The facility, which is operated by Southern California Edison, reported large plumes of smoke caused by some sort of explosion.
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"They thought a circuit breaker had blown inside the vault and that was the cause of all the smoke," says CAL FIRE Battalion Chief Brandon Markle.
SoCal Edison employees who working there at the time had to be treated on scene for smoke inhalation. Moving forward, investigators are using extreme caution for what they say can be a tricky situation.
"Eight of our employees who were in the underground facility were evacuated immediately without incident," says SoCal Edison spokesperson Ben Gallagher. "There were no initial injuries reported."
Hazmat crews were called in to help assess the area and make sure there was no danger to workers or the public.
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"We're unaware of what the hazmat exactly is, that's why we've called them out," Markle said.
Southern California Edison is also reporting that there are no safety concerns for the surrounding community at this time, and electric service has not been interrupted.