Asymmetrical Interrupting Current Rating of Medium Voltage Circuit Breakers



As discussed in the last Powell Technical Brief, the 1999 revision of ANSI Standard C37.04 and C37.09 changed several definitions relating to the rating structure of medium voltage circuit breakers. The earlier revision of the circuit breaker standards utilized the “S factor” as a multiplying factor that defined the total current a breaker was rated to interrupt at contact part. The newest revision of C37.04 replaced the “S factor” with the “%dc” as the method of evaluating the asymmetrical current offset. The “%dc” is utilized to calculate the total interrupting current.