Officials said they could not explain the cause of the failure, but it spread quickly through a cross-border power grid that has become as intertwined as the World Wide Web.
Conflicting early reports suggested the problem might have started at a power plant in New York State, and spread from there. Ontario imports power directly from New York State through a major power connection between grids on both sides of the border.
New York State Governor George Pataki yesterday blamed a system failure for the widespread outage. He told reporters that to his knowledge, the outage started in New York City and spread, creating a cascading power blackout.
"We had this happen back in the sixties. It wasn't supposed to happen again, and it did happen again."
Nonetheless, he said the system had worked in the sense that power had tripped out without damaging hydroelectric plants.
Critics have been calling for breakers to be installed throughout North America's grids for some time, only to be told by the grid operators that there is no money to fund such a massive infrastructure overhaul, an energy analyst said.
William Lacey, who covers the power industry for First Energy in Calgary, said he wasn't surprised by the widespread outage because North American energy markets have become so closely linked.
"That's one of the quandaries [observers] have had, that they're all connected and when one goes down, you have this whole cascading effect that runs through the entire grid," Mr. Lacey said.
Paul Choudhury, manager of system control at B.C. Transmission Corp., which manages the transmission system for the province, said if there is an outage on a grid system, a transmission line can be disconnected from the network. But the remaining lines have to carry the same amount of power, and they can overload.
He said there is equipment in place that monitors the amount of electricity going through the lines. If they get overloaded, they're taken out of service to protect the remaining transmission lines so they don't burn down. This can repeat itself, and the result is a cascading shutdown.
"One element at a time will become overloaded and has to be taken out of service, and it just cascades, and finally you've got no connection between your generator and your city," he said.
It can take three to eight hours to restore power, Mr. Choudhury said, because the lines have to be restored one piece at a time in order to make sure the equipment is safe to operate. And they have to arrange for power to be delivered to them.
Yesterday's failure was not the first of its kind. On Aug. 11, 1996 -- almost exactly seven years ago -- a squirrel found its way into a grid on the U.S. West Coast and knocked out power in nine states and parts of Mexico, demonstrating the weaknesses in the system.
In 1965, a power outage involving Ontario and New York was caused by a line tripping out in Canada on the border between Canada and the United States, Mr. Choudhury said.
"This is very serious because these kind of events are supposed to be looked at and planned for, and they've designed the power system to be able to withstand the loss of any one element," he said.
Energy specialist Gordon Laird, who wrote the book Power: Journeys Across an Energy Nation, said yesterday he was not shocked by the widespread system failures, nor that the crisis happened at the peak of summer demand.
He said the higher degree of interdependence and the "criss-crossing" of power lines in North America has left the system "brittle".
