This guide applies to low-voltage ac power circuit breakers of the 635 V maximum voltage class with
separately-mounted current-limiting fuses for use on ac circuits with available short-circuit currents of
200 000 A (rms symmetrical) or less. Low-voltage ac fused power circuit breakers and combinations of
fuses and molded-case circuit breakers are not covered by this guide.
This guide sets forth recommendations believed essential for the selection of current-limiting fuses (see
NEMA FU-11 and UL 248-1) for use in combination with low-voltage ac power circuit breakers, rated in
accordance with IEEE Std C37.13™ and applied in metal-enclosed low-voltage power circuit breaker
switchgear in accordance with ANSI/IEEE Std C37.20.1™.
NOTE—The combination of a circuit breaker and separately mounted fuses is limited to 600 V based on fuse
maximum voltage ratings.