About 2,400 customers lost power in the area of Second and Bryant streets shortly after 2 p.m., according to PG&E.

Police and firefighters initially said the outage was caused by a transformer failure, but PG&E spokeswoman Tamar Sarkissian later clarified that it was caused by clamps that failed. She said the clamps hold overhead wires to an insulator.

"It wasn't an explosion," Sarkissian said. "These failures can sometimes cause a noise."

At about 2:10 p.m., an ambulance was sent to 275 Brannan St., where the noise was heard and smoke was seen, but no injuries were reported, fire spokesman Deputy Chief Pat Gardner said.

Most customers had their power restored shortly after 3 p.m., Sarkissian said.

No estimated time was available for when the 30 remaining customers' power would be restored.