AC DC grounding

The best solutions are always simple, but most often it was long and not straight way to get to them.

I agree with you, but in example in my area, I was told by one specific equipment seller, that I was the only private person ever asking about the isolation transformer for boat, and even many boats for proffessional use do not have them. My wild guess is that it is a rare animal on boats, especially on recreational boats. People do invest in inverter because of comfort/convenience (refrigireator, air condition unit, electric tools, etc.), but how many are ready to invest in isolation transformer because of safety/bonding/etc, how many boat users even have any clue about it?

Regarding standards, I think most of them have legacies which are difficult to let go, even when rewritten for relatively new applications. Could be that writers do not want to risk missing some legal issue from the past that could be overseen.

Obviously we are both coming from similar proffesion. Though, marine sphere is just my personal interest/hoby which I enjoy.

Just as a side note, seacocks on my boat are made of gunmetal (closest available to bronze that I found on the market) and left alone (unprotected).

And something I have already noted in one of the previous messages, mind the responsibility of unofficial advising about electrical installations!

 

Most people with Class A boats have isolation transformers, but uncommon in most recreational smaller boats.

 

You don't often see isolation transformers on recreational boats in the USA until you get up in size to where they pretty much run everything with AC, or higher DC voltage (48 V and up) systems. Most smaller recreational boats have a galvanic isolator. They are relatively cheap, easy for a boat owner to install, and take care of the DC leakage problem. But AC is being installed on smaller and smaller boats now, because owners want Air Conditioning and real refrigerators, Big screen TV's etc. To see this all you have to do is go to a boat show that has houseboats, or yachts much longer than 30 feet. I've even seen washers and dryers on houseboats. It's amazing the stuff manufacturers put into their boats now. Personally I'll stick with 12V DC.

 

An AC not bonded system is basing safety on the belief that the AC system is properly grounded. The neutral can have a potential to kill a person if the supply is not properly grounded. Even if it is bonded to ground at the entry in the marina, the resistance of the ground/water between the ground rod and the boat is enough to create a significant potential.

 

Not when RCD/GFCI protection is enabled.
How many times must it be repeated?
Those devices trip with an imbalance between hot and neutral.
The path(s) to "ground" do not matter whether it be you or a wire.
The only thing that matters is a difference in electron flow between hot and neutral, (black and white).

 

How about the AC ground from the outlet at the dock?

 

I believe that fairly recent legislation and or codes now require dockside outlets to have GFCI protection.
It's been far too long in coming.
This video may explain just what you're addressing.
As an aside, Kevin Ritz has been a friend of mine for well over 20 years, and he is a court certified witness for issues pertaining to ESD and electrical corrosion problems in the marine environment.
He is the one who "wrote the book" on these subjects, and has taught classes at the Coast Guard Academy, and "Teaches the teachers" for the ABYC electrical certification classes.

 

Yes the regulation exists, but is not enforced in most places. It will take a while.

 

If AC and DC grounds aren’t tied together on a boat, what happens during an AC fault—where does the fault current go, and will the breaker still trip, especially when systems like mini splits are involved?

 

Question is irrelevant.

If the AC system hot faults to the ungrounded (as above) DC system AND the AC system is ungoverned by a GFI, then the DC system has potential and can harm persons or devices.

It isn’t about the likelihood of it happening, but what happens if the system fault is NOT corrected by GFI.

So, just consider the post from @Nidza 11/15/22 here and read it. Here, he explains well the reason for the rule. It isn’t done so a corded appliance on a GFCI shorts to the DC system. It is done so any AC system short to DC results in a short to the AC system; not just GFCI outputs.

all the best

 

@protynan

In case you are trying to justify your system setup; it is a technical error here in the US.

While at first it seemed nicer to me to keep the DC and the AC systems floating free of each other; the proper way is to run a line between AC ground and DC. I ran into a lot of heat on Facebook over this issue, but floating the AC system is not allowed here; despite what Facebook warriors tell you.

 

Just use GFCI on all AC power on the boat, the problem of shock is solved.
GFCI legally and functionally does not need even need a safety ground wire to function 100%
No ground wire requires it to be labeled.

As soon as GFCI detects 5ma or more current flow difference outgoing to incoming, instantly is tripped off the AC power.
That means current is being siphoned off onto an alternative path than the wires, and your body might be the path.
Brilliant really to prevent electric shock and deaths.
I have almost all AC on my boat using GFCI breakers, including the AC heat pump and water heater, all outlets and built-in electric wall heaters.

You can actually do it using one GFCI shore power supply cord to the boat.
Disadvantage is you then have a little more work tracking down the AC current leak.
Flip off the breakers till it stops tripping off the GFCI, and you found the defective circuit.

GFCI can be combined with AFCI, arc faulting. Which means it can also test for arcing faults in wires and connections which can cause a fire.
All my GFCI breakers are combination with AFCI, except for one, cause I already had it.
But at the time I figured why not go for the best protection.

I hope someday all marina shore power pedestals are GFCI and AFCI
If your AC system is so bad it leaks current constantly, and you say I don't want this, then your AC system is hazardous.

 

You’ve missed the point. This is a discussion about tying the AC and DC grounding systems. If the AC system wire ahead of the GFCI chafes; the GGCI tripping does nothing.

 

The ABYC standard of connecting the 12V neg buss to the AC ground buss, (green wires,) dates back to the time when boats had quite rudimentary DC systems and shore power was an extension cord to run a battery charger and a few outlets, and if you had a bigger boat you might have a water heater.
In those days the engine was "Grand Central Station" for all the DC grounds, and provided a low resistance path to the water.
To keep people on the boat from being shocked they connected the AC ground buss to the DC ground buss for the same purpose, to provide a low resistance path to the water.
That was all well and good in the old days WHEN BOATS HAD ALL THEIR UNDERWATER METALS OF BRONZE, (highly noble).
Today boats are mix-masters of less noble metals all just waiting for a few electrons to start eating away at some fitting.
But as the vid I posted shows, putting AC into the water is not good if there is somebody in the water nearby, and sending electrons down your prop shaft doesn't help the underwater metals either.
Today we can do better, we have RCDs, GFCIs, and other devices.
To repeat, a GFCI does not need a ground to work, it senses imbalance between the black and white.
But we want the green wire, it's a SAFETY ground, it's the low resistance path back to the source of the power, (the dock,) should a GFCI fail in the "On" mode.
PUT A GFCI BREAKER ON THE DOCK POWER TO PLUG YOUR BOAT INTO, that's your first line of defense.
Use a GFCI breaker in the boat at the inlet of the AC, that's your second line of defense.
Put GFCIs on EVERY AC device in the boat, that's your third line of defense.
Keep your AC wiring up high and dry in the boat, don't run AC wiring in a bilge.
STOP ANY AC ELECTRONS FROM GETTING TO YOU OR THE WATER.
If you have to connect the AC and DC busses together for the surveyor, (usually for insurance purposes,) do it.
If you don't have to do it make your own decision.

 

It shuts off the AC power, then no shock hazard saving your life.
Are you referring to DC faults causing some kind of galvanic corrosion?