Pacific Gas and Electric Co. today released information on the cause of an underground fire last month that knocked out power to a San Francisco neighborhood, filled the streets with smoke and sent flames shooting from a manhole.

A PG&E spokesman also said the utility has paid more than 250 individuals and businesses to settle claims related to the fire.

“We received 288 claims as a result of this,” PG&E spokesman David Eisenhauer said. “We’ve accelerated the processing of those claims.”

He said 254 claims have been paid but declined to release a total dollar amount, citing customer confidentiality.

Eisenhauer said an investigation into the fire by a third-party firm found the cause to have been the failure of a cable that dates back to the 1920s.

The fire started around 11:30 a.m. on June 5, when someone called 911 to report black smoke coming from a manhole at Polk and O’Farrell streets.

The blaze burned for hours, filling the neighborhood with smoke and disrupting power to thousands of PG&E customers as the flames burned through underground cables.

Firefighters were called to several buildings to help people trapped in elevators.

At the fire’s most dramatic point, flames shot 20 feet into the air from a manhole. Residents were told to shelter in place.

A fire spokeswoman said at the time no injuries were reported.

The firm hired by PG&E to investigate the fire, Menlo Park-based Exponent, found that a DC, or direct current, cable had failed, allowing a copper conductor inside the cable to burn through and start a small fire.

The fire grew and burned through other cables in the electrical vault, causing mineral oil from an oil switch to escape from its container and catch fire, Eisenhauer said.

It was the burning oil that sent black smoke billowing from the manhole, he said.

The cable that failed was part of a network of DC cables installed in the 1920s to power specialized electrical equipment, including elevators.

“Last year we started implementing a program to replace all of those DC cables throughout the city,” Eisenhauer said. “That is expected to be done by the end of 2010 … though we’re looking at trying to accelerate that.”

He said the overhaul of the DC cable network is part of a three-year, $200 million infrastructure upgrade project by PG&E that started in 2007.

The report by Exponent found that the electrical vault where the fire started had been inspected annually in the past few years, including five weeks before the June 5 fire.

The report states that it would have been hard for PG&E inspectors to detect any weakness in the cable.

“The cable failure that occurred would not be expected to have exhibited evidence of an imminent failure in the time leading up to its failure,” the report reads.

The fire initially knocked out power to 8,600 PG&E customers.

Streets around the intersection were shut down for days after the fire.

Please make sure your comment adheres to our comment policy. If it doesn't, it may be deleted. Repeat violations may cause us to revoke your commenting privileges. No one wants that!