Volkswacht Bodensee - Fire that closed Heathrow traced to unrepaired transformer

Water entering a transformer and causing a short circuit "most likely" started the fire, according to the National Energy System Operator (NESO) report, adding that elevated moisture readings were detected in July 2018 but "mitigating actions appropriate to its severity were not implemented".

The fire broke out at the North Hyde substation in West London, around two miles (3.2 km) north of Heathrow, at 11:21 pm on March 20, resulting in a complete loss of power supply to part of the airport's private internal electrical distribution network.

The airport was closed for most of March 21, affecting around 270,000 journeys and raising serious questions about the reliability of one of the UK's most critical pieces of infrastructure.

Using forensic analysis from the National Grid Electricity Transmission and London Fire Brigade, the government-commissioned report concluded that "a catastrophic failure" on high voltage "bushing" components caused a fire to ignite on the transformer.

"This was most likely caused by moisture entering the bushing causing a short circuit. The electricity likely then 'arced' (causing sparks) which combined with air and heat to ignite the oil, resulting in a fire," it added.

An elevated moisture reading in one of the bushings had been detected in oil samples taken in July 2018, indicating "an imminent fault and that the bushing should be replaced".

However, "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 said.

"This includes an opportunity in 2022 when a decision was taken to defer basic maintenance. The issue therefore went unaddressed," added the report.

- 'Risks were not addressed' -

Energy Minister Ed Miliband called the report "deeply concerning".

"Known risks were not addressed by the National Grid Electricity Transmission, and (energy regulator) Ofgem has now opened an official enforcement investigation to consider any possible licence breaches," he said in a statement.

The National Grid responded in a statement that "we fully support the recommendations in the report" and that "we will also cooperate closely with Ofgem's investigation".

Around 230,000 passengers use Heathrow every day -- 83 million a year -- making it one of the world's busiest airports. Planes from Heathrow serve around 80 countries.

The airport welcomed the report, which it said "sheds further light on the external power supply failure".

"A combination of outdated regulation, inadequate safety mechanisms, and National Grid's failure to maintain its infrastructure led to this catastrophic power outage," a spokesperson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

"We expect National Grid to be carefully considering what steps they can take to ensure this isn't repeated."

M.Betschart--VB