A substation behind a Sainsbury's caught on fire on Wednesday night, with people inside the store having to be evacuated.
Firefighters were called to put out the blazing transformer behind Sainsbury's in Clapham High Street, close to Clapham Park Road, at 10.45pm.
They spent two hours battling the fire before it was put out at 12.45am.
About 50 people working inside the Sainsbury's were evacuated.
Nobody was hurt.
A spokesperson for the fire brigade said: "It was part of the cooling system, there was a bit of an explosion before the fire started."
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