Hello You must test the power cutter with its test button; Prior to that, to hook it, disconnect the output cables. If the circuit breaker engages, and passes the test test, then you certainly have a ground leak. Triggering the differential circuit breaker with the cut phases does not necessarily mean that it is neutral, since some circuit may be closed and the leakage will occur the same on the active cable (do you understand this?) Anyway, in the neutral or in the phase, you must look for the ground fault. You must proceed as follows: Disconnect two of the phases and leave only one bypassing the others; In this way you proceed to look for the damage as if it were a single-phase installation, without risk of losing the neutral and having a damage, or risk of accident. At this point, look for the fault. I just uploaded the method in a recent post (an hour ago), so check out what I describe. Here the link: circuit breaker-differential-skip-and-disconnect-everything I don't know what degree of knowledge you have to look for the fault, but if you have doubts, comment.