What's behind the hydro vault fire that forced Sheppard station to be evacuated Sunday?

Toronto Hydro is investigating to find out what caused smoke from one of its hydro vaults to seep into the Sheppard-Yonge Station on Sunday, forcing it to be evacuated and causing trains to reroute for about an hour.

Fire crews were called just before 3:30 p.m. Sunday afternoon for reports of smoke coming from a construction site at the intersection of Yonge Street and Sheppard Avenue. A total of seven trucks and 30 firefighters responded to the scene, along with Toronto Hydro.

Toronto Fire Services arrived to the sound of large banging noises and thick, dark grey smoke in the TTC subway tunnel.

​Capt. Adrian Retushniak said five TTC staff were treated for smoke inhalation. No one was injured in the incident, police say.

The incident comes less than two weeks after another hydro vault fire, when a failed, burnt-out cable sent smoke billowing through Toronto's financial district.

'Not symptomatic of a greater problem'

Toronto Hydro spokesperson Brian Buchan told CBC Toronto that incident was in no way linked to an explosion in the same area in May, when the Royal Bank of Canada building had to be evacuated for a blast caused by water finding its way into the vault after heavy rains in the city.

"It's not symptomatic of a greater problem," Buchan said at the time.

But at least two former hydro workers have raised concerns, claiming "inadequate maintenance" is to blame for recent vault incidents.

In an open letter to Toronto Mayor John Tory in May 2017, retirees Paul Kahnert and David Grant, called on the city to ban any restaurant seating on top of hydro vaults, demanding a "thorough review" of the utility's maintenance practices.

Patios atop grates 'can be a safety concern'

"The vault explosion on King St. West on May, 1, 2017, exposed the public to great danger," the letter reads. "The resulting impact on neighbouring businesses is also a serious concern."

The mayor's office has since responded, saying Tory is "confident that Toronto Hydro is committed to ensuring public safety and maintaining a reliable electrical," in a statement on July 17.

But earlier this week, Toronto Hydro spokesperson Tori Gass said the utility does suggest avoid putting patios and outdoor equipment on top of vaults. "The risk to public safety from hydro vaults is extremely low," Gass said.

Gass added placing patios on grates can limit access for maintenance crews and increase the risk of debris like cigarette butts entering vaults, "which can be a safety concern."

"We respect the concerns that have been raised by our former employee as we know he's only thinking about public safety, but they don't reflect current realities," Gass said of the retired workers' letter.

Maintenance systems 'evolving'

The utility has a "comprehensive" vault inspection and maintenance program, she said. The vaults involved in the fire earlier this month and in May are considered "network vaults," said Gass, and are inspected twice a year at a minimum.

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.

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

As for the retired workers' claims that maintenance issues are a long-term systemic problem, Gass said the way the utiltiy maintains its system is different today than it was even five years ago.

"It's always evolving as Toronto continues to grow. That means we're having to work smarter by taking advantage of new technologies, new types of equipment and we're using our workforce more efficiently in order to maintain the system," Gass said. "And while this can be challenging, we have an excellent safety record."

She added that the utility received "top marks" in its most recent audit in May 2017 by the Electrical Safety Authority, which describes itself as a not-for-profit organization mandated by the Ontario government to promote electrical safety throughout the province.