There is one marina on Lake Ontario that uses Ground Interrupter circuit breakers on their main breaker box for the docks. When I plug in with all of my AC breakers off, I get the proper amber lights on my panel indicating normal polarity. The moment (or sometimes a couple seconds later), the GFI breaker on the dock trips. The only breaker that I had turned on was the main with no load. After opening my breaker panel, I observed the main AC breaker has three components. One for Hot, one for Neutral (return), and one that is stamped 65 VAC and is not a breaker. (Note: when I got the breaker to stay on using a generator, I measure 65 VAC across those terminals). The tabs on it seem to connect to earth ground and return, and to the indicator lamps on the panel. I have no idea what this device is and can take a photo if needed. The only other weird thing is that with all breakers off (AC and DC) and no AC input, I measure between the Ground Buss and the AC Return Buss and get a resistance that starts at 5K ohms and increases to about 15K ohms, as though it were charging a capacitor. I found that if I disconnected the three Ground wires that go to my AC sockets with nothing plugged into them, the resistance is infinity. Connecting the AC input and turning on the main breaker with those wires disconnected, the Main Box Breaker stays on. If I turn on my battery charger which is wired to a breaker with it's own ground connected to the buss, the Main Breaker on the dock trips. We used a generator as a substitute and all worked OK. We continued to many other marinas with no further problems using provided AC power. The attendant at the marina that had the problem said that this is a common problem with that box. However, there were several other boats to that same box with no AC problems. What gives? *o