SAN JUAN CAPISTRANO (CBS) — San Diego Gas & Electric says it has restored power to 1.4 million customers affected by a massive 12-hour outage that stretched from Arizona to Mexico.

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“Restoring power in the aftermath of the loss of the entire local grid serving San Diego and southern Orange counties was a monumental task and the ISO (California Independent System Operator Corporation), the region’s power plant managers and our employees really rose to the challenge,” said David Geier, vice president of electric operations.  “The restoration process, however, has left our local power grid very fragile and we are asking our customers to conserve electricity throughout the day Friday. ”

Thursday’s blackout affected nearly six million people in seven cities across southern Orange County, including San Clemente and San Juan Capistrano.

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The Coachella Valley, San Diego County, parts of Arizona, and even Baja California were also impacted.

The outage was apparently triggered by a single technician in Yuma, Ariz., who was working on a monitoring device at a small power substation, according to officials at Arizona Public Service Co.

Businesses were forced to close early Thursday and traffic was backed up throughout San Diego and Orange counties. Flights to and from San Diego International Airport were also canceled due to the outage.

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Several Orange County and San Diego schools will not be in session Friday. It’s unknown exactly how many customers are still without power.