Read post 3080, why it stopped at Valley Forge. It's an excellent article.
http://www.freerepublic.com/focus/f-news/964250/posts?page=3080#3080 Safety steps stopped spread of outage
Technicians in Valley Forge saw the sudden power surge. Circuit breakers tripped. And within four minutes, the electricity grid that serves Pennsylvania and New Jersey had clamped off the spike that blacked out much of the Northeast today, shielding Philadelphia and points south from the disruption.
As a result, the Mid-Atlantic grid, operated by Valley Forge-based PJM Interconnection L.L.C., experienced only a few spillover blackouts in sections of northern Pennsylvania and northern New Jersey.
It was the kind of moment PJM's technicians practice for, company president Phillip G. Harris said: "Coordination is rehearsed and drilled several times a year."
Heard that on local TV .