A utility company spokesman says the San Francisco power outage that left thousands in the dark was caused by a "catastrophic failure" of a circuit breaker that caused a fire at a power substation.

Pacific Gas & Electric spokesman Barry Anderson says power has been restored to about 23,000 of the 90,000 customers that previously lost power in a massive swath of the central city.

He says soot from the debris of the fire swept across the area and took out more customers.

In a statement posted on its website, PG&E says it expects to restore power to about 25,000 more customers "in the next few hours" and to remaining customers late in the afternoon.