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2017 Edition, January 1, 2017

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Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications

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Description / Abstract:

NEMA AB 4 sets forth, for use by qualified personnel,1 a number of basic procedures that may be used for the inspection and preventive maintenance of molded case circuit breakers used in industrial and commercial applications rated up to and including 1000 V 50/60 Hz AC or AC/DC. Note—consult the manufacturer for DC-only or 400 Hz circuit breakers.

The methods outlined may be used to verify specific characteristics of a molded case circuit breaker that was originally built and tested in compliance with the requirements of NEMA standards publication AB 1 (UL 489). These methods are intended for field application and are, therefore, non-destructive in nature. Accordingly, these methods cannot be used to verify all performance capabilities of a molded case circuit breaker since verification of some capabilities requires tests of a destructive nature.

Many tests, including those of a destructive nature, as defined in AB 1 (UL 489), are performed on representative samples of circuit breakers by the manufacturer, as part of a routine program of factory inspection.

The AB 4 standards publication is not intended, nor is it adequate, to verify proper electrical performance of a molded case circuit breaker that has been disassembled (broken factory seal or removal of rivets), modified, rebuilt, refurbished, or handled in any manner not intended or authorized by the original circuit breaker manufacturer. Such breakers should be removed from service.
ANSI/NEMA MW 1000, 2014