First-cycle duty of low-voltage fuse = MF × first-cycle symmetrical RMS current
MF = 1.0 is applicable whenever the first-cycle short circuit X/R ratio at the fault point (i.e., source side of breaker) is 4.9 or less. If the first-cycle short circuit X/R ratio is greater than 4.9, the following formula from IEEE Std C37.13-2008 can be used to find the MF.

The interrupting rating in RMS kA must exceed the first-cycle duty in asymmetrical RMS kA.

Note: The marked interrupting rating of a fuse is the tested single-pole interrupting rating, so derating to assess single-pole interrupting capability does not apply to fuses.

Example:

Let’s put numbers to the formulas above by considering the example from Part 1 of this series for which the first-cycle symmetrical RMS current and short circuit X/R ratio for a 3-phase fault at low-voltage Bus 3 are 23.910kA and 7.0, respectively.

First-cycle duty of Bus 3 unfused low-voltage power circuit breaker = MF × Bus 3 first-cycle (momentary) symmetrical RMS current = 1.012 × 23.910kA = 24.197kA (asymmetrical RMS)

First-cycle duty of Bus 3 molded- or insulated-case breaker = MF × Bus 3 first-cycle (momentary) symmetrical RMS current = 1.073 × 23.910kA = 25.655kA (asymmetrical RMS)

First-cycle duty of Bus 3 low-voltage fuse = MF × Bus 3 first-cycle (momentary) symmetrical RMS current = 1.078 × 23.910kA = 25.775kA (asymmetrical RMS)            

Mercede, P.E., is principal of Mercede Engineering LLC, based in Bryn Mawr, Pa. He can be reached at fmercede@mercedeengineering.com.