Hundreds of people were left without power near St. Clair Avenue and Spadina Road on Tuesday after Toronto's second underground electrical fire this week.

Tuesday's fire began while hydro crews were working on an electrical vault below a 15-storey apartment building at 260 Heath Street West, said Capt. Mike Strapko of the Toronto Fire Service.

"When they finished their work and re-energized the vault, the fire started," he told CTV.ca on Tuesday morning, saying the fire was small and "fairly routine" for firefighters to handle. "We got the call at 4:46 a.m... and cleared the scene in just over an hour."

Police believe the fire was caused by a blown fuse.

The electricity had already been turned off hours before the fire. Power has already been restored to some residents, but it is expected to remain off for much of the day for others in the affected units. Officials say the lights might be back on by 8 p.m.

While air quality in the area was a concern immediately after the fire, it was safe for residents to leave their units by about 6 a.m., Strapko said.

The fire pales in comparison to a vault explosion in East York on Sunday, which forced the evacuation of 900 residents from their homes and injured nine firefighters. Residents have not yet been allowed back into their apartments.