The National Energy System Operator (NESO) has published its final report from the review into the North Hyde substation outage which took place late on 20 March 2025.  

The consequence was the loss of all supplies from the 275 kV substation in Hayes, west London, impacting thousands of customers, including Heathrow Airport. More than 270,000 air passenger journeys were disrupted before flights were able to resume at 18:00 GMT the following day. 

The impact was also felt beyond the airport, with 66,919 domestic and commercial customers affected, as well as ‘essential services’ including road, rail and Hillingdon Hospital, NESO said.

Using forensic analysis from both National Grid Electricity Transmission and London Fire Brigade, the report states that a ‘catastrophic failure’ on one of the transformer’s high voltage bushings at the substation caused the transformer to catch fire.

The review found that this was most likely caused by moisture entering the bushing, causing an electrical fault. The report suggests that the electricity may have ‘arced’ and ignited the oil, which prompted the wider fire.  

The investigation reviewed almost 900 pieces of evidence and found an elevated moisture reading in the bushing had been detected in oil samples as early as July 2018. The report states that the issue went ‘unaddressed’ and ‘basic maintenance’ to fix the problem in 2022 was deferred.

Meanwhile, the report also found that none of the parties involved in supplying electricity to Heathrow knew how the airport’s internal electrical distribution network was configured, nor of the potential impact of losing a supply point.

The report says that Heathrow knew that power disruption could ‘greatly impact operations’ but assessed the total loss of power to one of its three supply points as a ‘high-impact, low-probability event’. The report recommends that Heathrow should diversify its configuration such that the loss of one supply point does not impact the entire airport.  

According to Heathrow, the report describes ‘clear and repeated failings’ which ‘could and should’ have prevented the fire. The airport said it expected National Grid to ‘take responsibility for those failings’ and is now said to be considering legal action.  

A prepared National Grid statement says that it has a ‘comprehensive asset inspection and maintenance programme in place’, and that further action has been taken since the fire. This includes an end-to-end review of the 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.

Responding to the publication of the report, Sue Ferns, Senior Deputy General Secretary of the union Prospect, said: ‘The outage at the North Hyde substation resulted from a regulatory regime which prioritises short-term cost-cutting and artificially contrived competition. Old infrastructure has not been effectively maintained, and the workforce has been under-valued and under-invested in. The consequences of this continued lack of investment are now becoming painfully clear.’

Ofgem approves £24bn investment  

In other news, UK regulator Ofgem has given the provisional green light to an initial £24bn investment programme to upgrade UK energy infrastructure, in what could be the biggest expansion of the electricity grid since the 1960s.  

The draft decision covers the 2026–2031 regulatory period and will include £15.3bn towards maintaining and operating the national gas grid and four regional distribution networks. Meanwhile, £8.9bn is being committed upfront to the high-voltage electricity transmission system – with another £1.3bn in reserve for ready-to-go projects.

The investment – around four times the current spending levels – will allow for 80 transmission projects across the UK to be completed within five years, involving 4,400 km of upgraded lines and 3,500 km of new circuits. These would support up to 126 GW of clean energy capacity.