AC DC grounding

[sdowney717]

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.

The green AC ground is the minimal and last line of defense for sure.
At least all AC wires have such a wire, so it's no big deal to hook it up as a mechanical ground.
GFCI definitely is an amazing device. The newer ones of the last few years auto test themselves even continuously. If the self test fails, then they turn off the power.
All mine are self testing, and they also have the manual test button.

These things have logic circuits, and built in firmware, they are aware when something is not right.

The AFCI GFCI Combo breakers are also tons better than when they first came out years ago, They are smart, they look at the AC waveform to determine if wires are arcing. I have not had any combo breakers shut off when running AC brushed motors.
Ss the GFCI protects your life, and AFCI prevents electrical fires caused by arcing wires.
My combo breakers are Square-D.

You can get an arc with loose screw connections to devices using AC power. The arc creates heat, and also points to high resistance connections, these things get so hot they cause things to smoke and burn.

An ARC Welder is a good example of a high current arc, and it is so hot it melts metals. AC wire arcing is a similar thing, just not as much current is flowing, but could flow 30 amps.

 

[fallguy]

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?

It shuts off AC power?

A GFCI breaker shuts off AC power from the breaker to everything after it, but nothing before. Same for outlet protected circuits. All lines after the outlet are protected, but not the line leading to the outlet.

This thread is an attempt to understand the rationale behind combining the grounding systems, not to argue against it. As I said earlier, my surveyor said it was to prevent any part of the DC system from becoming energized; ever, not just during normal use.

So, in the case of the AC inverter, the inverter produces AC power and that hot goes to a breaker panel. On the off chance that hot line is chafed and touches the DC system; if the DC grounds are not tied to the AC ground; there is nothing to turn off the system. And same for incoming loads unless of course they are GFCI protected. My boat also has an ELCI which I understand is designed to shut off if a current leak is detected. But I don’t believe it would detect a fault from the inverter. Perhaps I’m wrong and the ELCI would detect that fault as well. The surveyor did not care and said the rule is the rule.

At any rate, all we were trying to do is understand the reason for the requirement. I don’t believe the use of GFCI outlets is sufficient because the AC system would be live up to the outlet. And so, the AC system chafing to the DC system is possible. And the rule is to prevent ~any and all~ AC potential from entering the DC system, not just a chafed cord downline from a GFCI outlet.

It is also possible the inverter running onto a DC line may shut off the entire boat at the ELCI, but I’m not about to test it on $1500 equipment. I don’t think the inverter output is GFCI output because stacking GFCIs generally fails. And not all boats are equipped with ELCI which brings us back to the rule’s purpose again.

At any rate; there is no literature providing a purpose to float the two grounding systems either. So, if you don’t have a reason to float them other than your insistence it does not matter; it isn’t really a good reason eh? And we are getting into the hair splitting business of the ABYC electrical rules committee (it has a better name) without any credential to support it. Even a licensed electrician would not do well in that group.

 

[rangebowdrie]

The last time I read post #1 their was no mention of inverters.
As I wrote, the GFCI breaker ON THE DOCK is your first line of defense, when it trips their is no shock potential downstream.
One can argue "What Ifs" forever, at some point one needs to discern what is and what isn't important.
What's important is to keep electrons (AC or DC,) away from underwater metals and people.
Don't be this guy.

 

[fallguy]

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

~not a silly meme

 

[rangebowdrie]

Their isn't any single "Definitive" answer.
It's similar to; "It depends upon what your definition of "Is" is".
You're putting "grounding" and "bonding" and AC and DC all in the same mix, and no single answer covers all the bases for them, (now throw a lightning strike or an inverter into the mix and we've got another can of worms).
And even the so-called "experts" disagree on points.

 

[sdowney717]

It shuts off AC power?

A GFCI breaker shuts off AC power from the breaker to everything after it, but nothing before. Same for outlet protected circuits. All lines after the outlet are protected, but not the line leading to the outlet.

This thread is an attempt to understand the rationale behind combining the grounding systems, not to argue against it. As I said earlier, my surveyor said it was to prevent any part of the DC system from becoming energized; ever, not just during normal use.

So, in the case of the AC inverter, the inverter produces AC power and that hot goes to a breaker panel. On the off chance that hot line is chafed and touches the DC system; if the DC grounds are not tied to the AC ground; there is nothing to turn off the system. And same for incoming loads unless of course they are GFCI protected. My boat also has an ELCI which I understand is designed to shut off if a current leak is detected. But I don’t believe it would detect a fault from the inverter. Perhaps I’m wrong and the ELCI would detect that fault as well. The surveyor did not care and said the rule is the rule.

At any rate, all we were trying to do is understand the reason for the requirement. I don’t believe the use of GFCI outlets is sufficient because the AC system would be live up to the outlet. And so, the AC system chafing to the DC system is possible. And the rule is to prevent ~any and all~ AC potential from entering the DC system, not just a chafed cord downline from a GFCI outlet.

