Previously posted at: https://www.history.com/this-day-in-history/the-great-northeast-blackout
At dusk, the biggest power failure in U.S. history occurs as all of New York state, portions of seven neighboring states, and parts of eastern Canada are plunged into darkness. The Great Northeast Blackout began at the height of rush hour, delaying millions of commuters, trapping 800,000 people in New York’s subways, and stranding thousands more in office buildings, elevators, and trains. Ten thousand National Guardsmen and 5,000 off-duty policemen were called into service to prevent looting.
The blackout was caused by the tripping of a 230-kilovolt transmission line near Ontario, Canada, at 5:16 p.m., which caused several other heavily loaded lines also to fail. This precipitated a surge of power that overwhelmed the transmission lines in western New York, causing a “cascading” tripping of additional lines, resulting in the eventual breakup of the entire Northeastern transmission network. All together, 30 million people in eight U.S. states and the Canadian provinces of Ontario and Quebec were affected by the blackout. During the night, power was gradually restored to the blacked-out areas, and by morning power had been restored throughout the Northeast.
On August 14, 2003 another major blackout occurred which affected most of Eastern Canada as well as most of the Eastern United States.