Toronto Hydro says a failed, burnt-out cable in one of its vault was to blame for what police described as an explosion in Toronto's financial district Friday evening, which saw smoke billowing through the area.

Spokesperson Brian Buchan told CBC Toronto said hydro crews located the cable after they went into the vault to investigate, but that there was no evidence of an actual explosion.

He went on to say Friday's incident was in no way linked to a blast in the same area just two months ago, when a vault exploded under the Royal Bank of Canada building, forcing it to be evacuated. The May explosion was believed to have been caused by water finding its way into the vault following heavy rains.

"They're not related at all," Buchan said Friday evening.

"The first one was based on a severe weather event ... This was a vault that had a cable that failed within it," he said, adding that such situations do not happen regularly, with the last such fire occurring back in 2015.

"It's definitely not linked and it's not symptomatic of a greater problem."

Emergency crews are on the scene of a hydro vault explosion downtown in Toronto's financial district, police say. 0:23

No injuries were reported in Friday's fire. One witness told CBC Toronto she heard a "massive bang," followed by a flash out of the corner of her eye.

"Jesus, what was that?" Shauna Martin recalled saying at the time.

But while it wasn't immediately clear what had happened, no one appeared to be running, Martin said.

"For downtown Toronto on a Friday evening, it's not really what you'd be expecting," she said. "But because of what happened on King Street a [few] weeks ago, everyone kind of had an idea of what was going on.

Toronto Fire Platoon Chief Kevin Aucoin told CBC Toronto emergency crews arrived on the scene near the end of business hours Friday evening, responding to reports of smoke coming up through a sewer grate.

Toronto Fire arrived on the scene, responding to heavy smoke from the explosion in Toronto's financial district. (@torontodan/Twitter)

Bystanders in the area were made to move back 30 metres as fire crews secured the scene, he said, leaving Adelaide Street closed at Bay Street. The hazard has since been cleared and all roads have reopened.

Buchan told CBC Toronto just one customer appears to be without power at this time and that crews expect to be able to repair the feeder damaged in the incident within the next six to eight hours.

Police say there are no reported injuries. (@torontodan/Twitter)

May wasn't the first time hydro vault fires paralyzed parts of downtown. Last March, subway service was suspended during the morning commute following a hydro vault fire.

And back in 2014, a similar issue sent staff rushing out of Toronto city hall. Three people had to be treated for smoke inhalation.

The intersection of Bay and Adelaide is closed as police investigate a hydro vault explosion in Toronto's financial district. (@AnnRichards1/Twitter)

Toronto Hydro says it has some 11,000 vaults in all, and the majority are downtown.

Buchan says the vaults are formally inspected every couple of years but that the utility also performs informal checks especially on downtown vaults more frequently.

He wouldn't say whether Friday's failed cable was a new or old one, saying crews are focused on restoring power.