The Los Angeles Department of Water and Power, the city owned utility, said the outage occurred while its employees were cutting wires at an electrical transmitting station in Toluca Lake while installing a new electrical monitoring system.

The mistake rippled through the electrical grid, threatening to overload another transmission station and two electrical generating plants -- the Scattergood generating station south of Los Angeles International Airport and the Haynes generating station near Long Beach.

DWP shut down the generating facilities to avoid damage, sharply reducing the amount of power available for the city. That caused blackouts in neighborhoods scattered across the city, with heavy concentrations in parts of the San Fernando Valley, south Los Angeles and the downtown area. Large parts of Burbank and Glendale, whose electrical systems are linked to DWP's, also lost power ...

The new, automated system workers were installing was meant to detect surges or drops in voltage, said Ed Miller, the DWP's director of power systems, operations and maintenance.

"They cut a bundle of wires," Miller said. "The supposition is that by cutting them together they created a short that triggered the circuit breakers." Miller said cutting wires one by one might have avoided an electrical short.