California's worst power failure started in the Arizona deserts

Replacing faulty equipment at an energy substation triggered events that led to power shortage

The most extensive power outage in California history started in the western Arizona desert.

The replacement of faulty equipment at a power substation apparently triggered a series of events that knocked out Southwest Powerlink, one of two major transmission links that connect the San Diego area to the electrical grid for the western United States.

That, in turn, sent a jolt through the local grid that cut off power at San Onofre Nuclear Generating Station and tripped the other transmission link, this one leading from San Diego north to the Southern California Edison service area.

The result: a full system blackout for 5 million people across San Diego County, southern Orange County, western Arizona and northern Baja California.

[Graphic: How the blackout started; How a power grid works]

The outage exposed weakness in the electricity grid that supplies San Diego County with its power.

The grid is part of a web of powerlines stretching from Canada to northern Baja California, connecting electricity plants with their customers. Because of San Diego’s position in a geographical cul de sac, it is connected to the grid only through two major energy lines: a northern line connected to the San Onofre nuclear plants and an eastern line connected to power plants in Imperial County, Arizona, and northern Mexico.

The problem apparently began at an Arizona substation that is a major delivery point of electricity between power plants in Arizona and San Diego.

Just before 3:30 p.m., a worker at the substation replaced monitoring equipment that had been causing trouble earlier in the day. That created a short-term power outage for about 56,000 customers in Yuma and western Arizona, said APS, Arizona’s largest electric utility, which runs the substation.

Ten minutes later, workers at the substation unsuccessfully tried to restore power to the region, shorting the circuits. That led to a disruption in the electricity lines across Imperial County to San Diego — one of the county’s two major sources of electricity.

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