For customers in Maryland and neighboring areas, the system worked.
The most widespread blackout in the nation's history, which darkened parts of the Northeast and Midwest as well as Canada last evening, didn't spread much farther south thanks to circuit breakers in the multi-state power grid that carries electricity to consumers between New Jersey and Virginia.
The exact cause of the blackout remained unknown last night, although it was believed to have been triggered by a "massive disturbance" in northern New York or Ottawa, according to the head of PJM Interconnection. The PJM grid helps balance the flow of electricity in an area that includes Maryland, Delaware, New Jersey, Ohio, Pennsylvania, Virginia, West Virginia and Washington, D.C.
"This is the biggest thing since the '60s, a very big outage," stretching as far west as Cleveland, around the Great Lakes, said Phillip G. Harris, president and chief executive of PJM Interconnection, based outside Philadelphia. North America's oldest and largest power grid, begun in 1927, is named for three of the original members: Pennsylvania, New Jersey and Maryland.
Circuit breakers in the grid helped isolate the affected area, he said. Power was out temporarily in northern New Jersey and parts of Pennsylvania, but the system stabilized after dropping to 56,000 megawatts from 61,000 megawatts, Harris said.
Local utility companies serving Maryland said their customers experienced no power failures. Demand for power was high in Baltimore as the temperature reached 90 degrees, but never reached peak level, officials said.
"We continued to monitor the situation as the minutes and hours wore on, and our system remained stable," said Baltimore Gas & Electric Co. spokesman Robert L. Gould.
Similarly, at Allegheny Power Inc., which serves Western Maryland, no customers lost power, said Allen Staggers, a spokesman for the Hagerstown-based utility company, which serves 1.4 million customers in five states. "The problem seemed to be far enough away from our system that we didn't see any direct effect on customers. We were on alert, but we didn't have to do anything."
Robert A. Dobkin, a spokesman for Washington-based Potomac Electric Power Co., said demand reached a high of 6,042 megawatts yesterday, well below the record demand of 6,421 megawatts on July 29, 2002. Pepco, along with its Wilmington, Del.-based sister company, Conectiv, provides electricity for 1.8 million customers in Maryland, New Jersey, Delaware, the Virginia's Eastern Shore and Washington, D.C.
When a swath from New York to Toronto to Cleveland lost power after 4 p.m. yesterday, "there were some frequency changes gyrating on the lines, and the operators noticed that, but it had no noticeable effect," Dobkin said.
The University of Maryland's College Park campus lost power for about 20 minutes. The university, a customer of Pepco, uses its own generators to supply about half its power, but the generators automatically shut down because of disruptions on the power grid, Dobkin said.