the Scientific American provided this information in August 2017:
August 14, 2003, was a typical warm day in the Midwest. But shortly after 2:00 P.M. several power lines in northern Ohio, sagging under the high current they were carrying, brushed against some overgrown trees and shut down. Such a disturbance usually sets off alarms in a local utilitys control room, where human operators work with controllers in neighboring regions to reroute power flows around the injury site.
On this day, however, the alarm software failed, leaving local operators unaware of the problem. Other controllers who were relaying, or wheeling, large amounts of power hundreds of miles across Ohio, Michigan, the northeastern U.S. and Ontario, Canada, were oblivious, too. Transmission lines surrounding the failure spot, already fully taxed, were forced to shoulder more than their safe quota of electricity.
To make matters worse, utilities were not generating enough reactive poweran attribute of the magnetic and electric fields that move current along a wire. Without sufficient reactive power to support the suddenly shifting flows, overburdened lines in Ohio cut out by 4:05 P.M. In response, a power plant shut down, destabilizing the systems equilibrium. More lines and more plants dropped out. The cascade continued, faster than operators could track with the decades-old monitoring equipment that dots most of the North American power grid, and certainly much faster than they could control. Within eight minutes 50 million people across eight states and two Canadian provinces had been blacked out. The event was the largest power loss in North American history...