"Right sizing" circuit breakers in motor starters

Instantaneous-trip, molded-case circuit breakers (MCCBs) have been applied successfully for low-voltage motor protection for many years. However, the changes in motor-starting characteristics have created more difficult challenges for the application engineer. The major challenge is successfully starting the low-voltage motor without causing the instantaneous-trip breaker to falsely trip without exceeding equipment ratings or violating limitations in the National Electrical Code (NEC). "Right sizing" circuit breakers for use in low-voltage combination motor starters is critical to the successful, reliable operation of plant and process facilities. If not properly applied, false tripping on motor starting and contact welding of contactors may occur. Moreover, changes in the NEC and improved motor efficiencies have impacted equipment selection and protection. The following information is intended to guide application engineers in their selection of the right breaker and its setting for more reliable motor protection: combination motor-starter components, circuit breaker characteristics, circuit breaker and combination. starter testing, motor starting characteristics, and component selection using coordination examples. The examples show how selecting the right breaker and breaker setting can avoid false tripping during starting, mitigate contact welding, and provide backup protection for overload conditions.