A massive power outage that hit downtown San Francisco on Friday was caused by a fire in the Larkin Street substation. Equipment failure led to a circuit breaker igniting insulation, the San Francisco Business Times reports. About 88,000 residents and businesses were without power during the day. Power was restored to all customers by 5 p.m. No injuries were reported.

The outage caused a brief closure of the Montgomery BART station and traffic jams throughout downtown. It also shut down parts of Union Square and the Financial District. Muni bus lines that run on overhead electrical lines were down as well. The outage shut down 19 schools and several buildings, including the Phillip Burton Federal Building and the U.S. Courthouse, the San Francisco Chronicle reports.

PG&E spokesperson Tamar Sarkissian told the San Francisco Business Times the Larkin substation will be modernized by early 2018. The utility company is replacing and upgrading infrastructure in the substation to increase reliability and capacity.