While the actual blackout occurred at 4:11 p.m. Thursday, here are some key problems that occurred earlier that day:Excerpted from : http://www.ohiocitizen.org/campaigns/electric/2003/attention.htm2 p.m. - FirstEnergy’s Eastlake Unit 5, a 680-megawatt coal generation plant in Eastlake, Ohio, trips off. On a hot summer afternoon, "that wasn’t a unique event in and of itself," said Mr. DiNicola. "We had some transmission lines out of service and the Eastlake system tripped out of service, but we didn’t have any outages related to those events."
3:06 p.m. - FirstEnergy’s Chamberlain-Harding power transmission line, a 345-kilovolt power line in northeastern Ohio, trips. The company hasn’t reported a cause, but the outage put extra strain on FirstEnergy’s Hanna-Juniper line, the next to go dark.
3:32 p.m. - Extra power coursing through FirstEnergy’s Hanna-Juniper 345-kilovolt line heats the wires, causing them to sag into a tree and trip.
3:41 p.m. - An overload on First Energy’s Star-South Canton 345-kilovolt line trips a breaker at the Star switching station, where FirstEnergy’s grid interconnects with a neighboring grid owned by the American Electric Power Co. AEP’s Star station also is in northeastern Ohio.
3:46 p.m. - AEP’s 345-kilovolt Tidd-Canton Control transmission line also trips where it interconnects with FirstEnergy’s grid, at AEP’s connection station in Canton.
4:06 p.m. - FirstEnergy’s Sammis-Star 345-kilovolt line, also in northeast Ohio, trips, then reconnects.
4:08 p.m. - Utilities in Canada and the eastern United States see wild power swings. "It was a hopscotch event, not a big cascading domino effect," said Sean O’Leary, chief executive of Genscape, a company that monitors electric transmissions.
4:09 p.m. - The already lowered voltage coursing to customers of Cleveland Public Power, inside the city of Cleveland, plummets to zero. "It was like taking a light switch and turning it off," said Jim Majer, commissioner of Cleveland Public Power. "It was like a heart attack. It went straight down from 300 megawatts to zero."
4:10 to 4:25 p.m. - Power plants and high-voltage electric transmission lines in Ohio, Michigan, New York, New Jersey, and Ontario shut down.
In summary, this kinda 'came down' like a number of other large blackouts have come about - loss of a large generating plant or 'tie' lines into a relatively high-demand area, a cascading effect of 'overloaded lines' other lines/facilities that replace the lost line(s)/or generating capacity ...
As reference I would recommend the on-line resources to be found on the net that cover both the '65 North East Blackout and the '77 New York City blackout. In particular, the reports issued by the 'Federal Power Commission' at the time are particularly good at describing the events and causitive factors in the '65 and '77 blackouts.