Sporadic power failures Tuesday afternoon darkened a broad swath of downtown San Francisco, an area dotted with internet companies whose servers rely on a steady supply of electricity.

About 51,000 homes and businesses in San Francisco and south of the city lacked electricity at the height of the failure, which started at about 2 p.m. and lasted a few hours, said Pacific Gas & Electric spokeswoman Darlene Chiu.

The utility rerouted customers to backup circuits as crews worked to identify the failure's cause, Chiu said. It was eventually traced to several power surges the system experienced as PG&E tried to keep electricity flowing through a substation where transmission line breakers had failed, she said.

"Every single time we tried to restore service, multiple times, then it impacts the system and the outages occur," Chiu said.

Power was restored throughout the city by 5 p.m., she said.

Six Apart Inc., a blog-hosting service, said its sites began failing shortly before 2 p.m., and the company sent an e-mail to customers blaming the city's "power issues." The company reported that power resumed shortly afterward, but that employees were checking data and computers.

Several other internet sites with offices in San Francisco had problems Tuesday afternoon, including Technorati, Yelp, Red Envelope and CNet. It was unclear whether the problems were related to the outage.

San Francisco-based Linden Labs, which operates the popular virtual world Second Life, blamed the failure for sapping power from its data centre.

San Francisco-based classified site Craigslist.org was down Tuesday afternoon, but founder Craig Newmark said he didn't know why.

Although the failure didn't last long, experts say e-commerce companies can lose big money during even a minor blip. Estimates vary widely, but experts say high-profile dot-coms may lose anywhere from $1 million per hour to $1 million per minute when the power goes off.

Power failures that disable websites mean that customers can't place online orders, and that highly paid technology workers can't be productive. A blowout at Seattle-based Amazon.com during Thanksgiving weekend in November 2000 included one 20-minute blip that deleted about 20,000 product orders and $500,000 in revenue, according to investment firm Thomas Weisel Partners.

AT&T Park, home of the Giants, was also affected hours before a scheduled night game. The city's famed cable cars were out of commission for a time.

Bay Area Rapid Transit trains were operating normally, though one downtown station was running on backup batteries, a BART spokesman said. The San Francisco Police Department said it had not received additional calls related to the problem.

Gregg Fishman, a spokesman for California's Independent System Operator, which manages the state's power grid, said there had been no reports of widespread failures outside San Francisco.

"There's nothing that we know of on our system that would affect San Francisco," Fishman said.