Bushing failure most likely cause of fire that shut Heathrow

This is the finding of NESO (National Energy System Operator)’s final report into the North Hyde Substation outage that took place on 20 March.

As well as Heathrow, the loss of all supplies from North Hyde 275kV substation impacted services including road, rail and Hillingdon Hospital, plus thousands of homes and businesses. 

In its interim report published in May, NESO noted that on March 20, one of three supergrid transformers (SGT3) at North Hyde 275kV substation and its associated circuit - connecting it to the wider transmission system – tripped and was later confirmed to have caught fire.

Shortly after, the adjacent transformer (SGT1) also tripped, resulting in the simultaneous loss of connection to the remaining transformer (SGT2). Consequently, all supplies from North Hyde 275kV substation were lost, impacting thousands of customers including Heathrow Airport.

Using ‘forensic analysis’ from National Grid Electricity Transmission and London Fire Brigade, the fire was most likely caused by moisture entering STG3’s bushing, causing an electrical fault.