Routine work on a substation apparently led to a power outage that left more than 1.4 million customers – about 5 million people – without power for 12 hours Sept. 8 and 9 in Southern California,
Arizona and Mexico.

According to the Arizona Public Service, the outage began about 3:30 p.m. PDT Thursday when a major 500-kV line from Arizona to Southern California tripped off. The outage forced the automatic shut down of the San Onofre Generating Station, a nuclear powerplant. The powerplant and power imported from Arizona on the 500-kV line are the region’s primary sources of power. Without that electricity, San Diego Gas and Electric, and other utilities in the region, did not have enough power to keep the lights on.

It was the largest outage in SDG&E history, but it was about one-tenth the size of the 2003 outage in the Northeast.

Power had been restored to most all customers in the Southwest by 3:30 a.m Friday, though NERC said 200 MW of the initial 4,300 MW had not been restored.

San Diego Gas and Electric and the California Independent System Operator asked customers to conserve energy as they spend the next few days ensuring system integrity.

“The restoration process, however, has left our local power grid very fragile and we are asking our customers to conserve electricity,” said David Geier, vice president of electric operations for SDG&E in a statement.

Arizona Public Service said the outage apparently occurred when one of its employees was doing some work on a substation on the 500-kV t North Gila – Hassayampa  transmission line. System protections that should have isolated the outage did not work, the utility said.

On Friday the Federal Energy Regulatory Commission and the North American Electric Reliability Corporation announced a joint inquiry into the incident. “This inquiry is an effective way for us to protect consumers and ensure the reliability of the bulk power system,” FERC Chairman Jon Wellinghoff said.