The report said the fire appears to have been caused by a buildup of condensation on a piece of equipment called a “barrier insulating board,” which is supposed to act as a protective barrier against moisture, air infiltration and fire.
The report said that a month before the fire, PG&E had observed “burned spots” and “warping” on the board, as well as metal erosion of a circuit breaker — a piece of equipment meant to halt the flow of electricity when a circuit becomes overloaded, preventing electrical fires. That circuit breaker failed a November test, and was replaced and reported as suspected “water damage,” though the Exponent report concluded that that damage was likely due to other causes related to the insulating board.
Exponent also said a heater in the room where the fire broke out — which can help mitigate humidity — was likely turned off. The Mission Street building is ventilated with unconditioned outside air, and its system lacks heating, cooling and humidity control — meaning its interior is highly affected by changes in the outside temperature.
City Attorney David Chiu slammed the utility company over the report Friday, saying PG&E had knowledge of equipment damage prior to the fire, and “chose to do nothing about it.” He said the company has a pattern of failing to invest in aging infrastructure and fixing faulty equipment.
“San Franciscans paid the price for PG&E’s pattern of inaction and indifference during the December blackout,” Chiu said in a statement. “This report makes clear that meaningful improvements are essential to protect San Franciscans from future failures.”
The report comes as San Francisco renews efforts to transition away from the public utility, in favor of a city-controlled power grid. The December incident sparked backlash from city leaders, who probed PG&E leaders about the outage, among other incidents, earlier this year.
