About 8,600 customers initially lost power around 11:30 a.m. Friday when something caught fire beneath a manhole at O'Farrell and Polk Streets .
The fire spit out thick black smoke as tall as seven stories for hours.
A 20-foot fireball swirled in the area for about 45 minutes.
The area did not cool down enough for crews to go inside until Saturday afternoon. The initial assessment did not shed any light on what might have sparked the blaze.
Fire Lt. Mindy Talmadge said the flames started because the equipment "wasn't fully de-energized, and that's something PG&E needs to answer."
Deputy fire Chief Pat Gardner had said the fire was close to an underground oil switch that was holding 75 gallons of oil, which is what caused the smoke to be so black.
The fire caused building alarms in the area to sound and the outage stopped elevators, trapping several people for a time.
San Francisco Mayor Gavin Newsom said he was glad noone was hurt. "We'll get to who's right, who's wrong, who should be explaining themselves, who shouldn't," Newsom said.
Streets in a one-block radius of the intersection were still closed as of 10 p.m. Saturday so investigators could continue to work.