At 11:43 a.m., part of the wire on the outbound 5-Fulton line was seen drooping for unknown reasons, causing complications for bus movement in the area of Third and Mission streets, Muni spokesman Paul Rose said.
One person suffered minor injuries from shrapnel at the scene, according to San Francisco Fire Department spokeswoman Mindy Talmadge.
The injured person refused aid, Rose said.
The 14-Mission line is the only affected route, with buses moving around the delayed 5-Fulton line, Rose said.
The investigation into what caused the downed wire is ongoing, he said.