Since well into the 20th century, when regional power grids in the U.S.'s lower 48 states were completed, Americans have come to expect an uninterrupted supply of electricity to power everything from factories to traffic lights to household appliances. But that has only made it all the more disconcerting when large parts of the system have broken down, which happens with surprising frequency. In the U.S. and Canada, there were 57 breakdowns each involving at least 30,000 customers between 1965 and 2009, according to a paper published by the Institute of Electrical and Electronics Engineers, a technical professional group. Here are some of the biggest blackouts in that period that were caused mostly by grid failure, rather than by natural disasters such as hurricanes.
Nov. 9, 1965: An errant setting on a protective device near Niagara Falls started a cascading outage that cut off power for nearly 30 million people in the U.S. and Canada. It took 13 hours to fully restore electricity.
July 13, 1977: A lightning strike caused transmission failures and shut down lines, causing the Indian Point No. 3 nuclear generating plant in New York State to go offline. A second lightning strike compounded the damage by knocking out two more important lines, causing a cascade of surges and overloads, which were compounded by human error. Nine million people in New York City lost power for up to 26 hours.
Dec. 22, 1982: High winds knocked a transmission tower into another tower in the West, setting off a dominoes-like effect that caused three other towers to fall as well, and those struck two other power lines. At that point, a cascading failure led to five million people in the western U.S. losing power.
July 2-3, 1996: A power line in Idaho overheated and sagged into a tree, tripping a protective device. Other relays tripped two Wyoming coal plants. Ultimately, two million people in the U.S., Canada and Mexico lost power for up to several hours.
Aug. 10, 1996: Several power lines in Oregon and Washington short-circuited when they brushed against trees, causing 13 hydroelectric turbines at the McNary Dam to fail. As a result, 7.5 million customers lost power in seven U.S. states, two Canadian provinces, and part of Mexico for a period that ranged from a few minutes to six hours.
July 25, 1998: A lighting strike in Minnesota caused a power line to fail, which in turn caused other lines to overload. When a second lightning bolt hit another power line, the cumulative result was a cascading failure that cut off the entire northern portion the Midwest from the Eastern grid. About 52,000 people in Midwestern states and Canada lost power for up to 19 hours.
Aug. 14, 2003: After four high voltage power lines in northern Ohio brushed against trees and failed, a computer error compounded the effects, resulting in a cascade of failures that left 50 million people without power across the U.S. and Canada for as long as two days. The biggest blackout in North American history caused billions in economic damage and contributed to at least 11 deaths.
Sept. 8, 2011: A technician’s mistake on some switching equipment led to an outage in California and Arizona that deprived 2.7 million people of electricity for up to 12 hours.