A Fire Department dispatcher said firefighters responded after the cover at 11th and Howard blew into the air around 5 p.m. Wednesday.
Pacific Gas & Electric crews were called out, but PG&E spokeswoman Jana Morris said the equipment that caused the blast belonged to San Francisco Muni.
A Muni spokesman said the blast was caused by a problem with underground electrical equipment.
There were no injuries from the blast, but in a separate incident that took place at almost the same time, police said a pedestrian was hit by a car about two blocks away.
That person was taken to a hospital with possibly life-threatening injuries. Police said the two incidents appear to be unrelated.