Short circuit protection of energy efficient motors

Many years ago, the National Electrical Code established a maximum setting for this instantaneous trip breaker of seven times motor full load amperes (FLA), the theory being that that value was just above the typical 6X locked rotor current. In a 1986 IEEE IAS paper Scheda noted that the two most commonly encountered problems in applying high-efficiency motors are (1) replacing a standard motor with a high-efficiency motor, and (2) the need to use thermal sensors on larger machines since higher settings for electronic control would violate the NEC. His paper concludes: “It is suggested that work be done in the industry and standards organizations to serve the needs arising from the use of electronic detection of instantaneous currents for motor protection.” This article addresses some of the work done in the industry to serve these needs and, in particular, how the use of an “electronic detection” inverse time circuit breaker can be appropriately applied when an instantaneous trip breaker would nuisance-trip