The Heathrow fire report is not just damning for National Grid

Telegraph

03-07-2025

The report into the substation fire that brought Heathrow to a standstill in March is damning for National Grid. It reveals that the blaze was caused by a fault detected seven years ago but not fixed.
But the findings also raise serious questions about Ofgem, the energy regulator.
Ofgem's job isn't simply to rubber-stamp investment plans, it's to challenge risk management and resilience plans. Letting a known-defective transformer remain in service for years is not just bad engineering, it's a management and regulatory failure.
The regulator has now announced an investigation, saying it expects network companies to 'properly maintain their equipment'. The words 'horse' and 'bolted' spring to mind – the regulator should be monitoring this continually, not just issuing fines after things have gone wrong.
The National Energy System Operator (Neso)'s final report into the fire at the North Hyde substation in west London on March 20 details that at 11.21pm one of three supergrid transformers owned and maintained by National Grid tripped and was later confirmed to have caught fire.
Soon after, the adjacent transformer also tripped. A third was further away and so was not affected, other than it was unable to be used as long as the substation was shut off to tackle the fire.
Transformers are some of the most important pieces of equipment on the electricity grid. They allow us to move electricity in bulk over long distances by changing the voltage.
Higher voltages reduce losses from power lines heating up. But high voltages are dangerous – we wouldn't want or need them in our homes – so we use transformers to change the voltage up for long distance transmission and then down for local networks and into homes and businesses.
The actual bit that does the voltage transformation consists of lots of wires wrapped around a metal core. This can get very hot and heat degradation could lead to short circuits that would stop the transformer working properly, so the coils or windings need to be kept cool.
This means placing them in a tank of cooling fluid. It must be runny enough to be pumped in and out to take the heat away and also an electrical insulator – i.e. it won't create short circuits between the windings. It also needs to be stable in the normal UK temperature range, chiefly not freezing in winter.
The fluid settled upon is oil. Unfortunately it is very flammable, so if there is a fault causing sparks it can catch fire or even explode.
The part of the transformer that failed at North Hyde is called a bushing. This is essentially a sleeve that lets the high voltage cable enter the metal oil tank without touching the walls and becoming earthed.
If moisture gets inside the sleeve, a short circuit can happen and you can get arcing, which is where electricity jumps across air gaps.
The particular transformer that caught fire in North Hyde had known issues: moisture had been detected as far back as 2018. National Grid's own guidance said the level of moisture indicated there was a risk of 'an imminent fault and that the bushing should be replaced'.
In addition, basic maintenance of the transformer was overdue and had been consistently put off.
The Grid decided to extend the interval between basic maintenance from four to five years, and for major maintenance from 11 to 15 years. The most recent basic maintenance of the transformer had been in 2018, so it was well overdue on both the old and new schedules.
Fire suppression equipment at the substation was found not to be working in 2022. A further inspection in 2024 showed it still didn't work and repairs had not been carried out at the time of the fire, although London Fire Brigade suggested it would not have made much difference had it been working.
The sequence of events at North Hyde suggests a wider, systemic problem with delayed maintenance, and little follow-up in terms of the risks associated with these delays.
The report identifies that the transformer was installed in 1968, making it 57 years old. National Grid believes that transformers can last 60-65 years. Despite the North Hyde transformer approaching National Grid's end-of-life window, it had not clocked this and set in motion plans to replace it.
All this raises questions about Ofgem's role. While it has been critical of National Grid's business plan in respect of aging infrastructure, its response lacked teeth.
Why did Ofgem not insist that asset lives be more rigorously analysed and that maintenance of older critical equipment was carried out on time? Was Ofgem's focus on renewables connections a factor in the approach taken by grid owners to legacy equipment, knowing that consumers can't pay for everything at once during a cost of living crisis?
We need urgent action: Ofgem must demand a comprehensive review of all legacy transformers and other critical grid equipment and enforce swift maintenance or replacement. It also needs to be far more proactive about ensuring grid owners are assessing asset life appropriately and make sure that maintenance procedures are both rigorous and properly adhered to.
We were lucky this time. Incidents such as the North Hyde fire could easily cause explosions, threaten local communities, and even trigger cascading grid failures if large generation assets or import cables are disconnected as a result. There is no room for complacency.