The Northeast blackout of 2003 was a widespread power outage that occurred throughout parts of the Northeastern and Midwestern United States and the Canadian province of Ontario on Thursday, August 14, 2003, just after 4:10 p.m. EDT.
Some power was restored by 11 p.m. Many others did not get their power back until two days later. In more remote areas it took nearly a week to restore power. At the time, it was the world's second most widespread blackout in history, after the 1999 Southern Brazil blackout. The outage, which was much more widespread than the Northeast Blackout of 1965, affected an estimated 10 million people in Ontario and 45 million people in eight U.S. states.
The blackout's primary cause was a software bug in the alarm system at a control room of the FirstEnergy Corporation, located in Ohio. A lack of alarm left operators unaware of the need to re-distribute power after overloaded transmission lines hit unpruned foliage, which triggered a race condition in the control software. What would have been a manageable local blackout cascaded into massive widespread distress on the electric grid.