Editor’s note: This story by David Baker appeared June 16 in the San Francisco Chronicle.
A faulty power line from the 1920s caused the spectacular June 5 underground fire that shot flames 30 feet into the air above a manhole in San Francisco’s Tenderloin district, Pacific Gas and Electric Co. said Wednesday.
The old, direct-current line melted at a point where it had been spliced, and hot debris falling from the line triggered a cascade of problems that created the blaze. That’s the conclusion of a consulting firm hired by PG&E to find the cause of the fire, which knocked out power to 8,600 customers.
PG&E, which is based in San Francisco, won’t say that the power line’s great age caused it to fail. But the company last year started replacing all DC cables of the same vintage that lie beneath the city’s downtown.
“There are obviously DC cables that aren’t failing, so age alone may not be the factor,” said PG&E spokesman Jonathan Marshall. “The reason for the replacement program is that it’s our expert opinion that age will make failures more likely.”
All the old DC cables – which are clustered in the Tenderloin, South of Market and Union Square areas – are due to be replaced by late next year, he said.
The electrical vault at the intersection of Polk and O’Farrell streets was last inspected five weeks before the fire. The consulting firm – Exponent Inc. – concluded that inspectors were unlikely to find the problem, because the power line that failed was wrapped in a protective sheath, Marshall said.
After the line melted, flaming debris from it ignited another power line that was not protected by a sheath. The heat then melted the seals on a nearby electrical switch that contained oil, and the oil ignited. PG&E’s explanation didn’t pacify critics, who have complained for years about the utility’s service record. PG&E customers typically endure more-frequent and longer-lasting blackouts than do customers of California’s other big utilities.
“PG&E’s acknowledgement of their failed stewardship as a monopoly is a step forward,” said state Sen. Mark Leno, D-San Francisco.