If you thought that the nation's electrical grid was designed to prevent a single, localized malfunction from triggering a blackout for millions of people, you'd be right.
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Workers serve pizza to customers outside of Filippi's Pizza after a massive blackout hit Southern California September 8, 2011 in San Diego, California.
Sandy Huffaker
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Downtown is dark after a massive blackout hit Southern California September 8, 2011 in San Diego, California.
Sandy Huffaker
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People waited out traffic during the blackout at Downtown Johnny Brown's on Sept. 8, 2011.
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Traffic in downtown San Diego around rush hour during the outage on Sept. 8, 2011.
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Power outages throughout San Diego County as of 9 p.m. Thursday.
KPBS
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SDFD work on getting the woman out of the elevator on Sept. 8, 2011.
Katie Orr
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Traffic backed up due to the blackout across San Diego County on Sept. 8, 2011.
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Traffic downtown after the blackout across San Diego County on Sept. 8, 2011.
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Trolleys around San Diego stopped when the power went out at 3:48 p.m. on Sept. 8, 2011. The MTS said service would be restored sometime Friday, and told passengers to check schedules for buses which would be operating normally.
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Flights were grounded at San Diego International Airport while some arriving flights were diverted to other airports during the blackout on Sept. 8, 2011.
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Downtown San Diego during the massive power outage on Sept. 8, 2011.
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As the power outage on Sept. 8, 2011 went into the night, residents in San Diego relied on candlelight.
Leah S.
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A long exposure from the blackout on Sept. 8, 2011 by San Diego resident Andrew DaRe.
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Traffic on 1st Ave. in downtown San Diego right after the blackout at about 4 p.m. on Sept. 8, 2011.
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Cars were all leaving downtown during the blackout.
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Trolley service was stopped when the power went out on Sept. 8, 2011. Here, a trolley near San Diego State University is stopped on the tracks.
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A Vons grocery store in La Mesa closes because of the power outage. Employees were sent home and grocery carts were lined up to keep people out.
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Claudia Bateman is a resident of The Springs in La Mesa, a low income housing facility for seniors. Air conditioning went out during the blackout, and a generator powered auxiliary lighting. Bateman says she planned to spend the evening sitting by the fire entrance knitting.
Angela Carone
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The view of a darkened East County from the top of Mount Helix.
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Shoppers pick up supplies at a 99-cent store during the blackout on Sept. 8, 2011.
Digitaria
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The night sky from University Heights during the blackout on Sept. 8, 2011.
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Neighbors and friends took advantage of the blackout to grill food and relax under the stars. Here, neighbors gather in University Heights on Sept. 8, 2011.
Andy Trimlett
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Mike Niggli, the president and chief operating officer of SDG&E, speaks at a press conference during the blackout on Sept. 8, 2011.
Erik Anderson
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Mayor Sanders speaks at a press conference on Sept. 8, 2011 during the blackout that left all of San Diego County, parts of Orange and Imperial Counties, areas in Arizona, and northwest Mexico without power.
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A family sits out on the porch during the San Diego blackout on Sept. 8, 2011.
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Almost the entire city was dark during the blackout on Sept. 8, 2011.
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Normal Heights during the San Diego blackout on Sept. 8, 2011.
Nathan Gibbs
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Jayne's Gastropub in San Diego is lit by candlelight during the blackout on Sept. 8, 2011.
Nathan Gibbs
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Cyclists on the road during the blackout in San Diego on Sept. 8, 2011.
Nathan Gibbs
But that didn't prevent that exact event from happening Thursday in San Diego, parts of Arizona, and Mexico's Baja peninsula. Phoenix-based Arizona Public Service Co. said the blackout started when a piece of monitoring equipment was removed at a substation in Yuma, along the border with Mexico.
Andrew Philips, director of transmission and substations at the Electrical Power Research Institute, said "any one piece of equipment at any part of the system should not have caused this to happen."
To make matters worse, the power fluctuation caused two reactors at the San Onofre nuclear power station to shut down — cutting off even more of San Diego's power supply.
The Arizona utility declined to speculate on the exact chain of events, pending an investigation.
Mike Niggli, chief operating officer of San Diego Gas & Electric Co., said it's possible extreme heat in the region also may have caused some problems with the transmission lines.
Anjan Bose, a professor of electrical engineering at Washington State University, agrees that if the system was near capacity because of high usage, it could have contributed to the problem.
Bose said the main transmission lines from Arizona to Southern California might have been tripped by whatever happened at the substation, causing the power to reroute to lower-voltage lines that in turn overloaded and tripped.
"Whether it was human error or some malfunction of equipment, we don't know," Bose said. "Usually in these cases it is a bit of both."
Bose said the sort of outage that occurred Thursday is very unusual.
"When someone is fixing something in a substation, they would have to switch off certain things, and there's all sorts of procedures to that," he said. "So, it could have been human error or it could some piece of equipment malfunctioned."
A similar recent problem was caused by human error. In February 2008, about 1 million utility customers in Florida were cut off after a cascading effect with similarities to the Arizona outage. In a report, Florida Power & Light Co. concluded that a field engineer had disabled the systems designed to isolate problems to one piece of equipment or one location.