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PG&E's report blames insulating-board deterioration; supervisors demand full root-cause analysis and corrective plan
Summary
PG&E told a San Francisco committee that a December 20 substation arc and the citywide outages most likely stemmed from a combination of moisture, contamination and design factors that degraded an insulating board; supervisors and the fire department pressed PG&E for a full root-cause report, timelines and proof fixes are applied across critical substations.
At a May 14 continuation of earlier oversight work, the Public Safety and Neighborhood Services Committee pressed Pacific Gas & Electric over the December 20, 2025 Mission Substation incident that left as many as 120,000 customers without power in parts of San Francisco.
PG&E presented an independent technical review by Exponent that concluded the event most likely resulted from deterioration of an insulating board caused by a combination of environmental, physical and design factors (high humidity and condensation, contamination and interactions with a breaker arm) that led to electrical tracking and an arc flash. PG&E said Exponent's testing and operational audit found no single, isolated cause but rather several converging conditions.
PG&E's regional team described immediate mitigations: more than 3,000 enhanced inspections across 31 San Francisco substations, over 1,500 priority maintenance upgrades, targeted transformer and breaker testing, installation of continuous temperature and humidity monitors at indoor substations, additional dehumidifiers and heater thermostats, and new sensors and fault monitoring. "We are committed to doing better for San Francisco families and business owners," Regional Vice President Jake Ziegelman said.
Supervisors repeatedly pressed PG&E on why damaged insulation and evidence of moisture noted in a November inspection were not replaced rather than merely cleaned, why basement supply fans and filtration issues persisted for months prior to the event, and why the loss of a single cubicle cascaded to impact a large portion of the city. PG&E responded that prior inspection standards aligned with industry practice, that some back-of-board contamination is not visible during routine service, and that the company is revising inspection standards and retraining crews.
San Francisco Fire Department Deputy Chief Patrick Grama raised first‑responder concerns: the arc occurred at 1:04 p.m., but SFFD dispatch did not record notification until 2:16 p.m.; ventilation and ponding inside the substation, saturated filters and nonoperational supply fans created hazards and complicated response and access. Grama urged improved mapping, alarm systems and preplanned access for firefighters.
Supervisors said Exponent's direct‑cause report does not answer the committee's central questions about why a local failure cascaded and what management or system‑level conditions allowed it to happen. Supervisor Alan Wong moved to continue the hearing until PG&E provides a full root‑cause analysis, a public corrective action plan with deadlines, and evidence that fixes are in place across other critical substations. The committee voted to continue the hearing to the call of the chair; the roll call recorded three ayes and the motion passed.
PG&E committed to further testing with Exponent, to post findings publicly and to share corrective actions with regulators and elected officials. Committee members said they will reconvene once PG&E provides the promised root‑cause analysis and detailed corrective plan.
