Six fire engines and 35 firefighters and officers were called to reports of electrics arcing at a block of flats in Olympic Way, Wembley.
Residents on the first floor had reported lights flickering and flashes from appliances and other occupants also reported similar issues.
Crews searched the property and found heat damage to some plugs, but there was no fire apparent.
Around 45 people were evacuated as a precaution and power to the whole block was isolated. There were no reports of any injuries.
The Brigade was called at 0902 and the incident was over at 1155. Fire crews from Wembley, Park Royal, Stanmore and Willesden fire stations attended the scene.
The cause of the issue is believed to be a fault in an electrical distribution board.