A joint task force is investigating the causes of a blackout last week that left 3.5 million people in Southern California, Arizona and Mexico without power for 12 hours.

The California Independent System Operator Corp. (ISO) on Friday afternoon launched the task force, which will include utilities Arizona Public Service, San Diego Gas & Electric (SDG&E), Southern California Edison, Imperial Irrigation District and Comisión Federal de Electricidad, which serves customers in northern Baja.

"Our first priority was to help restore electricity to customers in the San Diego area who were hard hit by this devastating blackout and we appreciate all the entities that teamed up to perform Herculean efforts overnight," ISO President and CEO Steve Berberich said. "We now turn our focus to root-cause analysis to investigate the reason for the series of events that triggered the widespread power outage."

The investigation will probe, among other factors, why the outage that started in Arizona spread to Southern California and Mexico. Devices in the electrical grid are supposed to stop blackouts from cascading, experts said, but those safety mechanisms did not function as they were supposed to last week.

"There's equipment in the system to isolate [an outage] so that it doesn't become wider-spread," said Arizona Public Service (APS) spokesman Damon Gross. "We don't know why that didn't work and where that didn't work."

A utility worker in Arizona inadvertently triggered the outage on Thursday. The Arizona Public Service (APS) employee, at a substation northeast of Yuma, was working on equipment that regulates voltage. That maintenance somehow tripped a 500,000-volt line connecting areas northeast of Yuma to those west of Phoenix. The line shut down and then knocked out power across parts of Arizona, Southern California and Mexico (Greenwire, Sept. 9).

One of the mysteries is the timing of the blackout, Gross said today.

The 500,000-volt line tripped offline at 3:27 p.m. local time, Gross said. No one lost electricity at that time, he said. Then, about 10 minutes later, power went out in part of Yuma, and then the blackout cascaded into San Diego and Mexico.

"That's puzzling to us," Gross said. "Something happened in between there. What we need to determine still is what kind of relation the work in the substation had to this outage and why it became so big."

When electricity demand surged in San Diego, that trigged a "high current trip" at Southern California Edison's San Onofre nuclear power plant, located about 50 miles north of downtown San Diego. That safety feature within the switchyard caused the plant to detach from SDG&E's transmission lines, according to the Nuclear Regulatory Commission.

The ISO would not say whether San Onofre's disconnecting from SDG&E stopped the blackout from cascading farther north in California. Southern California Edison and SDG&E referred questions to the ISO.

One reactor at San Onofre was brought back online yesterday, and the company is "very close" to reconnecting the plant's second unit, said Gil Alexander, a Southern California Edison spokesman. Throughout the outage, no customers within the plant's service area -- 15 counties stretching from Orange County south to central California -- were without service, he said.

The Federal Energy Regulatory Commission (FERC) and the North American Electric Reliability Corp. (NERC) also are investigating the blackout.

Reporter Hannah Northey contributed.