Tami Mairs was driving along Western Road this morning when she saw an explosion at the top of a hydro pole just before 9 a.m.
For three hours after that, thousands of London Hydro customers were left in the dark as crews worked to restore power to northwest London and other smaller pockets of the city.
"It looked like a huge welding light going off. No flames, just an explosion. A sizzling sound and a slight bit of a bang," said Mairs.
What Mairs saw was likely a flashover, caused by the equipment failure while London Hydro crews worked in the area of Western Road and Oxford Street, said Nancy Hutton, spokesperson for London Hydro.
That flashover caused the Hydro One transformer station to fail.
The massive outage left 15,000 customers without power, including Western University.
The power went out just before 9 a.m., right during the late part of the morning commute.
None of the crew members was hurt in the explosion.
Customers in northwest London were most affected.
As of noon, 250 customers were still without power. Hutton said she couldn't predict when it would be restored.
Today's power outage is not connected to a similar incident last week, which knocked out power for more than an hour in north London.
London Hydro's updated outages map is available here.