'Catastrophic failure' that led to fire and Heathrow power outage revealed

A fire at an electrical substation that shut down Heathrow Airport for most of a day was most likely caused by moisture which had been present in a transformer since 2018 but went "unaddressed", a review by the National Energy System Operator (Neso) has found. The report said moisture had been detected in oil samples at the North Hyde site in 2018, but action was not taken to replace electrical insulators known as bushings.

Because of the blaze, which started late the previous night, no flights operated at the West London airport until about 6pm on March 21. The final report by Neso said there was a "catastrophic failure" in one of the transformers, "most likely caused by moisture entering the bushing, causing a short circuit", which ignited the oil. The report said: "An elevated moisture reading in one of SGT3's (the transformer's) bushings had been detected in oil samples taken in July 2018.

"According to National Grid Electricity Transmission's relevant guidance, such readings indicate 'an imminent fault and that the bushing should be replaced'. While the reading was recorded in National Grid Electricity Transmission's online system, the 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. This includes an opportunity in 2022 when a decision was taken to defer basic maintenance on SGT3. The issue, therefore, went unaddressed."

The Neso review also found the design and configuration of Heathrow Airport's internal power network meant the loss of just one of its three supply points would "result in the loss of power to operationally critical systems, leading to a suspension of operations for a significant period".

Plans were in place to respond to the event, but included the reconfiguration of the network which would take 10 to 12 hours, but the loss of a supply point was "not assessed to be a likely scenario by Heathrow due to its expectation of the resilience of the wider network".

The review said: "This meant that its internal electrical distribution network was not designed or configured to take advantage of having multiple supply points to provide quick recovery following such a loss and relied on manual switching."

A spokesperson for Heathrow said the National Grid's "failure to maintain its infrastructure" led to the power outage.

The spokesperson said: "Heathrow welcomes this report, which sheds further light on the external power supply failure that forced the airport's closure on March 21. A combination of outdated regulations, inadequate safety mechanisms, and National Grid's failure to maintain its infrastructure led to this catastrophic power outage.

"We expect National Grid to be carefully considering what steps they can take to ensure this isn't repeated. Our own review, led by former Cabinet minister Ruth Kelly, identified key areas for improvement and work is already under way to implement all 28 recommendations."

The National Grid said there were "important lessons to be learned" about cross-sector resilience following the review into the North Hyde substation fire.

A National Grid spokesperson said: "As Neso's report sets out, in Great Britain we have one of the most reliable networks in the world, and events of this nature are rare.

"National Grid has a comprehensive asset inspection and maintenance programme in place, and we have taken further action since the fire. This includes an end-to-end review of our oil sampling process and results, further enhancement of fire risk assessments at all operational sites and re-testing the resilience of substations that serve strategic infrastructure.

"We fully support the recommendations in the report and are committed to working with NESO and others to implement them. We will also cooperate closely with Ofgem's investigation.

"There are important lessons to be learnt about cross-sector resilience and the need for increased coordination, and we look forward to working with government, regulators and industry partners to take these recommendations forward."

fgem said it was launching an official enforcement investigation into the incident at the North Hyde substation "and take further action as necessary".

The regulator said the cause was a "preventable, technical fault" and will review whether the National Grid Electricity Transmission (NGET) complied with legislation and licence conditions over the development and maintenance of the site.

Ofgem will also commission an independent audit of NGET's assets and their status and whether "the failings identified in NESO's report into North Hyde were one-off in nature, or more systemic across the National Grid estate".

Akshay Kaul, director general for infrastructure at Ofgem, said: "The North Hyde substation fire resulted in global disruption, impacted thousands of local customers, and highlighted the importance of investment in our energy infrastructure. As a result of the report's findings, we have opened an investigation into National Grid Electricity Transmission (NGET). We have also commissioned an independent audit of their most critical assets.

"Ofgem will also further examine the incident and its causes and take further action as appropriate.

"We expect energy companies to properly maintain their equipment and networks to prevent events like this happening. Where there is evidence that they have not, we will take action and hold companies fully to account.

"Britain has one of most reliable energy systems in the world and thankfully incidents like this are rare. We must continue to invest in the system to maintain that resilience."