LOS ANGELES — The day after the largest power failure in California history left millions in Southern California, western Arizona and northern Mexico without electricity during one of the hottest weeks of the year, local and federal officials promised Friday to investigate the cause.

The blackout was concentrated in San Diego, and San Diego Gas & Electric, which supplies power to most of the region, said it was still examining how the failure had spread so widely. But the initial cause, it said, appeared to be human error in Arizona that took down a line that provides power to the San Diego area. A major nuclear power plant in San Onofre, about 50 miles north of San Diego, also shut down, causing more failures, officials said.

“On their own, these things would cause virtually no impact,” because there would be backup power, said Michael R. Niggli, the president of the utility company. “Having each of these things happen at one time is not something we have considered. That’s not the way the system is designed.”

Six million people lost power, and Mr. Niggli said he had seen nothing on that scale in his three decades at the company. The investigation could take weeks or even months, officials said.

Arizona’s largest utility, APS, said that a worker had been performing maintenance on a substation line when the failure happened, but that it was not clear whether that had been a direct cause.

“Operating and protection protocols typically would have isolated the resulting outage to the Yuma area,” the company said. “The reason that did not occur in this case will be the focal point of the investigation into the event, which already is under way.”

Power in most areas was restored by 3:30 a.m. Friday, less than 12 hours after the failure began and far more quickly than officials had initially expected. But they said the electrical grid was still “fragile” and urged residents to conserve electricity through the weekend.

All of San Diego Gas & Electric’s 1.4 million households were without power at the peak on Thursday.

Federal Energy Regulatory Commission officials will investigate with the North American Electric Reliability Corporation, other federal agencies and state regulators.

The blackout all but shut down San Diego’s airport and caused major traffic jams through the area as drivers struggled to navigate roads without stoplights and working train crossings. Drives out of downtown that typically take 20 minutes stretched to nearly two hours as hundreds of workers streamed out of their offices. Firefighters had to rescue several dozen people who were stuck in elevators.

But no injuries or major crimes related to the blackout were reported, San Diego County officials said, although vandals shattered the windows of one taco shop and there were scattered complaints of loud parties. (Students at California State University, San Marcos, improvised a Slip ’N Slide using tarp and held milk-chugging contests near campus.)

The power failure also shut down several pumping stations in San Diego, prompting 1.9 million gallons of sewage to spill into a lagoon near beaches and causing city officials to close about 10 miles of oceanfront for the weekend. City officials also urged residents to boil their water before cooking with or drinking it.

Schools in San Diego County were closed on Friday, a decision officials had made the night before when the blackout was still widespread. And grocery stores threw away countless pounds of meat and other spoiled products.

But for the most part, life had returned to normal by the Friday morning commute. Travelers who had been stranded the night before streamed into San Diego International Airport.

Lois and Roger Brindisi of Illinois had been just an hour away from landing on Thursday when their flight was diverted to Phoenix. The couple, on their way to visit their great-granddaughter, quickly booked a hotel after the airline refused to pay for accommodations.

“We’re kind of old to sit up all night,” said Mrs. Brindisi, 75. “Nobody seemed to know what was going on.”

Rob Davis contributed reporting from San Diego.

 

(From The New York Times)

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