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Report Blames Condensation for Arc Flash at PG&E’s Mission Substation

The finding now feeds a still‑open CPUC probe into PG&E’s upkeep practices.

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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.
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Overview

  • Exponent’s report, released Friday, says condensation on an insulating board set off an arc flash and a fire at PG&E’s Mission Street substation.
  • The report ties the moisture buildup to open ventilation, a quick warmup after a cool spell, and poor filtration inside the switchgear room.
  • Investigators noted earlier warning signs, including burned and warped insulating board, a circuit breaker that failed a November test, and a room heater likely turned off.
  • PG&E says it has inspected every San Francisco substation, replaced equipment, added humidity and temperature monitors, and is installing weatherproofing, heaters, and dehumidifiers while briefing state regulators.
  • The December 2025 outage cut power to about 130,000 customers for days, shuttered shops, darkened traffic signals, disrupted transit, and led Waymo to pause driverless rides.