Q. What is a current-limiting overcurrent device?
Find the Answer
A. An overcurrent device (typically a fast-acting fuse) that reduces the fault current to a magnitude substantially less than that obtainable in the same circuit if the current-limiting device wasn’t used. See Sec. 240.40 and Sec. 240.60(B) [240.2] for additional information.
A current-limiting fuse is a type of fuse designed for operations related to short circuits only. When a fuse operates in its current-limiting range, it’ll begin to melt in less than a quarter of a cycle, and it’ll open a bolted short circuit in less than half a cycle. This type of fuse limits the instantaneous peak let-through current to a value substantially less than what will occur in the same circuit if the fuse is replaced with a solid conductor of equal impedance. If the available short circuit current exceeds the equipment/conductor short circuit current rating, then the thermal and magnetic forces can cause the equipment circuit conductors, as well as the circuit equipment grounding conductors, to vaporize. The only solutions to the problem of excessive available fault current are to:
• Install equipment with a higher short circuit rating, or
• Protect the components of the circuit by a current-limiting overcurrent device such as a fast-clearing fuse, which can reduce the let-through energy.
A breaker or a fuse does limit current, but it may not be listed as a current-limiting device. A thermal-magnetic circuit breaker typically clears fault current in less than three to five cycles when subjected to a short circuit or ground fault of 20 times its rating. A standard fuse will clear the same fault in less than one cycle and a current-limiting fuse in less than half of a cycle.
