A city worker told firefighters a two-story fireball rose from a manhole cover in Pacific Heights just after 12:30 in the afternoon.
"I was just at home hanging out watching TV and there was a really loud kind of boom. I thought they had just dropped something because they're doing construction up here, but then the TV and all the lights went out," recalled resident Risa Stein.
"Initially 6,600 of our customers were affected, but within five minutes we were able to restore about 2,100 through switches, automatic switches," explained PG&E spokesman Joe Molica.
PG&E believes another switch inside of the manhole caused the fire but the utility will not know the exact nature of the problem until forensic investigators examine the damaged parts.
A medical building at Saint Francis Memorial Hospital switched to an emergency generator for a short time, but the power never went out in the main hospital.