LONDON

A devastating fire at a substation that wreaked havoc on Heathrow Airport earlier this year was caused by a preventable technical fault that National Grid was aware of seven years before, a recent report by the National Energy System Operator (Neso) showed.

The blaze in March resulted in the cancellation of over 1,350 flights and affected nearly 300,000 passengers.

The final investigation by the Neso concluded the fire was “most likely” caused by moisture seeping into insulation around electrical wiring, sparking a fault.

The fire knocked out power not only to Heathrow’s private electricity network but also to 67,000 homes.

While power was restored to the surrounding area relatively quickly, Heathrow's systems took hours to reboot—and another seven hours before the airport could resume operations.

Investigators found that National Grid, which owns the affected North Hyde substation, had detected “elevated moisture” in a key transformer component—the bushing—as far back as July 2018.

Despite this, the report says: “Mitigating actions appropriate to its severity were not implemented.”

National Grid has since launched a comprehensive review of its oil sampling process to prevent future failures.

The incident has prompted the energy regulator, Ofgem, to open a formal investigation into National Grid. It will examine whether the company failed to meet legal and licensing obligations related to system maintenance.

Although Heathrow was severely impacted, three data centers affected by the same outage were able to continue operations with backup generators.

Energy Secretary Ed Miliband called the report's findings "deeply concerning," citing a failure to address known risks.

He confirmed that Ofgem, the energy regulator, has launched a formal enforcement investigation to examine whether National Grid may have breached its license conditions concerning the development and upkeep of the North Hyde substation, which dates back to the 1960s.