The circuit breaker that is tripping is an RCD or Residual Current Device. It trips when it detects a even a minute difference between the current in the (normally) Hot leg and the current in the (normally) Neutral return. In a properly functioning AC electrical circuit, the current would be the same in the Hot and Neutral return. If the Hot was somehow shorted to ground at the appliance (like to the appliance case) the current would be returned to the source via the Ground wire instead of the Neutral wire. The RCD breaker would trip when it detected that the current in the HOT wire was NOT returning to the source via the Neutral wire.
Obviously, you no longer have a Neutral return but I am fairly certain that the RCD still works with two Hot legs. So assuming your RCD is working properly, then it appears that you have more current on one Hot leg then you have on the other WHEN ON SHORE POWER ONLY. Given that your voltage readings were the same, that would mean that the resistance of one Hot leg is higher than the other. In my experience, the most likely cause of higher resistance in a circuit like this is usually a bad "mechanical" electrical connection. Think bad crimp or a loose screw terminal.
Given that everything works fine when you are running on the Generator, I would be looking at every "mechanical" electrical connection in the Shore Power wiring including:
- Wire Connections inside the Automatic Transfer Switch
- Wire Connections at Shore Power Plug
If everything looks OK, I would probably try to bypass the Automatic Transfer Switch and see what happens.
Usual disclaimer that I am NOT a marine electrician and I do not even play one on TV! Good luck.
SM #440 Cara
Deale, MD USA