Los Angeles- The power outage that affected more than 2 million people in and around Los Angeles on Monday was triggered by an unlikely source: A utility crew installing a system upgrade.

The Los Angeles Department of Water and Power workers cut several cables incorrectly, slicing the thin wires as a group, rather than one at a time, said Ed Miller, director of Power System Operations and Maintenance for the department. That triggered a short and tripped circuit breakers.

The surge in electricity to the remaining lines overloaded the system and caused a shutdown at the power receiving station where the DWP employees were working. The station converts high voltage power from area generating stations to low voltage power used by businesses and residents.

The systemwide alert prompted two generating stations and other receiving stations to shut down as well. As a result, the department began “shedding” customers, cutting power to people across the city to stabilize lines.

Cutting power quickly when these sorts of problems happen “is the correct thing to do,” said Jan Smutny-Jones, executive director of Independent Energy Producers, a Sacramento-based trade association that represents power plants.

“If you don’t do that, you end up with what happened in the Northeast,” he said, referring to the nation’s worst blackout in August 2003. “It starts jumping into other regions and control areas and then you have big problems.”

When the outage began, the workers were installing automated monitoring controls to provide department officials with immediate information about the status of its system, Miller said.

The upgrade will prevent such widespread outages in the future, but it won’t be completed for five more years, he said.

“If you can imagine you’re sitting in a control room with hundreds of alarms and flashing lights, when an event starts occurring,” Miller said. “It’s happening so fast, it’s hard to decipher what’s happening. Automated systems give you the information instantly.”

By around 2 p.m., the department had restored power to most customers by firing up quick-start generating stations in the Los Angeles Basin and increasing output at its Castaic hydro-generating plant north of Los Angeles. By late afternoon, about 40,000 out of 1.4 million customers remained without power.

“The real questions are how big is the effect and how quickly did they recover,” Smutny-Jones said. “They responded quickly, and it was limited in scope and duration.”