It is also possible the inverter running onto a DC line may shut off the entire boat at the ELCI, but I’m not about to test it on $1500 equipment. I don’t think the inverter output is GFCI output because stacking GFCIs generally fails. And not all boats are equipped with ELCI which brings us back to the rule’s purpose again.

At any rate; there is no literature providing a purpose to float the two grounding systems either. So, if you don’t have a reason to float them other than your insistence it does not matter; it isn’t really a good reason eh? And we are getting into the hair splitting business of the ABYC electrical rules committee (it has a better name) without any credential to support it. Even a licensed electrician would not do well in that group.

My opinion is my own, I actually like what I have so far on the boat for my AC system..
GFCI breakers on my boat protect only the branch circuits.
The shore cord is not GFCI, but it could be GFCI.
I have a GFCI power cord, when I plug that into the dock and then into the boat, not tripping. Maybe I ought to just use that and not that thick yellow 30 amp shore power cord.
There is one worry for me though, what if my fridge nuisance trips off? That could be bad.
So it's a trade of good and bad.

I agree that the shore power cord is a risk, I had one get damaged, got pinched between boat and dock.
AC current flowed into the salt water and when I cut the cable open, the current had eaten a hot wire, about 6 feet worth of copper wire disintegrated.

I said earlier, I wish the shore power pedestal had the GFCI breaker, then everything downstream of it is protected. Honestly, I don't know why it is not illegal to gave unprotected shore power on a dock.

An ELCI breaker is not going to save your life. It is for equipment protection only. The trip off amps rating is too high (like 100 ma) to prevent you from being killed when there is a fault in the wires

 

[sdowney717]

I have an AC inverter. I can using my DBDT relay switch box use it to power the boat.
I have it because, I don't like the idea of running the generator for light duty AC loads.

All the GFCI-AFCI work with it.
And this inverter is not a full sine wave inverter.
The Modified sine inverters actually put power on the hot and neutral wires, 60 volts on each wire. They do not have a true ground.
But the Square-D GFCI do work fine with it. GFCI don't care about grounds, they measure incoming and outgoing current on the hot and neutral wire. If the currents differ by 5 ma or thereabouts, they flip off the AC power. If power differs that means power is leaking somewhere, and that is bad.

In my experimenting with the MSW inverter, some GFCI outlet brands buzzed, the Square-D do not buzz.
Hubbell was ok, no buzz.
Leviton I think worked no buzz.
Not certain about GE, may have buzzed, Pass and Seymour buzzed.
I do recall the buzzy noise I thought odd a designed GFCI would buzz on MSW inverters. I may have even contacted them to complain.
An AC inverter can kill you dead just as easily as shore grid power. You would want a GFCI to work with them.
The buzzing was annoyingly loud enough to hear, and disconcerting, who wants to trust that.

But that info is over 5 years old when I was testing things.

Some MSW inverters have better wave forms than others.
I would maybe today buy a full sine wave inverter instead of a modified one.

 

[fallguy]

You seem to be going through a lot of steps to justify floating the two systems. But that wasn’t the assignment. I was trying to understand the reasons FOR the rule, not enter the committee debate room over reasons against.

If dock power is not protected by GFCI and the AC line is chafed to the DC system, then the DC system can be energized by the AC.

And go…

 

[sdowney717]

You seem to be going through a lot of steps to justify floating the two systems. But that wasn’t the assignment. I was trying to understand the reasons FOR the rule, not enter the committee debate room over reasons against.

If dock power is not protected by GFCI and the AC line is chafed to the DC system, then the DC system can be energized by the AC.

And go…

Sure could!
Do you dock a boat?
Is the shore power GFCI to the boat?

Seems to me, the whole idea of a 'ground' wire is purely for human safety reasons, as in an aide to preventing shock deaths.
It is called a safety ground, but a safety ground using just wires requires the wiring system to remain intact and also not be compromised by water!
Which on boats, well, they get wet on the inside pretty easy.
Safety grounds to trip a breaker off, has to have the full current flow. The standard breaker is there only to protect wires, not lives.
The system without GFCI is definitely not very safe is it, in that just a tiny amount of current flowing through you can kill. Having a green safety ground wire, can kill you, on a boat, as can joining AC and DC grounds, if there is no GFCI to shut off the power.

A compromised AC hot wire to ground, with high resistance, won't trip a standard breaker, and then you touch connected metals, if your body can conduct that small current back to the source, your going to feel it. I have been shock bunches of times over my decades of life. But not yet with 240vac.

One time before I put GFCI breaker on the AC cruisair, I reached down to feel the water temp coming from the cruisair outlet flow, and wow shocked!
Opened it up, and the electrical box, the AC pump wire was wet, so AC current was leaking into the water flowing through the heatexchanger and discharged out the boat.

I think rain water had flowed down some wire, or tube, and gotten into the electric box. A GFCI would has seen that and not allowed power to the AC system.

 

[fallguy]

Take it up with the ABYC electrical committee or get back to the woody!