One, near Los Angeles, has been accused of sparking January’s Eaton Fire, which leveled the community of Altadena and killed 17 people. Another, in Northern California Wine Country, triggered a 2019 blaze that destroyed more than 370 structures. 

But no California agency knows how many miles of idled or abandoned power lines cross the state’s hillsides and forests, which have become increasingly prone to burn as the planet warms. State regulators don’t track exactly where they are. 

Utilities nationwide have been rushing to fortify and expand their power grids, as extreme weather strains the aging systems and electricity demand rises from data centers, factories and plug-in cars. Even as they add transmission lines, however, some older ones fall out of use, either because they’ve been replaced or because the power plants they once connected to the grid shut down.

“If idle lines are a risk, we need to be looking all around for these,” said Michael Wara, director of the Climate and Energy Policy Program at Stanford, who is an expert on utility wildfire mitigation practices. “We should be keeping a closer eye on the entire inventory of the overhead power system.”

Sometimes the utilities that own idled lines disconnect them from the power grid, cutting off the flow of electricity through the cables. Other times, they don’t, keeping them energized. California regulations require the companies to remove power lines they don’t anticipate using again, but there’s no set deadline. The companies can keep idled lines in place if they foresee needing them in the future, so long as they’re regularly inspected and maintained. 

Even those that have been unplugged from the grid can potentially pose a danger.

The line under scrutiny due to the Eaton Fire is owned by Edison International’s Southern California utility and was retired in 1971. Under normal circumstances, it carries no electricity, according to the company. Lawyers suing Edison allege that a Jan. 7 power surge on a nearby, active line created a magnetic field that briefly re-energized the dormant cable, setting off the sparks that ignited the blaze. Edison said in a state filing that it is investigating that theory. State investigators have yet to announce a cause for the fire. 

“This is going to be a big question: Why did they choose to keep those lines up for the past fifty years, and what kind of maintenance were they doing on them?” said Ali Moghaddas, an attorney for Edelson PC, which has filed suit against Southern California Edison.

The utility has patrolled and inspected the line each year, spokesman David Eisenhauer said. Asked why Edison didn’t remove the equipment, Eisenhauer said the investigation of the fire is ongoing and the utility is cooperating and sharing information with authorities as requested. Edison declined to disclose how many miles of idled lines it has and where they are located. 

California utility regulators told state lawmakers at a hearing Wednesday that there isn’t a comprehensive list or map of abandoned or idled lines, nor is there a requirement for utilities to report them.

“The service territories are so large and the pieces of equipment are so numerous that a registry of specific equipment may or may not exist,” Rachel Peterson, executive director of the California Public Utilities Commission, said at the hearing. “We will ask our expert teams within the commission to see what kinds of data exists about idled or abandoned lines.”

The commission has faced the issue before. One October night in 2019, a line in Sonoma County snapped in strong winds, shooting sparks onto the dry hillside below. The line once tied a nearby power plant into the state’s grid, but the plant closed in 2001. The line’s owners hadn’t removed it, and it stayed connected to the grid, energized. The resulting blaze, called the Kincade Fire, scorched 77,700 acres (31,444 hectares). 

Two years later, the utilities commission fined owner PG&E Corp. $40 million for not removing the line, among other violations.  PG&E, the state’s largest utility, agreed to remove 72 permanently abandoned transmission lines — representing about 260 miles (418 kilometers) of dormant cables. The work could cost as much as $268 million spread over 10 years, the company estimated at the time. PG&E had removed 64 of the idled lines by the end of 2024, according to a spokesman.

“At the right conditions, failing idle facilities can pose significant wildfire and safety risks,” PG&E said in its plan to remove the equipment. 

Sempra’s San Diego utility has five transmission lines and 8 partial transmission segments that are idled and maintained for potential future use. San Diego Gas & Electric said it regularly inspects and maintains those lines. 

A high-voltage transmission line can transfer electricity to a nearby idled line through a process called electromagnetic induction. The magnetic field generated by a current on the energized line creates a current on other, nearby lines, said Larry Pileggi, a professor of electrical engineering at Carnegie Mellon University. A surge of current could increase the size of the magnetic field, Pileggi said. 

Southern California Edison detected a fault on its transmission system near the time of the start of the Eaton Fire, according to a company filing with state regulators. The fault caused an increase of current on energized lines running parallel to the company’s inactive line in Eaton Canyon, where the blaze is thought to have started. SCE said the current surge remained within design limits. The utility added it was looking at whether its idle line could have become energized through induction, according to the filing.

Mikal Watts, a Texas-based attorney who has sued Edison on behalf of some fire victims, said his investigators found alleged evidence of electrical arcing and damage on the idled line and a steel tower carrying it near the suspected origin point of the fire. 

“That stuff shouldn’t have even been up there,” Watts said at a February 8 town hall with victims.