Heathrow substation fire ‘caused by fault first identified seven years ago’

The root cause of the substation fire that shut Heathrow airport in March was a preventable technical fault that National Grid had been aware of seven years ago but failed to fix properly, investigators have concluded.

The final report by the National Energy System Operator (Neso) into the incident found said the fire that cut power to the airport, affecting more than 1,350 flights and almost 300,000 passengers, was “most likely” caused by moisture entering the insulation around wires sparking the electrical fault.

The state-owned body said its investigation had found that National Grid – the owner of the substation that caught fire, which also cut power to 67,000 homes – had been aware of the issue in the bushing – the insulated structure through which current flows in and out of a transformer – in 2018.

“An elevated moisture reading in the bushing had been detected in oil samples taken in July 2018 but mitigating actions appropriate to its severity were not implemented,” the report said. “National Grid has since initiated an end-to-end review of its oil sampling process, with a view to ensuring that it is robust.”

In response to the findings, Ofgem, the energy regulator for Great Britain, has opened an investigation into National Grid for what it called a “preventable technical fault”.

Ofgem said it would review whether National Grid complied with legislation and licence conditions relating to the “development and maintenance” of the electricity system at North Hyde.

It has also commissioned an independent audit into National Grid’s critical infrastructure assets and their status.

Ofgem said that the aim was to identify whether the “failings identified in Neso’s report … were a one-off in nature, or more systemic across the National Grid estate”.

Akshay Kaul, the director general for infrastructure at Ofgem, said: “The North Hyde substation fire resulted in global disruption, impacted thousands of local customers, and highlighted the importance of investment in our energy infrastructure.

“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.”