On Aug. 14, when subways ground to a halt, elevators froze between floors and traffic lights went dark, it wasn't hard to tell that something had gone seriously wrong with the state's electrical grid.
But before that point, the public was blissfully unaware as grid operators grappled 66 times over the past seven years with other potentially debilitating power plant and transmission line breakdowns classified as "major emergencies."
A problem on the grid is deemed a "major emergency" when it has the potential to cascade and cause widespread blackouts.
But in each of the previous incidents, controllers at the New York Independent System Operator - the organization that controls the state's grid - reacted quickly enough to solve the problem before the lights went out.
Nineteen emergencies were caused by problems that originated outside the state and traveled here over the interconnected grid, according to reports compiled by the Independent Operator and the New York State Energy Planning Board.
Others began with an in-state power line or generator unexpectedly tripping out of service.
On Dec. 11, 2002, for example, a 345-kilovolt underground line carrying power from Westchester County to a substation in midtown Manhattan suddenly tripped offline at 10:28 p.m.
As the city continued to demand power from upstate, the flow jumped to a parallel line from Westchester that feeds into a substation in Queens.
The flow quickly surpassed that cable's safe operating limits and threatened to take it out of service.
The loss of the cable to Manhattan was suddenly threatening the stability of the statewide grid, and technicians working in the system operator's control room near Albany declared a major emergency.
The controllers immediately instructed Con Edison to ramp up all of the generators in the city to their full output and to draw an emergency infusion of electricity from three lines connected to a neighboring system in New Jersey.
With local sources helping to meet the city's demand, the draw on the Westchester-to-Queens line receded to safe levels, and at 10:38 p.m. - just 10 minutes after the initial line trip - the operators announced the emergency was over.
Meanwhile, residents of the city never knew anything had happened.
Experts say that if such a disturbance isn't quickly quashed, it can multiply through the interconnected grid, overwhelming controllers and the computer programs that guide them.
Instead of considering one or two breakdowns, operators suddenly have to consider 100 or 1,000 scenarios that might occur.
And the longer they take to respond, the worse a problem can get.
Ian Dobson, a professor of electrical engineering at the University of Wisconsin, says the escalating level of difficulty resembles a chess match, which has a finite number of opening moves but a vast range of strategies once the opening gambits are completed.
"When you get into the midgame, when things propagate, the combinations are just overwhelming," Dobson said. "You're looking at rare things and, by definition, they're unlikely and hard to anticipate. And there are just huge, huge numbers of combinations that quickly bring computers to their knees."
