nakile,
a properly designed grid would assure multiple redundancy and nominal load of no more than about 1/3 of the maximum capacity. in such a fail-safe design, when a link fails, power would get routed around the bad spot.

but the North American system isn’t really a power grid. it is an ad-hoc collection of separate companies, that trade power with each other. no one is in charge of assuring that nominal load never gets above 50%. so cascading failures can occur, like the northeastern blackout of 2003.