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8h ago

PG&E-Commissioned Probe Blames Humidity and Open Design for San Francisco Substation Fire

The findings point to fixes for moisture control at aging indoor substations.

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Temporary generators are seen outside the Pacific Gas and Electric Co substation at 24th Avenue and Balboa Street in the Richmond neighborhood of San Francisco following the massive blackout that left nearly a third of the city’s residents without power for hours.

Overview

  • PG&E, which released an Exponent report Friday, said the Dec. 20 Mission Street blaze stemmed from multiple interacting conditions that led to the mass outage.
  • Investigators said condensation formed on a barrier insulating board inside the open‑ventilated building, touched a circuit breaker arm, created electric tracking, and triggered an arc flash.
  • Records cited by KQED note burned spots on the insulating board and a failed circuit breaker test in November, signs now linked to moisture problems.
  • PG&E reports it added dehumidifiers and space heaters at Mission, installed humidity and temperature monitors, weatherproofed indoor sites, and replaced breakers and transformers.
  • The failure left about 130,000 customers without power for days across San Francisco, disrupting traffic signals and driverless taxis, and the CPUC says its investigation is still active.