SAN DIEGO (AP) - A power outage that affected 7 million people last month in the Southwest U.S. and part of Mexico was not solely caused by a utility worker doing a minor repair job, as originally thought, utility officials said Wednesday.
Federal investigators, and officials from California and Arizona public utility companies said they still don't know exactly what happened the afternoon of Sept. 8, when power was knocked out for up to 12 hours from Arizona to southern California and into the northern part of Mexico's Baja California.
While the utility worker's repair of a transmission line at an Arizona substation was the first action in the chain of events, it should not have triggered a massive blackout because a complex system involving five electric grids is built to quickly compensate for such glitches and prevent the problem from spreading, utility officials said.
Investigators have discovered that at least 20 problems took place across the five grids within an 11-minute period, said Stephen Berberich, president and CEO of the California Independent System Operator, or ISO, which operates the state's wholesale power system.
"This blackout should not have happened," Berberich said Wednesday during an oversight hearing of the California State Assembly Committee on Utilities and Commerce, and the Joint Legislative Committee on Emergency Management held in San Diego.
Donald Robinson, president and chief operating officer of Arizona Public Service Co., said it was wrong to pin the blame solely on a utility worker at his company's substation.
"There are still many things we do not know that happened on those days," he said.
But the probe over the past two months has revealed one fact, Robinson said: "This event was not caused by the actions of a single utility worker. That is an unfortunate perception that came out early in the process. The system is built to withstand an event like that."
The utility companies involved, under ISO's direction, have formed a joint task force to investigate. Berberich said it could take up to a year before investigators find the exact cause.
The Federal Energy Regulatory Commission and the North American Electric Reliabilty Corp. also have opened a joint inquiry into the outage.
The outage knocked out traffic lights, causing gridlock on the roads in the San Diego area. Two reactors at a nuclear power plant up the California coast went offline after losing electricity. Nearly 3.5 million gallons of sewage spilled into the water off San Diego, closing beaches in the eighth-largest U.S. city.
The National University System Institute for Policy Research has estimated the outage cost the San Diego-area economy more than $100 million.
Many people, on both sides of the U.S-Mexico border, had to spend the night struggling to fall asleep in high temperatures, which reached up to 115 degrees in the desert areas.
There were no reported deaths or injuries related to the outage.