By Marty Graham
SAN DIEGO (Reuters) - Power was restored on Friday to some 7 million people plunged into darkness by a blackout, blamed partly on "human failure," that snarled traffic, idled elevators and sent workers home early in parts of California, Mexico and Arizona.
The outage, traced to a botched maintenance procedure by an Arizona utility worker, left all of San Diego -- California's second most populous city -- without power for at least seven hours and ranks as one of the biggest U.S. blackouts in recent history.
But it paled in comparison to a massive 2003 power failure that left about 50 million people without electricity in the eastern United States and Canada.
San Diego's lights began to return shortly before 11 p.m. Thursday, and Sempra Energy's San Diego Gas & Electric power company said it had restored power to all 1.4 million of its customers -- homes and businesses roughly equivalent to 3.5 million people -- by early on Friday.
Across the border in Baja California, where 3.5 million people lost electricity, power had been restored to 1.1 million customers by early morning, Mexico's Federal Electricity Commission said in a statement.
"Service is practically back to normal, with only some specialized industrial clients awaiting reconnection," it said.
Another 56,000 people in and around Yuma, Arizona, where the trouble started, had their power restored Thursday night.
The outage initially unnerved some residents cut off from the media days before the 10th anniversary of the September 11 attacks on the World Trade Center and Pentagon. But jitters gave way to a sense of calm, and even a festive atmosphere in some communities where residents held impromptu cookouts with neighbors on their front lawns.
Fire departments were kept busy rescuing people from stalled elevators, and traffic gridlocked as throngs of workers suddenly given the rest of the day off converged on streets without functioning stoplights.
As darkness fell, police increased their presence in the darkened communities of San Diego, the Mexican military patrolled government buildings in Tijuana, and the night passed without reports of civil disturbances or looting.
But lingering problems remained. Beaches were closed and contamination signs were posted along 12 miles of the coast near San Diego after two municipal sewage pumps failed, dumping nearly 2 million gallons of raw waste into Pensasquitos Lagoon, which empties into the ocean.
The pumps were later returned to full service, according to San Diego wastewater spokesman Arian Collins, who earlier put the spill volume at 3.2 million gallons.
Residents in a half-dozen areas were advised to boil tap water before drinking it because of the risk that reduced water pressure could lead to contamination, county emergency operations officials said.
San Diego International Airport, where all outbound flights were canceled Thursday evening, was back to full operations on Friday morning.
But passengers were advised to expect continued delays as airlines caught up with a backlog of stranded travelers, regional airport authority spokeswoman Scarlett Swanson said.
Restaurants and grocery stores were especially hard hit by the loss of refrigeration.
"We closed all our restaurants," said David Cohn, CEO of the Cohn Restaurant Group, which operates a chain of 14 upscale eateries. He estimated 5,000 meals were lost.
Utility officials urged customers to continue to use electricity sparingly for the time being.
The power-restoration process "has left our local power grid very fragile, and we are asking our customers to conserve electricity throughout the day Friday," David Geier, SDG&E vice president of electric operations, said in a release.
The outage was set off on Thursday afternoon when a worker for the Arizona Public Service Co. was changing out a piece of faulty equipment at a Yuma substation, said Damon Gross, a spokesman for the utility.
That procedure triggered the failure of a high-power line supplying electricity to Southern California before unleashing a domino effect, including shutdown of reactors at the San Onofre Nuclear Generating Station.
The station, the second-biggest source of power to the San Diego area, was expected to remain offline through Friday.
Gross said experts were puzzled at why a routine operation at the substation should have caused a region-wide blackout.
"There just should not have been any outage in Yuma, let alone California," Gross said. "There appears to be two failures here -- one is human failure and the other is a system failure. Both of those will be addressed."
The utility said could it take weeks, if not months, to pinpoint the exact cause of the blackout.
(Additional reporting by David Schwartz and Tim Gaynor in Arizona, Scott DiSavino in New York, Eileen O'Grady in Houston and Lizbeth Diaz in Tijuana; Writing by Steve Gorman; Editing by Greg McCune, Jerry Norton and Cynthia Johnston)