Short Circuit Current Duties of Circuit Breakers and Fuses — Part 1

Medium-Voltage Power Fuses

This discussion is limited to applications of medium-voltage power fuses for industrial and commercial power systems. The reader is directed to Chapter 6 of the IEEE Buff Book (IEEE Std 242-2001) for broader coverage of high-voltage power and distribution fuses.

E-rated power fuses are available as both expulsion and current-limiting types, while R-rated power fuses are available only as current-limiting type. E-rated current-limiting power fuses afford protection of power transformers, voltage (or potential) transformers, and capacitor banks; while R-rated power fuses afford short circuit protection in medium-voltage motor controllers. For symmetrical-current-rated power fuses, the first-cycle symmetrical rms current can be directly compared to the rated symmetrical interrupting capabilities in a manufacturer’s table only if the first-cycle short circuit X/R ratio at the fault point (i.e., source side of the power fuse) is 15 or less. In this context, “symmetrical-current-rated” implies that an MF to account for the DC component is unnecessary to adjust the first-cycle symmetrical rms current whenever the first-cycle short circuit X/R ratio is less than or equal to 15, because a certain degree of asymmetry is built into the rating structure. If the first-cycle short circuit X/R ratio exceeds 15, the manufacturer should be consulted for derating (reduction) of the published rated symmetrical interrupting capabilities. For the example at hand, the last statement applies, since the first-cycle short circuit X/R ratio for a 3-phase fault at Bus 4 is 16.4.

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