How poor maintenance, oil and paper sparked major fire - and chaos at Heathrow

The reliability of our grid is, it turns out, paper thin.

A fire at the National Grid substation at North Hyde, which supplies Heathrow Airport, was caused by the "catastrophic failure" of a bushing in a high-voltage transformer at the site. A bushing is made of paper and foil, soaked in oil.

The resulting fire, which knocked out the substation and in turn, power to more than 70,000 customers, including the west London airport, led to 1,300 flight cancellations affecting nearly 300,000 travellers. The bushing on which the blame falls is the insulated wire that at North Hyde was one of three carrying a whopping 275kV of electricity into the transformer.

In many transformers - especially older ones like this installed in 1968 - bushings are insulated by several layers of paper wrapped around alternating layers of metal foil, all soaked in a special insulating "bushing oil". But if air bubbles or moisture get into the absorbent paper, it loses its insulating power.

As the report found, the most likely cause of the fire at North Hyde was "moisture entering the bushing causing a short circuit." That short circuit caused a spark that ignited the oil in the bushing, and in turn, the 150 litres of oil that insulates and cools the transformer, resulting in an inferno. The result was a fire so catastrophic, it took out not just one transformer at the site but also its sister transformer next to it.

Oil and paper insulators date back to the turn of the last century, and they work perfectly well if properly maintained. And it is National Grid's maintenance that is the damning failure identified by this report.

The report finds that the highest "category 1" moisture reading was identified during inspection of the doomed bushing in 2018. According to National Grid's procedures, that reading should have resulted in its immediate replacement.

Yet, for some reason, no action was taken. In fact, the report finds, routine maintenance of the failing transformer was deferred in 2022 - potentially another missed chance to rectify the fault.

National Grid said in response to the report that it has already carried out an "end-to-end review" of its relevant inspection and maintenance processes. The review also found that no other maintenance red flags have been missed.

It's nonetheless a serious failing and explains why Ofgem, the energy regulator, has now ordered an enforcement investigation into our largest grid operator. It raises questions, too, about the level of investment made by privatised network companies in maintaining or replacing ageing infrastructure.

Read more on Sky News:Thunderstorms warning for UKMatt Hancock at COVID inquiryPartial verdict in Diddy trial Recent analysis by Common Wealth found that all our electricity network operators have underspent their budgets for replacing grid hardware. This is money they are allowed by Ofgem to add to customers' bills as part of "network charges".

Those charges are set to rise again in Ofgem's latest review. Other infrastructure companies, our water firms in particular, stand accused of "sweating" ageing assets to increase their profits.

A key question for Ofgem's investigation into National Grid is whether money for maintenance and replacement was instead going toward their profits, at the expense of customers and ultimately the resilience of our electricity supply..