A power outage cut electricity Thursday to a large swath of Southern California, as well as parts of Arizona and Mexico, causing a nuclear-power plant to shut down and exacerbating strains from a heat wave.
More than one million utility customers in San Diego County, parts of Orange County, parts of Arizona and the Mexican state of Baja California all lost power after a line transporting electricity from Arizona power plants to California customers failed, according to the California Independent System Operator.
The loss of electricity also forced an automatic shutdown of the San Onofre nuclear-power plant in San Clemente, Calif., which is operated by Edison International's Southern California Edison utility, spokesman Paul Klein said. SoCal Edison did not say when it would restart the plant. The plant's two units had enough grid electricity to operate the facility's safety systems, said Mr. Klein.
The loss of power from the transmission line and the nuclear plant caused all 1.4 million customers of Sempra Energy's San Diego Gas & Electric utility to lose electricity, said SDG&E spokeswoman Jennifer Ramp.
The 500-kilovolt North Gila-Imperial Valley transmission line tripped offline in Arizona, causing the outage, Ms. Ramp said. The line was back up and running late Thursday, but Ms. Ramp said it would take a day or more to reconnect all 1.4 million customers to the grid.
Arizona Public Service said the outage likely started at one of its substations near Yuma, Ariz. A utility employee was "doing some work" at the utility's North Gila substation, which caused a power outage in the Yuma area, said APS spokesman Damon Gross.
That outage appears to have spread to the transmission line, causing wider outages in southern California, Gross said. The utility is investigating the cause of the wider outage, and possible "human error" that may have started the outage, Gross said.
In San Diego County, which was hardest-hit, authorities rescued some people stranded on trains and elevators, law-enforcement officials said. No major injuries or deaths were reported. "Some people had to be taken off of trains that were stuck between stops," said Lt. Eddie Brock of the San Diego Sheriff's Department.
SoCal Edison customers in Orange and Riverside Counties were also hit by the outage. The utility didn't have an estimate for how many customers were without electricity, but the two counties have a combined population of more than five million people.
The outage hit during the height of a heat wave that has seen temperatures top 100 degrees, and during the afternoon rush hour snarling traffic across the region. "It's a mess," said Jim Amormino, a spokesman for the Orange County Sheriff.
FAA officials said all air traffic control facilities were operating normally. But San Diego International Airport was closed to outbound traffic due to a lack of runway lights, said controllers and pilots.