New York utility Con Edison said that the relay protection system at one of its substations did not operate as designed and caused a July 13 blackout that affected parts of Manhattan's west side, including Times Square.

The system that failed detects electrical faults and directs circuit breakers to isolate and de-energize those faults. The utility said the relay protection system is designed with redundancies intended to provide high levels of reliability.

In this case, primary and backup relay systems did not isolate a faulted 13,000 V distribution cable at West 64th Street and West End Avenue. The protective relay system failure ultimately resulted in isolation of the fault at the West 49th Street transmission substation, and the subsequent loss of several electrical networks, starting at 6:47 p.m.

The utility said that past experience with the transmission and distribution system led repair workers initially to believe the 13 kV cable fault was unrelated to the transmission disturbance. It said that although the cable fault was an "initiating event," the outages were the result of the protective relay system failure.

The utility said that more than half of the customers who lost power during the evening outage were restored in under three hours and all within five hours.

The investigation involved inspecting and testing transmission equipment, and analyzing data. Con Edison said that data analysis and relay protection equipment testing is continuing, and will provide more insight into why the system and its redundancies did not operate as designed.