By mid-evening, though, conditions had worsened. More than 150,000 customers in New York City and Westchester County were already off grid. The utility began turning off the power, as a precaution, to a section of lower Manhattan, including Wall Street, in an attempt to stem damage. Shortly afterward, the company began cutting electricity in parts of Brooklyn too; a total of 220,000 other customers were already in the dark.
Less than an hour later, more equipment flooded, sparks flew, and the blast boomed across the East River and throughout lower Manhattan from what ConEd believes was a circuit breaker at its flooded substation.
The flooded equipment had failed.
When live electric equipment is inundated with salt water, electricity escapes every which way, sending sparks flying and damaging equipment. "You see a huge blast just from the short circuit," says Arshad Mansoor, senior vice president for research and development at the Electric Power Research Institute, an industry-funded research group.
As day broke Tuesday, the company was busily assessing damage and fixing equipment. But downed trees and wires, as well as lingering flood waters, made it hard for repair crews to reach some areas. The utility was able to get at least 140,000 customers back on the grid within several hours.
But hundreds of thousands of others hunkered down for a longer outage. ConEd said customers served by underground equipment should be restored to service in four days. Those who get power from overhead lines are expected to wait a week. That's because there are so many fallen lines.
The most densely populated parts of the city, mostly in Manhattan and Brooklyn, are served by underground transmission wires. These offer protection from wind and falling tree limbs that plague overhead wires and make the suburbs far more vulnerable to outages.
But underground wires can flood and be more difficult to repair, especially in low-lying areas. It can be harder for workers to get to the wires because manholes flood. When water recedes, it can be harder to find problems, pull out wires and equipment, dry them, fix them, and slide them back into place.