New report shows what caused San Francisco's blackout in December

"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."

"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."

"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."

A power outage affected 120,000 PG&E customers in San Francisco on December 20. A report prepared for PG&E identified an arc-flash event and fire at the Mission Substation as the trigger. The most likely cause was moisture and surface contamination that led to equipment failure. The report found PG&E had observed damage to the incident insulating board in November and had taken corrective actions afterward. The substation building appeared prone to elevated humidity and moisture. Ventilation filters were in poor or degraded condition due to water saturation, which reduced filtration of outside ventilation air for the incident switchgear room. Local officials and business owners cited concerns about preventability and financial losses.

Read at ABC7 San Francisco