a catastrophic short circuit and fire at the North Hyde substation, operated by National Grid Electricity Transmission, leading to widespread outages affecting Heathrow, Hillingdon Hospital, three data centres, road and rail networks, and over 66,000 homes and businesses.

According to the report, elevated moisture readings were recorded in oil samples seven years earlier, but no appropriate mitigation was taken. National Grid has since launched a review of its oil sampling and asset maintenance protocols.

The outage forced Heathrow’s closure for nearly 24 hours, with delays affecting more than 270,000 passengers. The report also revealed that Heathrow’s internal power distribution system lacked redundancy, requiring 10–12 hours to manually reroute supply from its other two power sources.

In the wake of the report, Heathrow Airport is considering legal action against National Grid, calling the event preventable. Meanwhile, Ofgem has opened an investigation into potential systemic failures in National Grid’s maintenance practices.

NESO’s review includes 12 recommendations, targeting improvements in asset management, fire risk, incident protocols, and energy resilience for critical national infrastructure (CNI). The report also highlights a lack of communication between operators and utilities regarding CNI status, calling for better cross-sector coordination.

“This was a high-impact event rooted in missed maintenance and visibility gaps,” said Fintan Slye, NESO CEO. “Our report offers a clear roadmap to strengthen infrastructure resilience.”

Source: NESO, BBC, New York Times