Humidity at PG&E Substation Likely Cause of Massive December San Francisco Blackout

The fire at a PG&E substation that sparked a major outage in December, cutting power for about a third of city residents during the holiday weekend, was likely caused by a buildup of humidity, among other factors, according to an independent report released Friday.

The 70-page analysis by engineering consulting firm Exponent found that the South of Market substation was prone to elevated humidity and moisture, which appears to have caused damage to critical equipment that could have prevented the blaze. This was worsened by sudden temperature swings, like the weather that preceded the December fire.

“The most likely failure mechanism is degradation of surface insulation of the Incident Insulating Board, likely assisted by moisture and surface contamination,” the report reads.