A power outage that shut down Heathrow Airport earlier this year was caused by unresolved faults at a local substation first identified as far back as 2018, according to a new report.

The BBC reports that The National Energy System Operator (NESO) found that elevated moisture levels at the North Hyde substation, which supplies power to Heathrow, had been recorded seven years ago but were never addressed.

The moisture ultimately triggered a fire, leading to a mass power failure across west London and grounding thousands of flights at the UK's busiest airport.

Fire at the substation, which broke out on 20 March 2024, was reportedly sparked when moisture entered the bushing, an insulating component in the substation, causing a short circuit that ignited oil and led to the blaze.

Although the issue was first logged in National Grid's system in July 2018, NESO said that maintenance work was cancelled, and mitigation measures were not implemented.

The report described the fire as "most likely" caused by the moisture-induced fault and said controls designed to prevent such failures "were not effective".

The consequences were severe. Some 66,919 customers, including homes, businesses, and critical services, were affected by the outage.

In total, around 270,000 journeys were disrupted, with Heathrow completely shut down the following day.

Terminals remained without full power until nearly 11am, and flights only resumed in the evening after extensive safety checks.

NESO also highlighted broader concerns about the resilience of the UK's infrastructure.

Essential services such as road and rail transport and Hillingdon Hospital were among those affected. Some homes were evacuated during the incident.

Around 270,000 journeys were disrupted, with Heathrow shut down the following day (credit: SWNS).

Energy Secretary Ed Miliband described the findings as "deeply concerning" and confirmed that energy regulator Ofgem had launched an enforcement investigation into potential licence breaches by National Grid.

Heathrow Airport, in a strongly worded statement, accused National Grid of failing to uphold infrastructure safety.

"A combination of outdated regulation, inadequate safety mechanisms, and National Grid's failure to maintain its infrastructure led to this catastrophic power outage," the airport said.

National Grid has since said it has reviewed its inspection and maintenance procedures and is working to strengthen the resilience of key substations.

It also acknowledged that one of the three main power sources to Heathrow had failed, and that it was unaware this would affect the airport's critical systems so severely.

NESO's review found that energy companies are generally not informed when their networks serve Critical National Infrastructure such as airports, raising further questions about oversight.

Ofgem's infrastructure director, Akshay Kaul, said: "We expect energy companies to properly maintain their equipment and networks. Where there is evidence they have not, we will take action."

Nigel Wicking, chief executive of the Heathrow Airline Operators' Committee, told the BBC that the airport closure is estimated to have cost airlines between £80m and £100m.

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