Photo: Jessica Christian / The Chronicle Photo: Jessica Christian / The Chronicle Photo: Jessica Christian / The Chronicle

San Francisco is not at high risk of burning down in a wildfire, but if the weather gets windy and dry enough in the East Bay, Pacific Gas and Electric Co. could temporarily cut power to the city and its nearly 900,000 residents anyway.

PG&E mentioned that unlikely but alarming scenario in a recent regulatory filing as the company explained how it has dramatically expanded its program for intentionally shutting down power lines so they don’t spark more dangerous wildfires.

The company will consider turning off high-voltage transmission lines like the one that malfunctioned right before last year’s Camp Fire — which state investigators last week confirmed was ignited by PG&E equipment.

Because those types of transmission lines are so interconnected with the rest of the electric grid, turning them off could have a “cascading effect” on places far removed from the weather that prompted the shutoff, PG&E told state regulators. It would be like closing a major interstate to prevent traffic — or, in this case, electricity — from reaching smaller streets.

That means a place like San Francisco, which is “highly unlikely” to experience the kind of weather that would cause an intentional power shutdown, could end up in the dark, PG&E said.

More specifically, the city could lose power if “East Bay transmission lines were to be de-energized due to extreme conditions,” PG&E said in a filing with the California Public Utilities Commission.

“It is a real possibility,” said Aaron Johnson, a PG&E vice president who has overseen the company’s power shutoff program.

San Francisco is still far less likely to experience an intentional PG&E power shutoff than a community in an area where wildfire risk is high, Johnson said. But power shutoffs are “possible, certainly, anywhere in our service territory,” he said.

San Jose power lines could also be shut off because of the same spillover effect that might affect San Francisco, PG&E told the commission.

The idea that customers in San Francisco or San Jose could lose power because of fire danger elsewhere is consistent with PG&E’s public statements in recent months. The company said when it announced its shutoff expansion plans in February that any of its 5.4 million electric customers could be affected.

Still, the scenario described by PG&E is a jarring indication of the impact that the utility’s recent wildfire problems could have on everyone in its 70,000-square-mile service area, which stretches from Eureka to Bakersfield.

After the devastating 2017 Wine Country wildfires, PG&E implemented power shutoffs for the first time last year. But the company didn’t include high-voltage transmission lines like the one at the origin point of the Camp Fire, so even if PG&E had turned off power in the area on Nov. 8 — a step it considered but did not take — that line would have remained energized.

Now facing responsibility for the deadliest and most destructive wildfire in state history, which prompted PG&E to file for bankruptcy protection in January, the company doesn’t want to take any more chances.

Barbara Hale, the San Francisco Public Utilities Commission’s assistant general manager in charge of power, acknowledged that “many residents will be surprised” if PG&E intentionally turns their power off. But she said the drastic measure could happen.

“I’m not surprised, because I know it’s an interconnected grid, and if enough of these big transmission lines come down, there’s no way to keep the power on for everybody,” Hale said.

Electricity enters the city primarily from two places: an underwater transmission line that runs through the bay, and a series of other lines that come up the Peninsula. It’s not clear why PG&E’s regulatory filing mentions only the East Bay transmission lines as a potential trigger for a San Francisco blackout. It’s also unclear why the Peninsula lines couldn’t necessarily make up for a shutoff in the East Bay.

The city’s location and the interconnected nature of the grid means that if enough of the transmission lines it depends on are shut off, it would become “just not physically possible to keep the power up for everyone,” Hale said.

“We’re very dependent on those lines, and if the lines that feed them are turned off to avoid wildfires, then yes, we will lose power,” she said.

In light of PG&E’s broad problems, San Francisco is seriously exploring a potential purchase of the utility’s distribution lines that serve the city. Distribution lines are different from transmission lines: They take electricity at lower voltages directly to homes and businesses.

A report released last week by the city utilities commission said the step would cost billions of dollars but touted the benefits of public power, describing it as a system that would be “held accountable by San Franciscans through their local elected officials” for affordability and quality of service. A full-fledged municipal utility would also “eliminate the need to fight for fair treatment from PG&E” and help the city meet its climate goals, the report said.

City officials are continuing to study the move, which would make the power generated at Hetch Hetchy Reservoir available to the entire city, according to the report. Hetch Hetchy power currently serves San Francisco city facilities and Muni.

But even if the city goes in that direction, it will still be vulnerable to PG&E shutoffs, Hale said.

If “PG&E turns off the transmission lines, the electricity is not going to flow here, even if we own the distribution grid,” she said. “Even though our generators are pumping out green electricity at Hetch Hetchy, it’s not going to be able to get here if PG&E de-energizes to the extent that report references.”

California utility regulators are in the process of developing new standards for how utilities statewide conduct power shutoffs for wildfire-prevention, a measure that San Diego’s investor-owned utility has used for years.

Proposed rules would direct investor-owned electric utilities to use intentional blackouts only “as a measure of last resort,” and would set guidelines for how companies are to warn their customers, among other measures. The state utilities commission is scheduled to consider the proposal at its May 30 meeting.

Decisions about whether to turn off the power rest squarely with utilities. Even the California Independent System Operator, which manages most of the state’s electric grid, will not stop PG&E once it has made the call.

“They will notify us so we can plan our grid operations accordingly ... but it is entirely their decision whether or not to de-energize those lines to mitigate wildfire risk,” said Keith Casey, the system operator’s vice president of market and infrastructure development.

PG&E is trying to raise public awareness about how widespread the power shutoffs could be through advertisements, emails and letters to its customers. The utility is also attempting to keep shutoffs as limited as possible by installing devices that can allow it to de-energize a limited area, for example.

But the company’s message to its electric customers is simple: Be prepared.

Johnson, the PG&E vice president, said forced shutoffs are “not something we take lightly” and that the company is “trying to do it as thoughtfully ... as possible across all the communities we serve.” But, he said, the reality is that PG&E customers need to be prepared for shutoffs “just like any other potential natural disaster in the state.”

Hale, the San Francisco utilities commission official, echoed that point.

“Just like the city has encouraged folks to be ready for earthquakes, folks should be prepared to be without power, have their plans for what their family and friends are going to do to keep themselves comfortable,” she said.

Even with Northern California rain lingering into May, dangerous conditions are already on the horizon. Fire officials in Butte County, where the Camp Fire burned last year, announced Thursday that the 2019 wildfire season begins Monday at 8 a.m.

J.D. Morris is a San Francisco Chronicle staff writer. Email: jd.morris@sfchronicle.com Twitter: @thejdmorris