I have a question about generator wiring practice on boats. I have recently encountered three instances of boats with generators tripping ground fault breakers recently installed on many docks.
In all three it was found the ground wire (green) was connected to shore power neutral at the generator. In all three cases disconnecting this fixed the ground fault trips.
From articles I have read it never permissible to connect ground to neutral on a boat, though it is required by code in residential electric.
So my question is why did boat manufacturers connect ground to neutral on generator installations and is it ok to remove the connection?
If I remove the connection do I need to provide a way to reconnect when on generator power and disconnect on shore power?
In order for the breakers to trip, the neutral has to be grounded. Otherwise there is no path back to the source (in this case the generator) to complete the circuit so the breakers can trip. If the neutral is not grounded, then if there is a fault to ground nothing would happen and then a dangerous condition would exist if there was an accidental connection to ground, maybe through a person - hence shock hazard.
The tripping of the shore power GFI should be prevented by the selector switching between shore power to generator power by disconnecting the generator neutral when in shore power mode, and when in generator mode, the shore power neutral would be disconnected. This is how my panel works as I have two mains that have an interlock so only one is on at a time. The main breakers are two pole disconnecting both hot and neutral at the same time. If you had a transfer switch it must also transfer the neutral. The moral of the story is that the shore power neutral and the generator neutral cannot be landed together and must switch just like the hot wires. This would apply to a 120/240V system as well.
The transfer switch looks like it breaks both line and neutral. Must be the genny neutral is connected somewhere else to shore power neutral.
Thinking about this the generator does have a line and neutral. All the circuits are powered by this pair of wires. So the breaker would have a path to trip without a green ground wire involved. This is for an internal failure of a device or a short circuit or any fault where too much current is drawn
If a fault to ground occurred then no path to trip. Results in a hazardous situation
Is that the correct view of it?
Yes I think you’ve got it. A short circuit between hot and neutral would cause an immediate over current condition and should result in an immediate breaker trip. The same for any over current condition it just might take longer for the breaker to respond depending on the magnitude of the over current. The purpose for the green grounding conductor is to bond the exterior frames of equipment and appliances back to the source to establish a sure return path with a low resistance. The neutral is then intentionally connected to the ground in this case the generator frame where the green grounding conductor are also connected. This way if there is an unintentional connection between the hot and the frame of any equipment then we have a path to trip the breakers. In an ungrounded system the first fault doesn’t do anything so it would not be detected and could effectively energize the frames of equipment or appliances. Then someone touches the appliance and something that is grounded and gets shocked by completing the circuit.
A GFI works by sensing a balance between the hot and neutral. If there is any unbalance then it will trip. Anytime the neutral and ground get connected together downstream of the GFI it will cause an unbalance as some power will return through the ground.
You’ll need to check your neutrals from both the generator and shore power to make sure they are not connected together. When on shore power only that neutral should be connected through to the neutral bus bars in the breaker panel. When in generator mode the shore neutral should be disconnected and only the generator neutral connected to the neutral bus in the panel. That should resolve the issue. The green grounds should always be connected and can be all together.
Hope that helps!
I have another question concerning what to do with the green grounding wire on the boat.
Looking at the ABYC wiring practices standards it shows the green g=grounding wire from shore power cord connected to the AC ground bus in the boat. That makes sense.
It also shows the shore power grounding conductor (green) connected to the generator chassis. That makes sense to me too.
However it then shows it (shore power green grounding wire) also connected to the engine ground. That seems to me to connect it to the negative terminal on the battery and to the bonding system for all underwater metal. I have heard that you NEVER connect the green ground wire to the negative battery terminal.
Does anyone know what the correct connection is?
Shore power GND is connected to DC -ve to allow the AC breaker to trip, should a live AC hot connect with the casing of a DC device. Think of anything that is both AC and DC, refrigerator, inverter, charger, etc. Without this connection, the DC devices casing would become energized and dangerous. I believe galvanic corrosion is the reason people suggest not to do this, but it is an electrical risk if not done.