PG&E: Multiple factors led to massive San Francisco outage

SAN FRANCISCO - PG&E released findings from an independent investigation into a December power outage that left about 130,000 San Francisco customers without electricity, concluding the incident was caused by a combination of factors rather than a single failure.

The utility commissioned third-party firm Exponent to examine the Dec. 20 outage, which arose from a fire at PG&E’s Mission Substation.

Investigation findings

Dig deeper:

According to the report, the substation’s open-air design, combined with certain weather conditions, created an environment that led to the fire.

"The building’s open design, along with a specific weather pattern, caused high humidity levels inside the substation," the report said. "High humidity and low temperatures created conditions that allowed for condensation to occur on the back of an 'insulating board.'"

The report found that moisture, contamination, and contact between the insulating board and a circuit breaker arm caused "electric tracking," allowing electricity to flow where it shouldn't. This ultimately triggered an "arc flash" that ignited the fire and caused the outage.

The substation is designed with open ventilation to accommodate equipment such as transformers and to safely vent heat and smoke in the event of a fire. The building has had this configuration since the 1940s, with its current equipment design in place since 2010.

Customer impact

What we know:

The outage affected roughly one-third of PG&E’s San Francisco customers at its peak, leaving about 130,000 customers without power. Full restoration took several days due to the complexity of repairs, the utility said.

"The outage that occurred last December in San Francisco caused real frustration and disruption for our customers, and it was unacceptable," said Sumeet Singh, PG&E CEO and executive vice president of energy delivery.

The outage forced widespread closures of restaurants and shops and caused significant transit disruptions across the city as traffic signals failed.

Autonomous vehicle disruptions

Driverless ride-hailing company Waymo temporarily suspended service during the outage. Videos on social media showed its vehicles struggling at intersections without functioning traffic lights.

Some vehicles activated hazard lights and stopped abruptly, while others stalled in intersections, forcing human drivers to navigate around them.

Upgrades

What's next:

Since the incident, PG&E said it has made several improvements, including replacing circuit breakers and transformers and adding weatherproofing measures to reduce the risk of similar events.