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Electronic motor circuit protectors

Motor circuits have a dynamic load profile that creates a fine line between acceptable inrush currents and damaging short circuits. Achieving a proper balance of protection, along with gaining National Electrical Code compliance can be a delicate balance. The setting of the motor circuit protector can lead to decisions between limiting the potential of a nuisance trip on start-up versus providing an optimal setting to protect an electrical motor. Circuit breakers equipped with electronic trip systems have become a critical part of protecting electrical systems. This technology is well suited for applications that require adjustments to properly align trip curves with specific devices or to properly coordinate with other circuit breakers. Motor control applications have a unique load profile and are ideally suited for electronic trip unit technology. This technology can be referenced as an electronic motor circuit protector. This circuit breaker utilizes an internal microprocessor that is programmed with specific trip characteristics to protect an electric motor and align with the setting requirements based on the National Electric Code Table 430.52 for the maximum setting of branch motor protective devices. An evaluation of the time-current characteristics of motor start-up will be examined while comparing motor protection offered by both electro-mechanical motor circuit protectors and electronic motor circuit protectors. There will be a review of the requirements of the National Electric Code Table 430.52 and a discussion of several common mistakes people make when setting branch motor circuit protection which are: significance of motor full load currents; not accounting for energy-efficient motors; overlooking heavy service applications; and missing the short circuit current ratings. Data results from a field survey are included which also illustrate the need for a simpler, more flexible solution for protecting your motor branch circuit.



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