MELISSA BLOCK, Host:
The lights are back on for more than 4 million people in Southern California, parts of Arizona and Mexico. A massive blackout hit the region yesterday and power was out until this morning. The incident shines a bright light on some of the vulnerabilities of the nation's power grid. Here's Alison St. John of member station KPBS in San Diego.
ALISON ST JOHN: Art Arvizu was working at his computer in San Diego City Hall. He says the first few moments were tense.
ART ARVIZU: I think people were nervous because the date is close to September 11th, so I kind of braced myself. I thought there would be an explosion or something.
ST JOHN: Arvizu's building is just one of many high-rise office towers in San Diego. Traffic on the freeways snarled as everyone tried to get home to their families. Hospitals had to switch to backup power. At the Palomar Pomerado hospital's emergency room, nurse manager Michelle Gunnett reassured people in the waiting room.
MICHELLE GUNNETT: It will be dark, and it will be hot because we don't have air-conditioning, OK? So just to let you know what's going on, we're still seeing patients. We'll treat you as quickly as we can, and we have plenty of help to take care of things right now.
ST JOHN: Yesterday was one of the hottest and most humid days of the summer. Chief operating officer of San Diego Gas & Electric Mike Niggli later told reporters he had no warning anything was wrong until his own computer screen went dark. He says the Arizona utility that controls a 500-kilovolt transmission line traced the problem to a substation near Yuma, Arizona, which went out at 3:27 p.m.
MIKE NIGGLI: They tried to re-establish it at about 3:37, about 10 minutes later. And then, you still had the reverberations because it did not come back in. It did not reconnect to the system. And it had a major short circuit on it as well. So it caused fluctuations in the system.
ST JOHN: It appears the whole blackout was triggered by a technician in the Arizona power system who inadvertently tripped a relay in a substation. James Avery is senior vice president of power supply for San Diego Gas & Electric. He says that tripped relay interrupted a transmission line.
JAMES AVERY: This was not a situation where there was not adequate resources within the south. The problem is there was inadequate transmission to get those resources to San Diego.
ST JOHN: Avery says demand for electricity throughout the desert cascaded over into San Diego's transmission lines, which shut down the San Onofre nuclear power plant. The domino effect left the whole region in the dark. Murray Jennex is an electrical engineer at San Diego State University who worked with the power company to help plan for Y2K. He says even back then, people knew more safeguards against blackouts might be needed.
MURRAY JENNEX: Because back in the '90s, we changed our approach to reliability, and a lot of the redundancy that was built into the system through multiple lines was removed and put on a single pole, into a single system.
ST JOHN: Jennex says that means the system now is not as reliable as it was. He says there are two reasons the transmission system was changed: first to reduce costs and second because there is so much public opposition to erecting new power lines. San Diego Gas & Electric says it's reviewing contingency plans to avoid this happening again. California's statewide power regulator is investigating and so is the National Electric Reliability Council, because that lack of redundancy that caused yesterday's blackout is a problem throughout the United States. For NPR News, I'm Alison St. John in San Diego. Transcript provided by NPR, Copyright NPR.