Effect of DC offset on instantaneous operating characteristics of low-voltage circuit breakers

This paper addresses the effect of asymmetrical currents on factory adjustments and field verification of low-voltage circuit breaker instantaneous trip units. Both molded case circuit breakers (MCCB's) and power circuit breakers are pertinent. Factory adjustment of low-voltage circuit breakers, based on test results using asymmetrical current, may cause the circuit breaker instantaneous unit to perform improperly and result in inadequate protection and coordination of electrical system apparatus. Similarly, field testing conducted to verify proper performance of low-voltage circuit breakers, using asymmetrical current, may lead to the erroneous conclusion that the circuit breaker's instantaneous unit is out of specification. In addition to addressing the difficulties presented by this phenomenon, this paper also proposes an approach to identifying and overcoming these testing difficulties.