SAN DIEGO -- A blackout that swept across parts of the Southwest and Mexico apparently began with a single utility worker and a minor repair job.
Federal investigators are trying to determine how the problem then rippled from that worker in the Arizona desert, to Southern California and across the border, plunging millions of people into darkness, especially since the power grid is built to withstand such mishaps.
Before power was restored Friday, Thursday's outage left more than 4 million people in the dark. Two reactors at a nuclear power plant went offline. More than 2 million gallons of sewage spilled into the water.
The outage was a reminder that the nation's transmission lines remain all too vulnerable to cascading power failures.
In 2003, a blackout knocked out power to 50 million people in the Midwest and the Northeast. And in 2005, a major outage struck the Los Angeles area. That same year, Congress required utilities to comply with federal standards for the electricity grid, instead of self-regulation. Layers of safeguards were intended to isolate problems and make sure the power keeps flowing.
But that didn't happen Thursday. The Arizona Public Service Co. worker was switching out a capacitor, which controls voltage levels, outside Yuma, Ariz. Then a section of a regional power line failed, eventually spreading the outage to California and Mexico, officials said.
There was a roughly 10-minute gap between the time the power line failed and customers lost electricity, said Daniel Froetscher, vice president of energy delivery for Phoenix-based APS. The line has been "solid, reliable" with no history of problems, he said.
San Diego Gas & Electric Co. had no time to isolate the problem by shutting down the line because it had no warning, said the company's president, Michael Niggli. He said automatic circuit-breakers at San Onofre nuclear power plant prevented the blackout from spreading to Southern California Edison, which serves 14 million people.