San Francisco Fire Department Lt. Mindy Talmadge said the incident was reported as an explosion in a manhole at Post Street and Grant Avenue just before 9:30 a.m.
Swanson said an electrical cable failure in an underground vault generated pressure that pushed the 300- to 350-pound manhole cover off the manhole.
PG&E has redundancy built into the electrical system in that area and was able to avoid power outages from the incident, Swanson said.
No injuries or property damage was reported, Talmadge said.
Traffic was initially diverted around the area, but as of about 11:45 a.m. pedestrian and vehicle traffic had returned to normal on Post Street. Grant Avenue, however, was still blocked between Post Street and Campton Place while crews continued to work to repair the cable, Swanson said.
Swanson said PG&E is in the process of installing new manhole covers in San Francisco that are designed to prevent events like this from happening in the future.
The new covers have a spring-loaded locking system that allows the cover to raise up and safely vent pressure from inside the vault but prevents the cover from coming off, Swanson said.
Swanson said the utility started replacing the manhole covers in 2010 and has so far replaced about 100 of the 1,000 covers they intend to replace.