Eight fire engines and around 60 firefighters were called to a fire in the basement underneath a block of flats on Station Road in Hayes.
A small part of the basement was damaged by fire. There were no reports of any injuries.
Smoke alarms fitted inside the property alerted the residents to the fire, who then called 999.
Station Commander Brett Loft, who was at the scene, said: "On arrival, crews faced heavy smoke in the lobby and communal stairwell and discovered the fire in an electrical substation in the basement below a block of flats. Firefighters worked swiftly to confine the fire to electrical intake equipment and stop it from spreading.
"Around 20 people left the building before the Brigade arrived.
"Firefighters ventilated the property due to smoke logging affecting several floors of the building."
The Brigade was called at 1945 and the fire was under control by 2123. Fire crews from Hayes, Hillingdon, Heathrow, Southall, Feltham and surrounding fire stations attended the scene.
The Brigade's Fire Investigators believe the fire was accidental and caused by an electrical event inside the electrical intake equipment in the basement.