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 utility's 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.