Humidity at PG&E Substation Likely Cause of Massive December San Francisco Blackout

The fire at a PG&E substation that sparked a major outage in December, cutting power for about a third of city residents during the holiday weekend, was likely caused by a buildup of humidity, among other factors, according to an independent report released Friday.

The 70-page analysis by engineering consulting firm Exponent found that the South of Market substation was prone to elevated humidity and moisture, which appears to have caused damage to critical equipment that could have prevented the blaze. This was worsened by sudden temperature swings, like the weather that preceded the December fire.

“The most likely failure mechanism is degradation of surface insulation of the Incident Insulating Board, likely assisted by moisture and surface contamination,” the report reads.

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.

Separately, a number of small businesses and residents are suing PG&E over its response to the blackout.

PG&E spokesperson Lynsey Paulo said in a statement that the company had made $3 billion in capital upgrades in San Francisco over the last 20 years, and “continually invest[s] in the electric system with a focus on safety and reliability.”

“While this most recent outage was not the best reflection of our investment, that $3 billion has overall helped San Francisco to have one of the most reliable electric grids among California cities over the past decade,” she wrote. “We recognize that despite these substantial investments, there is still more to be done — and we are committed to delivering for San Francisco.”

After the fire, PG&E said, it completed thousands of inspections across its 31 substations in the city, and accelerated testing and maintenance work — including replacing circuit breakers and other electrical equipment.

It also installed humidity and temperature monitors, and is “weatherproofing” its indoor substations. Paulo said PG&E also added space heaters and dehumidifiers to the Mission Street substation, and is in the process of installing those at other locations.