The North Hyde substation fire that plunged Heathrow into chaos was caused by a single catastrophic failure — and the National Energy System Operator (NESO) says lessons must be learned fast.
In its final review published today, NESO blames the dramatic 20 March blackout on a high-voltage transformer bushing failure — a vital insulated pathway that lets power flow in and out of transformers.
The likely cause? Moisture that had been flagged as far back as 2018… but never dealt with.
Evidence shows the bushing failure sparked a fire that wiped out all supply from National Grid Electricity Transmission’s North Hyde 275kV substation.
More than 66,000 homes and businesses lost power. Hillingdon Hospital was hit. Three data centres went dark. Thousands were evacuated. Heathrow shut down.
By 12:24 the next day, all but two customers had power back. But the fallout was severe.
“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,” said NESO CEO Fintan Slye.
“NESO’s final report into the North Hyde Substation outage sets out the root cause and a clear set of recommendations to further improve the resilience of Great Britain’s energy system and the resilience of its critical national infrastructure.”
The investigation reviewed almost 900 pieces of evidence and found that moisture had been detected in the transformer’s bushing oil as early as July 2018.
But “mitigating actions appropriate to its severity were not implemented.” National Grid has now launched a full review of its oil sampling process and is checking historical samples to ensure any further risks are caught.
More worryingly, the review uncovered that Heathrow’s internal electricity setup was vulnerable.
Although the airport has three separate supply points, if one fails, some critical systems still go down. Reconfiguring takes 10 to 12 hours.
“This was less well-known by those outside the technical team within Heathrow Airport Limited,” the report states, “and it was not known to the energy companies.”
NESO’s review also warns that energy network operators do not always know whether customers are part of the UK’s Critical National Infrastructure (CNI) — nor are CNI facilities given any legal priority in terms of electricity access.
The government is now looking at cross-sector CNI interdependencies.
Twelve key recommendations are made across areas like asset maintenance, risk assessment, emergency access and CNI resilience.
NESO says these changes are vital to avoid another major disruption. The full report, it says, should be a “starting point” for stronger UK energy security.
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