Most power outages are accidents — a power line affected by a tree limb falling, a car crashing, a lightning bolt striking. But the “rolling blackouts” that Texans experienced Feb. 2, 2011, were no accident.

The problem wasn’t simply the record lows — 14 degrees in Dallas, with a high of only 21 — that caused heaters to work overtime. (More than two-thirds of Texans’ heat is powered by electricity, according to the state comptroller’s office.) The problem was that generators were failing.

“It was not only cold many days in a row, but the wind was blowing, and generators were getting into some freezing problems,” says Bill Muston of Oncor.

The freezing, which Muston compares to “a frozen pipe at your house,” forced coal and natural gas plants to go offline; wind turbines also were suffering from ice on their blades. All in all, 75 generators were out of commission that day, and ERCOT foresaw that Texans’ demand for energy would overreach the supply.

“If ERCOT has done all the generation it can and all-calls to neighborhoods to cut back, then neighborhoods go in the dark,” Muston says.

As a result, Dallasites experienced what Oncor describes as “the longest-lasting and farthest-reaching emergency load-shedding in Texas history.” For more than eight hours, as many as 275,000 Oncor customers at a time experienced rotating outages, each lasting about 15 minutes.

It was better than the alternative, Muston says — a system crash that would be unpredictable and would likely mean much longer blackouts.

“You don’t want to have a blackout like in the San Diego area a few years ago or in New York in 2003,” Muston says. “No one believes it, but it’s in your interest that they cut your power at your house for a few hours. What happened in that event was some really good planning that was executed well.”Rolling blackouts happen on purpose