What happened

An in-service supergrid transformer containing insulating oil caught fire at National Grid's North Hyde 275 kV substation. The incident cut all supplies from the substation, initially disconnecting more than 60,000 customers and businesses and disrupting part of Heathrow Airport's internal electricity system; the airport temporarily closed and resumed operations late on 21 March. Investigators found the fire was most likely caused by moisture entering transformer equipment and producing an electrical fault. Police did not treat it as suspicious. This was a technical failure, not sabotage.

When cited sources give different casualty, damage or spill figures for this event, they’re reported side by side above rather than merged into one number. See how confidence levels work.