Investigators have determined the electrical facility fire which forced the closure of London Heathrow was triggered by a short-circuit from moisture entering high-voltage infrastructure – a problem detected seven years earlier but left unaddressed.

The fire on 20 March, and Heathrow’s shutdown the following day, resulted in 121 flights being diverted and some 1,300 more being cancelled.

Three transformers at the North Hyde electrical facility were used to feed one of three substations at Heathrow.

Elevated moisture readings in one of the transformers’ bushings – which insulate the high-voltage conductors from other structures – were detected during maintenance in July 2018.

The reading was significant enough to indicate an “imminent fault” and require replacement of the bushing, states National Energy System Operator’s inquiry into the 20 March fire.

While the reading was recorded by National Grid Electricity Transmission, which owns the high-voltage network, the inquiry says “mitigations appropriate to its severity were not actioned”.

“The controls in place were not effective and failed to identify subsequently that action had not been taken in relation to the elevated moisture reading,” it adds.

Multiple attempts were made to schedule maintenance on the transformer but, owing to priority considerations and the need to keep systems running, none of these went ahead.

“The issue therefore went unaddressed,” the inquiry states.

Presence of moisture caused a “catastrophic” short-circuit, it states, with electrical arcing igniting oil used as a coolant.

Protections automatically disconnected the other two Heathrow-feed transformers, meaning that the airport lost one of its three substations. The airport’s management opted to reconfigure distribution from the two remaining substations, but this took several hours to carry out.

“The power outage and closure of Heathrow airport were hugely disruptive and our report seeks to improve the way parties plan for and respond to these incidents, building on the underlying resilience of our energy system,” says National Energy System Operator chief Fintan Slye.

The inquiry points out that National Grid Electricity Transmission has since initiated an end-to-end review of the oil-sampling process which should have alerted the organisation to the bushing moisture problem.

National Grid Electricity Transmission says the fire was a “rare” event, and insists it has a “comprehensive asset inspection and maintenance programme in place”, with further action – including the sampling review – undertaken since the incident. It adds that it “fully” supports recommendations made by the inquiry.