Occasionally even on a correctly coordinated system the main breaker beats the branch breaker on ground fault. Most molded case low current breakers have set trip curves and cannot be adjusted. clearing times can be longer than the higher quality and perhaps much faster mains.
I think a typical 10KA rated breaker clears in 4 to 8 cycles and the main might clear in 2 to 4 cycles The main ground fault setting at 30 amps could easily trip faster than a a branch breaker that needs 6 times rated current to trip in its instantaneous range. So a 15 amp breaker might need 75 to 90 amps of fault current to trip as fast as the breaker is designed and the main is tripping at 30 to 60 amps of ground fault current.
I always rely on a testing engineer to provide selectivity and coordination.
Last edited by mikesh; 07/25/13 07:13 PM.
