This is the second part of a two-part series on the methodologies used to find the short circuit current duties of circuit breakers and fuses. The first part, Short Circuit Current Duties of Circuit Breakers and Fuses — Part 1, which ran last month in the July issue of EC&M, provided the necessary background and illustrated the ANSI methodologies to determine the short circuit current duties of ANSI-rated medium-voltage circuit breakers and fuses. This part follows the same process steps but focuses on low-voltage circuit breakers and fuses. It should be noted that other ratings and application considerations beyond the scope of this article must be considered for the proper selection of low-voltage circuit breakers and fuses. Specifically, the reader is directed to the IEEE Blue Book (IEEE Std 1015-2006), Chapters 5 and 7 of the IEEE Buff Book (IEEE Std 242-2001), and the latest edition of the National Electrical Code (NFPA 70) for comprehensive coverage of this topic.
As discussed in Part 1, the interrupting ratings of low-voltage circuit breakers and fuses are classified on a symmetrical-current-rated basis. In this context, “symmetrical-current-rated” implies that a multiplying factor (MF) to account for the DC component of the short circuit current waveform is unnecessary to adjust the calculated first-cycle symmetrical RMS short circuit current whenever the short circuit X/R ratio is less than or equal to some limit, because a certain degree of asymmetry is built into the rating structure. However, if the short circuit X/R ratio exceeds the limit, a larger than tested DC component may exceed the built-in asymmetry allowance; and the first-cycle symmetrical RMS current must be multiplied by an appropriate MF before comparison with the interrupting ratings (i.e., 3-phase short circuit current ratings) in a manufacturer’s table. Listed below are the short circuit X/R ratio limit and the formula to calculate the MF (in the event that the short circuit X/R ratio exceeds the limit) for several categories of low-voltage circuit breakers and fuses.