PG&E spokesman Jason King said a crew is investigating the cause of the cable failure and said there are no associated service outages.
The fire occurred at about 2:45 p.m. in an underground vault in the area of Clay and Kearny streets, and firefighters and police officers responded to the scene.
Police had shut down streets in the area, but the streets have since reopened.
Kearny Street was closed between Clay and California streets; Clay Street was closed between Grant and Kearny streets; and the immediate area around Portsmouth Square was also closed.
