Norwich ― A power outage that began shortly after 5:30 p.m. Friday left more than half the city’s electricity customers without power for more than three hours.

The outage was caused by equipment failure in the utility’s 10th Street substation, spokesman Chris Riley said.

“This is a very significant issue,” Riley said.

Riley said about 11,000 of the city’s 21,000 customers lost power in the outage. Police were directing traffic at major intersections as traffic lights were not working.

The company’s Facebook post said that “Once power is safely restored, we will begin a thorough investigation of what caused this outage to occur.”

Power was restored by 9 p.m.