Fire at Mission Street substation causes multi-day outage in San Francisco

What happened

In December 2025, around the middle of the day, a fire broke out at the Mission Street substation, an urban distribution and transmission substation in San Francisco, California, following the failure of a piece of equipment. Approximately 130,000 customers lost supply, reported as roughly one third of the city's electricity customers. Pacific Gas and Electric, the operator, de-energised the affected plant so that the fire service could work safely, the grid was reported stable about three and a half hours after the fire started, and the fire was brought under control by the early evening.

Most customers were restored within hours, but two residential districts were without supply for up to three days. The event fell on a Saturday in the run-up to Christmas, one of the busiest retail days of the year. Traffic signals lost supply at busy intersections and businesses closed early. Pacific Gas and Electric described the damage to the substation as significant and extensive, and stated that a piece of equipment had failed, resulting in a fire.

Preventive maintenance at the substation had been completed in October 2025 and an inspection had been carried out on 5 December 2025. An independent engineering consultancy was retained to establish the cause. No injuries to utility workers or members of the public were reported. Local elected representatives called for a formal hearing into the outage and renewed calls for the city to take utility assets into public ownership. As of public reporting in early 2026, the root cause investigation remained ongoing.

Contributing factors

As stated publicly by Pacific Gas and Electric, a piece of equipment at the Mission Street substation failed and this resulted in a fire, with a circuit breaker among the plant involved. Pacific Gas and Electric stated that the root cause investigation was ongoing as of late December 2025 and that an independent engineering consultancy had been retained to carry it out. Reporting also noted that preventive maintenance had been completed in October 2025 and that an inspection had taken place on 5 December 2025. No confirmed failure mechanism had been published at the time of review, and no further causal factors should be inferred from the available sources. Restoration timing was reported to depend in part on the sequence of de-energisation agreed with the fire service before crews could begin work. An independent investigation by the engineering firm Exponent, commissioned by Pacific Gas and Electric and reported in May 2026, found the fire was started by high humidity and condensation building up inside the switchgear room. The report found that staff had found a damaged insulating board with burned spots and warping the previous November and logged it as a water issue rather than replacing it or inspecting it fully. The substation, built in the 1940s, had no interior drainage system, and ventilation fans in its basement had been broken for years.

Lessons learned

1. A substation fire requires prior agreement with the fire service on de-energisation sequence, since that step sits on the critical path to restoration.
2. Routine inspections must detect actual failure modes such as water damage in switchgear rooms, rather than logging faults without full investigation.
3. Urban substations built without interior drainage or with long-term broken ventilation need checks for condensation risks that can start fires.

More lessons on fire suppression and firefighting, detection and monitoring, from every incident on the register.

Not yet known

• Exact model and age of failed switchgear
• Total cost of damage and restoration
• Downtime for full substation repair

This entry is updated when these are reported. Get an alert for the next one like it.

Source

ktvu.com