Heathrow Airport, the UK’s busiest air travel hub, is considering legal action against National Grid after a report revealed that a devastating fire, which forced the airport to shut down in March 2025, was caused by a preventable fault at an electrical substation. The fire erupted at the North Hyde substation in west London, a crucial power supply point for Heathrow, and led to massive disruption affecting over 270,000 passenger journeys and numerous essential services in the surrounding area.
The National Energy System Operator (Neso) conducted an investigation and found that National Grid had been aware of elevated moisture levels in the substation’s electrical components as far back as July 2018. This moisture, which affected the bushings—insulating materials surrounding electrical parts—was a clear indicator of an imminent fault that should have prompted immediate maintenance. However, the report disclosed that despite these warnings, maintenance was repeatedly deferred, including a critical decision in 2022 to postpone basic repairs. This neglect allowed the moisture problem to worsen, culminating in a catastrophic transformer failure that ignited the oil inside and triggered the fire on March 20.
The fire’s impact was profound. Heathrow was forced to close completely, with no flights operating until the evening of March 21, causing thousands of cancellations and stranding passengers. The power outage also left 71,655 domestic and commercial customers without electricity and caused a complete loss of supply to parts of Heathrow’s internal electrical network. Beyond the airport, vital services such as road and rail transport, as well as Hillingdon Hospital, experienced disruption. Airlines based at Heathrow estimated losses between £80 million and £100 million due to the closure.
Fintan Slye, chief executive of Neso, emphasized that while the report did not seek to assign blame, National Grid was ultimately responsible for maintaining the equipment and ensuring its operational fitness. He highlighted a critical oversight: National Grid did not fully appreciate how vital the North Hyde substation was to Heathrow’s operations, even though the airport itself was aware of the substation’s importance. "When they identified those moisture levels back in 2018, what should have happened is that the transformer should have been taken out of service for a short period of time and that fault effectively repaired," Slye told the BBC. "That was missed. The moisture got worse over time and ultimately that's what caused the fire to happen. The story of our report really is probably one of missed opportunities."
Adding to the concerns, the report revealed that the fire suppression system at the North Hyde substation was inoperable at the time of the fire. A National Grid review in 2022 had already flagged the system as nonfunctional, and a subsequent assessment in July 2024 confirmed it remained out of service. The lack of an effective fire suppression mechanism allowed the blaze to escalate unchecked.
Heathrow Airport’s spokesperson welcomed the report, stating it shed important light on the external power failure that forced the airport’s closure. They pointed to a combination of outdated regulations, inadequate safety mechanisms, and National Grid’s failure to maintain infrastructure as the root causes of the outage. The airport’s own review, led by former cabinet minister Ruth Kelly, identified 28 key recommendations to improve resilience, with work already underway to implement these measures.
Energy Secretary Ed Miliband, who commissioned the Neso report, expressed deep concern over the findings. "Known risks were not addressed by the National Grid Electricity Transmission," he said. In response, the energy watchdog Ofgem has launched an official enforcement investigation into National Grid Electricity Transmission (NGET). Ofgem’s director general for infrastructure, Akshay Kaul, announced an independent audit of National Grid’s most critical assets and pledged to hold companies fully accountable if they fail to maintain their equipment properly.
Kaul remarked, "The North Hyde substation fire resulted in global disruption, impacted thousands of local customers, and highlighted the importance of investment in our energy infrastructure. As a result of the report’s findings, we have opened an investigation into National Grid Electricity Transmission. We expect energy companies to properly maintain their equipment and networks to prevent events like this happening. Where there is evidence that they have not, we will take action and hold companies fully to account." He also noted that Britain has one of the most reliable energy systems in the world, and incidents like this are thankfully rare, but underscored the necessity of continued investment to maintain resilience.
National Grid responded by acknowledging the rarity of such events and reaffirmed that it has a comprehensive asset inspection and maintenance programme. The company stated it has taken further action since the fire, including reviewing oil-sampling processes, enhancing fire risk assessments, and retesting the resilience of substations serving strategic infrastructure. National Grid expressed full support for the report’s recommendations and committed to cooperating closely with Ofgem’s investigation.
Another significant finding from the Neso review was the vulnerability of Heathrow’s internal power network design. The airport has three electrical supply points, but the loss of just one was enough to cause a power outage to critical systems, leading to the suspension of operations for a significant time. While plans existed to reconfigure the network to restore power, this process would take 10 to 12 hours and relied on manual switching. Heathrow had not assessed the loss of a supply point as a likely scenario, given their expectation of the wider network’s resilience. This gap in planning and network design contributed to the prolonged disruption.
Heathrow’s spokesperson pointed out that the energy network operators generally do not have clear knowledge of whether their customers are critical national infrastructure, complicating coordination and resilience efforts. This incident has therefore raised wider questions about the robustness of the UK’s major infrastructure and the need for better cross-sector collaboration.
As the investigation unfolds, the outcomes could range from no breach findings to compliance actions, penalties, or even licence revocations for National Grid. The episode has sparked urgent calls for systemic improvements to prevent similar failures in the future and ensure the reliability of essential services that millions depend on.
While the fire and subsequent shutdown exposed glaring weaknesses, it also serves as a stark reminder of the intricate dependencies within modern infrastructure and the high stakes involved in maintaining it. Heathrow, handling over 83.9 million passengers in 2024, remains a linchpin of UK transport and economy, making the lessons from this incident all the more critical.
