AC green to DC negative buss ?

[hjohnson]

So I’ll also take a contrarian position too much of what is posted here.

In my opinion, the Protective Earth conductor absolutely should be connected to earth, and thus also to your DC negative. However, to protect your underwater metal (shaft/prop) you should also have either a galvanic isolator or isolation transformer.

It all comes down to safety, both while alongside and connected to shore power, and while away from the dock running on inverter (if you have one).

Alongside, yes there is the potential for your boat to become the grounding rod for the whole marina. You do not want your prop to sacrifice itself to save your neighbour’s boat. This is what the isolator will protect against. Conversely, you want to be sure that PE is good, which is sometimes hard to know with the janky wiring in some marinas. Having your own solid connection to the ocean ensures that. You also want a quality ELCI on your power inlet, but that’s a separate topic. What you do not want alongside is your neutral bonded to PE.

Underway, you also want your PE at ground potential for the same reason you want it at home. You should always assume that your body is connected to earth. You do not want your body to be the lowest resistance way of completing a circuit.

Underway, assuming you have an inverter, you also want your neutral bonded to PE at a single point. On my boat this is handled by the bonding relay in my Victron, which is mutually exclusive from the shore power relay). Without this, you can wind up with a silent fault that suddenly makes the Neutral 120V with respect to earth. With everything bonded, any safety critical fault should trip a GFCI or ELCI.

So, anyhow, on my boat I have the PE connected to my negative DC connected to my starter negative connected to my engine and prop. We’ve had it setup this way for a year at this point, and when wen pulled the boat from the water last week, our zincs were in remarkably good condition. They’re actually now lasting longer than they used to, most likely due to the isolator.

Anyhow, that’s my $0.02.

[s/v Jedi]

Originally Posted by hjohnson

So I’ll also take a contrarian position too much of what is posted here.

In my opinion, the Protective Earth conductor absolutely should be connected to earth, and thus also to your DC negative. However, to protect your underwater metal (shaft/prop) you should also have either a galvanic isolator or isolation transformer.

You need to do more than voice an opinion: you need to support that with a case showing why, or at least a logic for the approach.

So on shore they ground the AC neutral, I get that. But you write that you need an isolation transformer or at minimum an isolator. When you do that, it means that you don’t have the connection to ground anymore, because you just isolated it. So you have opposite claims that don’t go together.

Also, why must a DC supply which, unlike cars, engines etc., uses two conductors for positive and negative…. be grounded? Your claim about safety makes no sense for this.

[Bowdrie]

Originally Posted by s/v Jedi

Also, why must a DC supply which, unlike cars, engines etc., uses two conductors for positive and negative…. be grounded?

You are correct, there is no reason at all for any DC to be "grounded" to the water.
Some electronics may, (as in maybe/perhaps/might,) want to see a discrete connection to something like a "Dynaplate", but that's a device specific installation, not a general thing.
Lightning?
You don't just connect your aluminum mast/compression post to a keel bolt or "bonding" system.
You get one of those special interrupter gizmos, (I've forgotten the name,) they are non-conductive until they are hit with massive voltage, then they conduct the electrons.
I use the cheap method.
As the mast steps on the keel about 6in. from a keel bolt I have a bracket with an electrode that has an air-gap of ~1/8".
If a strike occurs that gap will ionize and conduct into the keel.
This keeps the mast isolated from normal stuff.
Perfect? Of course not, nothing is perfect when lightning is in the mix.
You do what you can, (or can afford,) and move on to more important things like keeping-up the varnish.

[hjohnson]

Originally Posted by s/v Jedi

You need to do more than voice an opinion: you need to support that with a case showing why, or at least a logic for the approach.

So on shore they ground the AC neutral, I get that. But you write that you need an isolation transformer or at minimum an isolator. When you do that, it means that you don’t have the connection to ground anymore, because you just isolated it. So you have opposite claims that don’t go together.

Also, why must a DC supply which, unlike cars, engines etc., uses two conductors for positive and negative…. be grounded? Your claim about safety makes no sense for this.

So, my logic goes something like this:

1.) A boat floating in the saltchuck is floating on the single largest ground conductor on earth.
2.) The safest thing to assume on a boat is that your body (and other random things) is electrically connected to the water.
3.) You do not want to complete the circuit through your body, or if you do, you want safety equipment like a GFCI or ELCI to trip and protect you.

Now, you're going to say, "But hjohnson, because my boat's electrical system is isolated, you wont be completing a circuit through your body!" but can you be 100% sure of that? Even in my small 27' boat, there is a significant amount of electrical wiring running through bulkheads, through wiring harnesses, through lockers, through the bilge, and all sorts of other things. Can you be absolutely sure that, 5 years from now, it hasn't chafed a little somewhere, or otherwise made contact with something it shouldn't? are you ready to bet your life on that?

The good thing about isolated electrical systems is they let you have a "free fault" without killing you. The problem with isolated electrical systems is that "free fault" is silent, with no good way to detect it.

This is why while there is room made in the shoreside electrical code for isolated electrical systems in things like Operating Rooms, there is also a requirement for increased inspections and testing to ensure that the isolated electrical system is, in fact, isolated.

This is particularly bad if you are running a floating system where the PE and Neutral are bonded, and you wind up with a fault between your hot conductor and the hull/water/whatever. Suddenly everything connected to your PE is going to be hot. Grab the wrong thing, you're potentially going to be in a world of hurt as that PE is far more likely to be touchable. If you're running with 3 independent conductors (PE, N, and L) you're better off, but still you can have a silent fault that can bite you later.

