Customer has a detached garage, two circuits are run underground in 10/3 underground cable. Two pole 15 amp breaker in house trips maybe once a day for no apparent reason, I checked connections on both ends, replaced breaker, but it still trips, there is no load on when breaker trips. I,m thinking the underground cable might be damaged, but I don't know. I don't have time waiting around the house waiting for it to trip. How would you guys troubleshoot this problem, thanks for any replies.