Originally Posted by Paul Swanson
The green ground is connected directly to Earth at many places including your house and the boat’s bonding system. The neutral is connected to Earth at the power generating facility but nowhere else. At your house (or boat) it is possible to get a small potential between the neutral and Earth (green ground) by virtue of the current flowing through the circuit and the finite resistance of the wires. Note that the same current flows through both the “hot” and neutral wires.
Maybe it's just terminology, but as I'm reading it I don't think it's correct.
Ground and Neutral are supposed to be bonded (connected) at the "power source", and only at the power source. What constitutes the "power source" varies, especially on a boat. When you say "power facility" I take that to mean the power generation plant, but this may be where I'm just not understanding your terminology. The power coming out of a generation plan actually has no neutral.
Here are some examples of proper neutral/ground bonding under different circumstances found on a boat:
1) When on shore power, the power source is the land power system. Land power systems have neutral and ground bonded at the main breaker/meter panel, so the neutral and ground are already bonded when they reach the power stand at your dock. That is carried through to your boat, so when on shore power there should not be any neutral/ground bond on your boat.
A galvanic isolator does not change this. It carries the ground through to the boat - sort of. It isolates the shore and boat grounds as long as there is no more than 0.7V difference between them. But if there is a difference greater than 0.7V, as in the case of a fault being protected by the ground, it ties them together.
2) Running on the boat's generator. In this case, the generator is the power source, and there neutral and ground should be bonded there. The trick is to have this bonding be in place when powered off the generator, but not when powered off shore power. Most setups I've seen have ground and neutral bonded in the generator, but the power selector switch that selects between gen or shore power breaks the neutral when the generator is deselected, thereby also breaking the neutral/ground bond.
3) Inverter power. In this case the inverter is the power source, and neutral and ground should be bonded there. This is where it starts to get complicated because many inverter setups operate with and without shore and gen power. What is bonded where and when gets complicated.
3A) Inverter only. If shore and gen power are off and you are only running on inverter, like at anchor, then bonding should be at the inverter.
3B) Inverter with gen or shore power operating. When either shore power or the gen is operating and the inverter is either passing through AC power, or operating in charger mode, then bonding should be at the gen or shore power as previously described since they are the power source. Some inverters have internal relays that will create or remove the bonding connection based on the presence or absence of input AC power. Other rely on it being handled correctly external to the inverter.
This is probably more than you wanted to know, but might help you track down and fix the problem without creating a different problem at the same time.
But the bottom line is that you should not be seeing any voltage difference between neutral and ground.