[johnsjmc]

He said he is using direct burial cable in pvc. The romex is above ground in the buildings. Again terminology, using the general term romex to describe both types of non metallic sheathed cable. Here we have NMd (dry) and NMw (wet) which are similar to US types.
It appears he has more than one building fed by a single breaker in a subpanel in another building.
You could use an ohm meter if it turns out he has only a little leakage to ground (suggested by the 60 sec interval to trip. Not a dead short but he still needs to do some disconnecting to isolate the fault to a part of this large and rather unusual circuit.