- 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 programming error or "bug" in the alarm system at a FirstEnergy Corporation control room in Ohio. The lack of an alarm left operators unaware of the need to re-distribute power after overloaded transmission lines hit unpruned foliage, triggering a "race condition" in the energy management system software, a bug affecting the order of operations in the system. What would have been a manageable local blackout cascaded into massive widespread distress on the electric grid.