AC DC grounding

I am looking for a definitive answer on grounding and bonding and the AC and DC system.

My boat is a twin outboard powercat. I have one raw water bronze intake below water.

The main question is whether or not I am required to tie the AC and the DC ground bus. I was told by surveyor it was required, and so I did, but I got some flack recently on a Facebook forum for saying so.

My boat has a galvanic isolater and iirc, the ground off the iso heads to the AC distribution box and then I tied that node to the DC ground bus as per surveyor. Someone said the risk is that I could run AC short to ground back through the DC ground bus.

On a side note, I tied the bonding system to the DC ground buss as well.

Did I do anything wrong; despite the surveyor requirements?

I do have @Ike handbook, and plan to read it again..later today.

 

http://honeynav.com/wp-content/uploads/2016/06/Grounding.pdf

 

I did not see the article comment on my question, unless I missed it.

 

I'm not so sure that a "Definitive answer" exists.
Various answers exist depending upon each boats construction, wood/fiberglass/metal, (what metal).
As well as what metals are underwater.

"The main question is whether or not I am required to tie the AC and the DC ground bus."

Your surveyor is most likely basing their judgement on the ABYC recommendations.
That recommendation is one designed to save people from being electrocuted, (you are laying across a defective water heater and grasp a seacock).
I went a different route.
EVERY device that uses AC in the boat is wired thru a GFCI outlet, (breaker to line/load to device,) and also GFCI protection on the dock power.
I believe my system is superior at protecting life and limb.
But anything I say will not appease your surveyor.

 

The GFCI only works if the AC and DC have a common ground. That is the reason houses have grounding rods that are connected to the AC ground.

 

GFCIs, if there is any difference between the currant flow in the hot and neutral will trip.
On a boat the green goes back to the dock, and there the ground and neutral are connected.
ground and neutral are only connected together at the source. not on the boat.

 

Might be the terminology. The article describes how to run the AC and DC grounds and states that it meets ABYC requirements.

 

This is interesting to me.

So, if there is no earth ground when the boat is underway, how does a AC side GFCI work, or does it not?

Of course, if the AC ground is not connected to the DC ground or a ground plate, then it is floating, and gfci would not work, then the people suggesting AC grounds and DC grounds are separate would only be correct if the AC grounds had an actual path to ground..if I understand this right.

And so my surveyor is correct if I am inverting an AC current underway.

 

If you invert AC power, what creates a path to earth?

 

We tend to use terms like "Earth" and "Ground" interchangeably, but they technically are not the same.
Think of a vehicle sitting on rubber tires.
It uses the frame as a "ground", (a return path,) for the lights, but there is no real "earth" in the system, what we are calling a "ground" is really just the "2nd wire" for the return to the source, (the battery).
Electricity, (depending upon its source,) does not seek a ground or earth, per se, what it does do is seek
a path back to its source.
When you install an isolation transformer on a boat there is no path from the green/ground wiring on
the boat back to the shore, yet the GFCIs still work.
Why? because as I said before, an imbalance in the flow between hot and neutral is what makes the GFCI trip.
The "grounding" is all internal to the boat and device.
An inverter works the same way, the outlets are GFCI that use the boats "grounding".
You could haul your boat out on dry land and the inverter will still work without any connection to "Earth" or water.

 

Oh, I know it will work as described, but I am trying to deep dive to understand if the gfci is more or less likely to work if not attached to DC ground bus. But I think you are telling me that the gfci is working on voltage differences and doesn't really need to be connected to the DC bus. Maybe the gfci will be slower?

So, the downside risk to the bus being connected is a short to ground on the AC side could travel through the DC grounding system?

And maybe this is why an argument ensues over the practice..

 

Perhaps I can try to simplify.
At the shore side of the boat power, or at the main power supply for a house, the ground and neutral are connected together, because that is considered the "original source" of the electrons.
In an isolation transformer on the boat the ground and neutral are also connected together ON THE SECONDARY side of the transformer, because it is considered to be the "original source" of the electrons.
The inverter is the same.
Now, how does that work?
Electricity does NOT take the path of least resistance; it takes ALL paths available in its quest to return to its source, the flow being divided in proportion to the resistance of each available path back to the scorce.
It's the "Resisters in parallel" operation.
Ok, for simplicity, let's say that 100 electrons from the source flow thru the hot wire going to the load.
If there are no problems, then those 100 electrons will flow back thru the neutral wire back to the scorce.
The ground and neutral are connected together at the scorce but NOT at the load.
If there is any "leakage" of electrons into the ground wire those two wires, (neutral and ground,) become "parallel resisters/conductors", that means that some of those electrons are no longer flowing thru the neutral wire, that causes the
imbalance which trips the GFCI device.
The same thing happens if YOU become the "ground wire".
It's similar to a bird sitting on a 12,000-volt wire, they don't get shocked because they are not creating a return path to the scorce.

 

A few years back, after getting a lot of questions like this, I create a separate web page on my web site that will answer some of your questions (hopefully all) Frankly, it is too complext to answer in a few sentences or paragraphs. look at Boat Building Standards | Basic Electricity | Bonding Grounding Lightning https://newboatbuilders.com/pages/electricity14.html
Then if you have more questions I, and others here, will be happy to answer them. By the way, the surveyor was probably right, but we don't have enough information on your electrical system to give a definitive answer. We'd need to see a schematic of the system.

 

GFCI works on current, not voltage, difference between the two sides of the circuit. If the current is the same (zero difference) then there is no current leaking elsewhere.

 

Thanks for the correction. It seems unlikely, in this context, the DC and AC grounds need to be joined.

The system is relatively simple. I don't have a drawing at hand atm. But, I can explain it well.

AC shore power 30A runs through galvanic isolater to inverter, then small AC breaker box.

DC side has 24v battery bank and four charging sources, solar, inverter, Victron Orion from each engine atm. The charge side runs through a switched bus. The distribution side includes 2 small 24/7 fuse banks, a switched analog fuse bank and a czone digital distribution system (coi).

The drawings are on my pc and I can work on them. I did not have the ac system and the dc system ground in the drawing and called the surveyor, so drawing probably needs updating,