Warning signs in the Midwest didn't come early enough to stop the nation's largest blackout from spreading, mid-Atlantic grid operators said Thursday, noting they got the first trouble call about 40 minutes before the big crash.

"Nobody saw this coming," said Michael Kormos, vice president of operations for PJM Interconnection (search), the mid-Atlantic regional power grid operator.

The call to PJM Interconnection came at about 3:30 p.m. EDT on Aug. 14 from American Electric Power Co. (search) officials, who notified the grid operator of a transmission line overload in Ohio, Kormos said, speaking at a PJM market committee meeting.

But officials were unable to pinpoint the cause of the trouble before blackouts began rolling across the Northeast, Kormos said, adding that before the line overloaded, PJM was experiencing a routine day, with no abnormal demands and plenty of reserve capacity.

"We were fat, dumb and happy, for lack of a better word," said Kormos, who briefed PJM stakeholders in a public meeting on the Aug. 14 blackout events.

A joint U.S.-Canada task force is investigating the blackout, which cut power to about 50 million people in eight Midwest and Northeast states and the province of Ontario, Canada, with electricity returning to most customers within a day or so.

No conclusions have been reached, but investigators are focusing on the failure of a power plant shutdown two hours before the blackout and transmission line overloads in Ohio on systems owned or partly owned by AEP and FirstEnergy Corp. (search), of Akron, Ohio.

After learning of the AEP line overload at 3:30 p.m., PJM officials began a series of conversations with AEP, First Energy and the Midwest Independent Transmission System Operator, the power grid operator in the Mideast and PJM's counterpart, Kormos said.

At the same time, PJM officials were trying to determine whether the overload warranted a transmission line load relief — known as a TLR — in which adjustments are made across a grid to avoid further potential problems.

Such requests are a daily occurrence, but sometimes can be based on faulty data, Kormos said, and a check showed no unusual occurrences on PJM's own grid system.

While PJM officials were trying to determine whether to curtail transactions on the power grid, Kormos said the AEP line tripped — or cut off from the grid.

Then, at about 3:45 p.m., PJM learned from the Midwest operating group that another line had been lost in First Energy territory, setting off a new round of troubleshooting.

"As the calls were being made, the other lines started to trip out," Kormos said. "At that time, everybody was just trying to assess what was happening. Things were happening too rapidly to even have conversations.

"I don't see any immediate actions PJM could have taken," Kormos concluded.

The first indication of problems within PJM's own grid system occurred about 4:10 p.m., when a 500 megawatt generating unit went down, he said. Less than a minute later, the first line in the PJM grid tripped — and the blackout peaked across the entire region.

When it was all over, PJM had lost some 4,500 megawatts across its grid region, about 7 percent of the 61,200 megawatt load at the time.

The bulk of the PJM outage affected northern New Jersey, where about 1 million people lost power, with significant outages also around Erie in northwestern Pennsylvania.

PJM officials said it was too early to tell whether the outages on their grid were attributable to equipment failure, or whether the grid simply is not designed to handle the dramatic fluctuations it experienced.

"There's going to be a lot of investigative work as to where it broke, why did it break there as opposed to someplace else," Kormos said. "Either there were missed operations, ... or it was something that was never designed for."

PJM operates a power grid serving 25 million people in parts or all of Washington D.C., and more than half a dozen mid-Atlantic states, including Delaware, Maryland, New Jersey, Ohio, Pennsylvania, Virginia and West Virginia.

PJM is also the Federal Energy Regulatory Commission's designated reliability coordinator for AEP, which is seeking to join the PJM grid system.