GFCI Breaker Tip

I had a similar situation as the past commodore's boat at Winslow; where I had passed the electrical safety inspection but was popping the new GFCI breakers on the dock. Hopefully this can help some of you who have MagnaSine inverters and are experiencing this problem.

This occurred even with all my panel breakers off; even the main AC breaker and inverter breaker. It did not trip the shorepower breaker when my generator/shorepower transfer switch was off. So this meant that the fault wasn't in the shorepower cable, since the transfer switch connects the cable wiring to the rest of the boat. 

Using an ohmmeter on the shorepower plug, I was able to determine there was a short between the AC neutral prong and the green wire safety ground, which only showed up when shorepower was selected on the transfer switch. Remember, all breakers were off, so somewhat puzzling. But remember that circuit breakers don't switch the neutral wire. I concluded that the transfer switch must switch the neutral to the boat.

It turns out that the MagnaSine 2812 inverter/charger has a neutral to ground jumper inside the AC electrical connection box. The installation manual leads one to believe that this unit actively disconnects this neutral/ground bond when shorepower is applied, but mine didn't work like that.

The net effect is that current to the boat flowed through the AC hot wire, as normal, but the return current got split between the green wire (safety ground) and white wire (neutral). The GFCI looks to see if hot wire current and white wire are balanced, and since some of the current was returning through the green wire, the breaker tripped.

In any case, removing the neutral to ground jumper on the MagnaSine 2812  solved this problem. The manual says this jumper "might be needed in some installations", which is pretty vague. All I know is that when operating from shorepower, neutral and ground can only be connected at the shorepower pedestal and nowhere else on your boat, or you will have this problem.

Craig