Con Edison: Relay system failure caused Manhattan blackout

These are computerized systems. And it failed? So why did it fail?

NEW YORK (AP) –
A relay protection system that should have isolated a faulty distribution cable but didn’t led to the loss of power that darkened Manhattan for several hours.

A preliminary review of Saturday’s blackout shows that the relay system at a West 65th Street substation should have kept the problem from spreading, Con Edison said. Instead, the electrical fault was isolated at a transmission substation at West 49th Street.