So if the dedicated inverter connects to +12 VDC at my DC refrigerator breaker and -12 VDC at the DC ground bus near the panel, where does the AC ground from the refrigerator power cord find a ground?
Is your refrigerator an AC fridge?
If yes, the refrigerator has three wires. A positive wire, a negative (neutral) wire, and a ground wire. On some residential appliances, the neutral and the ground are connected in the refrigerator wiring.
You will need to check the inverter to see how it is wired. Many regular marine inverters have a pass through from shore power. In that case, the inverter opens the connection between the neutral and the ground within the inverter when the inverter is in pass through mode. In that case, the ground/neutral connection is at the SP pedestal. Any stray current from a ground neutral connection at the fridge can follow either wire back to the SP, or through the single connection on the boat where DC/AC/bonding come together and then out through a through hull, or prop, or rudder of the boat. That is what happened in my case. It was coming through the rudder.
If the marine inverter is receiving power from the DC battery bank, then the connection between the AC neutral and ground is closed and stray AC current
from the fridge can flow back through the DC negative buss to the DC/AC/bonding connection. In my case, that also resulted in AC amps escaping through the water to get to ground.
Either way, it can create a dangerous condition.
I am not a marine electrician, but learned this through study when “the best electrician on the river” “fixed” mine by connecting the ground buss to the neutral buss at the panel. Of course, the SP breaker popped immediately. I removed it and started studying. Found the problem after a few hours of checking and testing, fixed it, and moved on. If an electrician wants to jump in and correct any of this, please do so, but this is the way I understand the issue.
