The causes of the fires ripping through Wine Country and beyond have not been determined, but investigators are looking into reports that strong winds knocked down numerous Pacific Gas and Electric Co. power lines between 9:20 and 10 p.m. on Sunday, about the time that some of the most damaging fires broke out in Napa and Sonoma counties, officials said Wednesday.
"We are investigating a number of potential causes, including whether reports of power lines falling down and electrical transformers exploding Sunday night may have caused some of the wildfires in the region," said Janet Upton, deputy director of the California Department of Forestry and Fire Protection, also known as Cal Fire.
PG&E spokeswoman Ari Vanrenen said the utility, which has mobilized nearly 800 repair workers to the North Bay, found downed power lines and poles broken by the windstorm. PG&E has reported all of those problems to Cal Fire, she said.
"These destructive winds, along with millions of trees weakened by years of drought and recent renewed vegetation growth from winter storms, all contributed to some trees, branches and debris impacting our electric lines across the North Bay," Vanrenen said in an email. She did not address whether the damaged equipment might have triggered any of the fires.
But contact between electrical lines and tree branches has often caused blackouts and fires. The massive August 2003 blackout that cut power to 55 million people from Ontario to New Jersey was caused, in part, by high-voltage lines coming into contact with overgrown trees. Four of the 20 most destructive wildfires in modern California history were sparked by power lines, according to data from Cal Fire.
Lines tangling with branches can spark, so utilities are required to trim or remove nearby trees and bushes. For the standard electrical line running through most neighborhoods, branches must be kept at least 4 feet from the wires and 10 feet from each power pole.
Conditions were ripe for fires on Sunday night. Dry winds out of the northeast gusted up to 50 mph, and the vegetation that flourished after last year's drought-ending rains provided ample tinder.
As the winds picked up Sunday, PG&E sent out warnings on social media, telling customers that strong gusts could topple power poles and knock out electricity. The utility urged customers to report any downed lines to 911 and avoid touching them, since they could still be carrying electricity.
The warning proved prophetic. Several of the blackouts that swept across the North Bay Sunday night and Monday morning were caused by downed lines or damaged equipment on power poles, according to a PG&E website that tracks outages.
As many as 114,000 PG&E customers in the area lost power by midday Monday. By Wednesday, the utility had restored power to more than half of them, with 53,000 customers still lacking electricity.
Wildfires triggered by power lines have been an issue for PG&E before.
In September 2015, a pine tree in Amador County came in contact with a PG&E power line and sparked the Butte Fire, which burned more than 70,800 acres, destroyed 549 homes and killed two people. California utility regulators this year fined the company $8.3 million over the incident, and lawsuits filed by area residents, county and state officials could eventually cost the company $750 million, PG&E has said in regulatory filings.
An investigation by the California Public Utilities Commission after a 1994 wildfire accused PG&E of diverting money from its tree-trimming program to boost profits, while managers received bonuses for cutting budgets. PG&E paid $29 million to settle the case, while not admitting fault, and agreed to boost its tree-trimming budget.
PG&E now appears to be spending roughly the same amount of money each year on tree trimming as state regulators have approved.
According to an update PG&E provided to the Public Utilities Commission in April, the company spent more than $198 million in 2016 on "vegetation management" across its sprawling territory, about $62,000 less than the commission had approved. The difference would be used to help keep future rate increases down.
PG&E's annual spending on vegetation management is approved by the commission every three years during what are known as general rate cases, when regulators look at how much money the utility needs to spend on its many operations. Because the tree-trimming program is an ongoing expense, the amounts approved tend to be based on the costs PG&E incurred in previous years, and they have risen slowly over time.
It remains unclear whether PG&E could be held liable for wildfire damage linked to one of its power lines even if the company followed all tree-trimming regulations to the letter. Asked what would happen in that scenario, a commission spokeswoman said only that the agency would look for potential violations by PG&E of vegetation management rules.
"CPUC staff are working in coordination with Cal Fire to look at PG&E's maintenance and vegetation management practices," said the spokeswoman, Terrie Prosper, in an e-mailed statement. "Once the facts have been established, the analysis can then take place to identify any violations that may have taken place and to consider any additional measures that can be put in place to mitigate fire risks in the future."