NEMA AB3-1996, Molded Case Circuit Breakers and Their Application, describes the types of available molded case circuit breakers (MCCBs), along with many of their applications. It also lists accessories and ratings, provides guidance on MCCB selection, and defines much of the associated terminology. Section 5 of NEMA AB3 is especially useful in selecting MCCBs. It includes information on electrical system parameters, environmental conditions (extreme temperatures, humidity, corrosive atmospheres), vibration shock, and harmonics.

NEMA AB4-1996, Guidelines for Inspection and Preventive Maintenance of Molded Case Circuit Breakers Used in Commercial and Industrial Applications, provides guidance for maintenance and field testing of MCCBs—explaining how to evaluate their condition. NEMA AB4 also provides a number of basic procedures for inspection and preventive maintenance of MCCBs rated up to and including 1000VAC, 50/60 Hz or AC/DC. You can use the test methods to verify specific characteristics of MCCBs originally built and tested in compliance to the UL 489 and NEMA AB1-1991 standards. However, NEMA AB4 is not intended, nor is it adequate, to verify proper electrical performance of an MCCB that’s been disassembled, modified, rebuilt, refurbished, or handled in any manner not intended or authorized by the OEM.

The NEMA AB3 and NEMA AB4 standards are valuable tools for the safe and efficient operation of MCCBs. They are indispensable to contractors, specifiers, plant engineers, and maintenance personnel. For copies of the standards, call NEMA at (703) 841-3201 or IHS at (800) 854-7179.