A consulting company hired by Pacific Gas & Electric Co. to investigate the cause of a massive power outage in San Francisco in April concluded that the problems involved equipment failure and a flawed design of a critical alarm system.

A fire inside an electrical substation on Larkin Street in the city’s Tenderloin district triggered a blackout, which left 88,000 customers in the dark for hours, shut down city streets and snarled traffic.

The NBC Bay Area Investigative Unit obtained a report prepared by Exponent, the consulting company that investigated the cause of the outage. The company found that a control switch malfunctioned at the substation, causing a circuit breaker to stay shut down for 27 hours. The switch didn’t work due to age and wear, the company said.

The report explains that the substation’s computer system detected the problem and sent an alarm to PG&E’s electric distribution control center. But the operator at the control center missed the alert; according to Exponent, it was a silent alarm.

Exponent’s report indicates that for more than a day, the Larkin substation wasn’t operating normally. The equipment became overstressed, which caused an explosion in the cabinet of a second circuit breaker. The explosion started a fire, and the fire knocked out power.

PG&E is upgrading the substation and creating a new building to house switch gears, which the company recently replaced. The project began 2016 before the power outage, and is scheduled to be completed by the end of 2019.

Exponent notes that since the blackout, PG&E has made changes to its alarm system so operators can identify abnormal conditions in the future.

The California Public Utilities Commission is also examining the Larkin Street Substation incident, but the agency has yet to finish its investigation.

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