Fire and hydro officials are still trying to get to the bottom of what caused a transformer station fire and massive amounts of smoke that shut down Highway 401 and forced people from their homes.

Two transformers burned for hours at Hydro One's Richview Transformer Station on Kelfield Street in Etobicoke until the blaze was finally extinguished at about 8:40 p.m. on Friday. The fire broke out shortly before 4 p.m.

At the fire's peak, the thick, black smoke formed a massive plume that could be seen from kilometres away as it drifted across Highway 401, resulting in a closure that created a traffic jam for about an hour during rush hour.

Residents of Longbourne Drive and Waterbury Drive were forced from their homes because they were seen as directly downwind from the site. A condominium complex at 44 Longbourne Dr., near highways 401 and 427, was evacuated with the help of TTC buses.

Toronto Fire Services, who reported no injuries, said the smoke didn't pose a health hazard to the public, and the blaze was fuelled by mineral oil that is used to cool transformers.

About 30 fire units and Hydro One's emergency team were on scene at the fire's peak.

People who work in the area say at least one explosion occurred as the fire broke out. Lights flickered in buildings across the west end of Toronto and some power outages were reported.

Hydro One says Toronto Hydro transferred the electricity load supplied by the transformers to other stations at about 5:40 p.m., restoring power to all affected customers.

The power went out at Pearson International Airport's Terminal 3, but the facility used a generator in the interim. Flights weren't affected by the situation, the GTAA says.

Police closed Highway 401 in both directions between Kipling Avenue and Highway 427 as a precaution because the plume was drifting across the road and it wasn't known if the smoke was toxic. The highway reopened once officials determined the smoke wasn't toxic.

One per cent of electricity lost

The Richview Transformer Station is a 230-kilovolt line distribution centre, which is considered to be one of the larger centres in the Greater Toronto Area.

The station is connected to transmission lines from six other transmission stations in the west end of the GTA, feeding electricity to thousands of homes and businesses.

The Independent Electricity System Operator says Toronto experienced a loss of 32 megawatts of power – or one per cent of the city's electricity – when the power failure occurred. The city usually uses about 3,500 megawatts during peak afternoon times.

There was an immediate reduction in voltage in the west end to compensate for the instant loss of power. That's why many buildings in the west end lost power or experienced power bumps.

The IESO says there was no problem re-routing electricity to other parts of the GTA.

The Richview Transformer Station was undergoing millions of dollars worth of upgrades and capital investments. Hydro One documents show $15 million was to be spent on air blast circuit breakers and another $9 million on transformer replacement in 2011 and 2012.

With a report from CTV's Paul Bliss