Enhanced safety: faster clearing times and lower let through currents
Breaker-based motor controller short circuit protection is usually provided by the instantaneous overcurrent setting of the motor protection relay. Although this type of protection operates with no intentional time delay, it is important to note that inherit delays exist due to relay and breaker operations or total clearing time.
To obtain the relay operating time, the protection engineer must account for the relay’s output contact and instantaneous protection operating time. Relays typically have an 8msec contact operation time with a 30 msec maximum pick up time. This means the relay’s total operating time is about 38 msec or about 2 ¼ cycles. Once the relay picks up and closes its output contact to trip the breaker, it will take an additional 50 to 83 msec (3 to 5 cycles) in order to fully open its contacts and clear the fault. Adding both the relay’s and the breaker’s operating times, the total clearing time is 88-121 msec (5-7 cycles).
Class E2 motor controllers utilize a main contactor to make and break load and overload currents, in addition to MV current limiting fuses for interrupting fault currents that exceed the breaking capacity of the main contactor.
Most 400 A MV contactors have interrupting ratings between 6,000 A to 8,500 A; and 800 A MV contactors have interrupting ratings of 7,200 to 12,500A. In order to obtain higher interrupting ratings, current limiting fuses are supplied as backup protection to interrupt and limit short circuit currents higher than the contactor’s rating. The motor starter design must ensure the contactor does not open above its interrupting rating, and instead allow the fuse to clear this fault.
The table below tabulates the minimum and total clearing times as well as the let-through current of common fuses utilized in 400 A and 800 A MV motor starters. As seen on this table, the higher the levels of prospective short circuit current at the fuse location, the faster the fuse takes to clear and limit the let through fault current feeding the fault. This current limiting feature clears a fault within ½ cycle or at least 10 times faster than a breakers’ instantaneous protection, greatly reducing the amount of arc-flash energy produced during a fault making it safer for its users.