While part of the Northeast plunged into darkness, lights remained on for millions more -- a victory for adjoining multistate power grids (search) that managed to beat back the cascading failure.
It's not exactly clear what the difference was between systems that stayed on and those that failed in the crucial seconds Thursday afternoon, said officials from PJM Interconnection (search), the company that operates a multistate grid serving 25 million people in parts or all of six mid-Atlantic states and Washington, D.C.
The grid's automatic circuit breakers shut off connections to Great Lakes-area grids as they began blacking out.
Robert Hinkel, an electrical engineer and operations manager at PJM, said that while each grid operator has similar criteria for protecting power lines, each may have different protocols for its automated systems to respond to individual events.
In PJM's suburban Philadelphia headquarters, engineers watched Thursday as electricity drained rapidly to the north and the grid lost 7 percent of its power.
Several power plants (search) in northern New Jersey shut down to protect equipment there against power surges. At about the same time, circuit breakers on up to eight high-voltage transmission lines carrying power to New York from Pennsylvania and New Jersey tripped off to prevent the cables from overloading, Hinkel said.
Effectively, breaking the transmission lines saved PJM's grid before it, too, became unstable and shut down. In PJM's grid, 1 million PSE&G customers in northern New Jersey and an additional 100,000 largely in northwest Pennsylvania lost power, the inevitable result of cutting off the transmission lines to save the rest of the grid.
"Physically speaking, it was impossible to save them," said Chika Nwankpa, director of Drexel University's Center for Electric Power Engineering (search) in Philadelphia. "Because the distance was so close, they were dragged in."
The situation was similar in New England, where circuit breakers severed grid connections with New York and Canada, shutting off power lines and substations. That left some border areas from Connecticut to Vermont dark, according to the grid operator for six states there.
The major trouble spot was southwestern Connecticut where 300,000 customers lost power Thursday and the failure of a transmission line carrying power into the region on Friday morning caused additional problems.
At two major hydroelectric power stations in New York, power stayed on, probably because those plants were designed to act as shock absorbers and handle power surges, said Joanne Willmott, a spokeswoman for the New York Power Authority.
The result was that parts of Buffalo and surrounding areas had power, while much of New York went black.
The nation's largest electricity generator, American Electric Power, based in Columbus, Ohio, said its automatic shut-off system kept its lines from failing as well, ensuring power for 5 million people in 11 states from Texas to West Virginia.
Elsewhere, from Michigan to New Jersey to the Canadian province of Ontario, an estimated 50 million people lost power. As investigators looked at power lines south of Cleveland on Sunday where the original failure may have happened, they also indicated that human error could have played a part in allowing the blackouts to roll from grid to grid.
The interconnected grids east of the Rocky Mountains are designed to protect themselves from cascading outages and equipment damage by separating themselves from areas where voltage reductions or surges occur, according to the North American Electric Reliability Council, a nonprofit industry group that is investigating the blackout.
"Ultimately this is what happened," spokeswoman Ellen Vancko of the North American Electric Reliability Council said. "From that standpoint, you could say that it worked as it was designed to."