This guide applies to unfused 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. In this guide, the term circuit breaker means unfused low-voltage ac power circuit breaker.
Purpose
This guide sets forth recommendations believed essential for the selection of current-limiting fuses (see NEMA FU-1 and UL 248-1) for use in combination with low-voltage ac power circuit breakers, rated in accordance with IEEE Std C37.13(TM) and applied in metal-enclosed low-voltage power circuit breaker switchgear in accordance with 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.
Abstract
Revision Standard - Active.Application recommendations to assist in selection of separately mounted current-limiting fuses to be installed in series with low-voltage ac power circuit breakers are provided in this guide.
