Britain’s National Energy System Operator said a catastrophic failure of a high-voltage bushing at National Grid’s North Hyde substation caused the fire and blackout that shut London Heathrow Airport on 21 March. Moisture that entered the equipment most likely triggered a short circuit, igniting oil inside the transformer and spreading flames that burned for days. The fault had been identified in July 2018, when elevated moisture readings signalled an imminent breakdown, but replacement and other basic maintenance were repeatedly deferred, the report found. A fire-suppression system at the 275-kilovolt site had also been out of service since 2022. The ensuing outage closed Heathrow for almost 20 hours, cancelled more than 1,300 flights and disrupted travel for about 270,000 passengers, while cutting electricity to some 67,000 homes and critical services in west London. Energy regulator Ofgem opened a formal investigation into National Grid Electricity Transmission following the findings, and Energy Secretary Ed Miliband called the lapses “deeply concerning”. National Grid said it supports the report’s recommendations, has begun retesting key substations and will cooperate fully with the probe. NESO urged tougher asset-management rules and better coordination between network operators and critical-infrastructure users to prevent similar failures.