A major power outage Tuesday in the San Francisco Bay area prevented several banks from opening and forced others to revert to primitive business techniques.

Seven branches owned by Sanwa Bank California were without power for several hours, leaving tellers unable to use computers.

"We had to do banking the old-fashioned way, with a pen and a piece of paper," said Keith Karpe, spokesman for Los Angeles-based Sanwa.

The outage, which occurred at about 8:20 a.m. Pacific Standard Time, was reportedly caused by a power surge at a San Mateo substation, 20 miles south of the city. Power was restored Tuesday afternoon.

Primarily affected was a 49-square-mile area of San Francisco, but counties stretching from Napa to San Mateo also were affected. The blackout cut off power to 375,000 people, according to a Pacific Gas & Electric spokesman.

In San Francisco's financial district, stranded workers gathered on street corners, unable to take elevators to their offices or public transportation home. Some were trapped in elevators for hours. However, several downtown buildings shifted to power from emergency generators.

About 200 BankAmerica Corp. automated teller machines were down during the morning, and some Bay Area branches were operating in a "limited fashion," according to spokesman John Keane. BankAmerica's data center and trading rooms were running on emergency power, he added.

"All in all we're dealing with the situation well," Mr. Keane said.

About 113 of Wells Fargo & Co.'s 240 branches in the area were without power, and an undetermined number of ATMs were out of service, bank spokesman Larry Haeg said. The company's data center and Web site remained fully operable, he said.

Jan Schlesinger, senior vice president of marketing at Bay View Capital Corp. in San Mateo, said the thrift's 13 branches and its ATMs in San Francisco did not open Tuesday.

A spokesman for Washington Mutual Inc. said the Seattle-based thrift decided not to open its 25 branches in the affected area until power was restored.

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