Four fire engines and around 25 firefighters were called to a fire at on Clove Crescent in Tower Hamlets.
A transformer conductor in an electricity sub-station on the ground floor of a building was destroyed by the blaze.
A number of residents left the block before the Brigade arrived and no injuries were reported. Residents were alerted to the fire when they heard a loud bang from the ground floor sub-station and saw smoke.
The cause of the fire was believed to be accidental.
The Brigade was called at 1511 and the fire was under control by 1646. Fire crews from Poplar, Plaistow, Millwall and Shadwell fire stations attended the scene.