According to the 77-page report conducted for PG&E, the blackout was triggered by an arc-flash event and fire at the Mission Substation, which is crucial in supplying electricity to the city. The report states that the most likely cause of the incident was "moisture and surface contamination" that caused a piece of equipment to fail.
The report also found that PG&E had observed damage to the incident insulating board at the substation in November. PG&E said they did take corrective actions after the November inspection. The report said the substation building appeared prone to elevated humidity and moisture and that filters for the ventilation system were in poor or degraded condition because of water saturation and were not providing filtration of the outside ventilation air for the incident switchgear room.
San Francisco Supervisor Alan Wong, who represents the Sunset District, commented on the findings, saying: "It appeared that there were many issues that create more questions than answers. We have seen that there are a number of things that appears that may have been preventable, whether it be maintenance issues, moisture issues, maintenance issues that PG & E may have been able to handle better to prevent this."
Jetto Seeto, owner of The Mochi Donut Shop in the Sunset neighborhood, said she lost thousands of dollars in ingredients and sales due to the outage.
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"I think (PG&E) do very good (most of the) time, right, but if something like this, I think they need to figure out what should they do to San Francisco not have that problem again," she said.
In a statement, Sumeet Singh, CEO of PG&E and Executive Vice President of Energy Delivery, said: "We responded by taking immediate actions and strengthening our controls, and we are using the findings in the Exponent report to guide further improvements."
The utility said they're committed to learning from the event to make its electric system in San Francisco more reliable and resilient.
Full PG&E Response.
Full report.