If, instead, you have a bonded electrical system, those kinds of faults will trip relatively simple things like an ELCI.

In effect, my logic boils down to the fact that if there is a fault in my system, I want to know about it as soon as it happens. I do not want to find out about it later.

Now, as far as the galvanic isolator and/or isolation transformer goes, you're right in regards to the transformer. That breaks the PE on purpose, and makes your own PE independent of the shore PE. I don't have one of those, so haven't put much thought into it. The Galvanic Isolator, though, effectively puts something like 4 diode drops on the conductor in either direction. Basically if there's less than a 3 or 4 volt potential between the PE on your boat, and the PE on shore, no current will flow. This blocks any galvanic currents that could come from all the underwater metal on your boat being electrically connected to your neighbour's boat. It still allows a high voltage fault to conduct through it.

[hjohnson]

Originally Posted by Bowdrie

You are correct, there is no reason at all for any DC to be "grounded" to the water.

Except that you usually are through your engine. Unless your engine has both an isolated starter motor and an isolated alternator, your DC negative is going to be connected to the ocean. Even with a drivesaver, there's still an electrical connection through the heat exchanger and raw water pumps.

[Shanachie]

[QUOTE=
Also, why must a DC supply which, unlike cars, engines etc., uses two conductors for positive and negative…. be grounded? Your claim about safety makes no sense for this.[/QUOTE]


I'm not an electrician, just a cautious person.


As I understand it, 110 volt electricity can leak into the 12-volt system through problems in equipment that uses both, either through shore power or possibly a generator or inverter.



No path to ground, you get electrocuted.

[Bowdrie]

Originally Posted by hjohnson

Except that you usually are through your engine. Unless your engine has both an isolated starter motor and an isolated alternator, your DC negative is going to be connected to the ocean. Even with a drivesaver, there's still an electrical connection through the heat exchanger and raw water pumps.

You're grasping at straws and minutia.
The electrical connection from a seacock feeding thru a water pump into a heat exchanger is quite tenuous, and highly overrated.
It's a "nonstarter" issue, but people latch on to it as though it was serious.
Where are the electrons? =Where are they present.
Where are they coming from? =What is the source.
Where are they going? =What is the path.
How many of them are there? =what is the amperage.
How hard are they pressing? =What is the voltage.
How long will they be there? =Is it intermittent or continuous.
When there are zero electrons from any source present on the engine, (or anything else,) there are no electrons to go anywhere.
It takes two to tango, no source=no flow.
Cranking a starter for a few seconds?
Look at the bird sitting on the high-tension wire.
Why isn't it shocked?
There isn't any path for the electrons to flow thru its body.
It's the same on a boat.
Keep the D**n electrons where they belong.
Let them leave a battery, go thru a circuit to operate a device and go back to the battery.
Making all the metallic objects in your boat some kind of "ground sink", or the desire to present a "common reference" to ground becomes an easy way to create issues that need not be.

[hjohnson]

Originally Posted by Bowdrie

You're grasping at straws and minutia.
The electrical connection from a seacock feeding thru a water pump into a heat exchanger is quite tenuous, and highly overrated.

A 1/2" column of seawater, which is flowing through to the engine has a reasonably low resistance, obviously nowhere near as good as a wire, but not insignificant. Especially true when you start looking at stray currents and everything else. By bonding everything to a big hunk of zinc, I absolutely know where my sacrificial anode is. But more to the point, my engine is permanently connected to my negative battery bus anyways by the 2awg conductor needed to make my starter motor work. The engine, in turn, is also connected to my prop shaft, and prop through the transmission. That's just the way the damned thing is built, and I'm fine with that. I just designed my electrical system such that my zincs will be dissolving first to protect the rest of the system, as it should be.

Originally Posted by Bowdrie

Making all the metallic objects in your boat some kind of "ground sink", or the desire to present a "common reference" to ground becomes an easy way to create issues that need not be.

I would argue the complete opposite. Having all sorts of different potentials and possible stray currents makes it really hard to figure out what's causing you problems. Tying it all together, and having a known sacrificial anode, means you don't have to look anywhere else.

[Bowdrie]

It's Ok, some boats are built/wired such that the electrons have to stay within the chosen path and not infect the boat.
And should some electrons stray from the righteous path the source is easily found and delt with.
Other boats are built/wired as an homogenized mass, (a pea soup,) if you will.
Then it becomes more difficult to identify the culprit, so the answer becomes, "Put on more zincs".
Over the years untold/countless threads/posts on this forum have been, (paraphrasing,) "My zincs are going away, and I have no idea why".
And tracking down the problem becomes a nightmare of detective work that only Chief Inspector Clouseau could solve.

[s/v Jedi]

Originally Posted by hjohnson

So, my logic goes something like this:

1.) A boat floating in the saltchuck is floating on the single largest ground conductor on earth.
2.) The safest thing to assume on a boat is that your body (and other random things) is electrically connected to the water.
3.) You do not want to complete the circuit through your body, or if you do, you want safety equipment like a GFCI or ELCI to trip and protect you.

Now, you're going to say, "But hjohnson, because my boat's electrical system is isolated, you wont be completing a circuit through your body!" but can you be 100% sure of that? Even in my small 27' boat, there is a significant amount of electrical wiring running through bulkheads, through wiring harnesses, through lockers, through the bilge, and all sorts of other things. Can you be absolutely sure that, 5 years from now, it hasn't chafed a little somewhere, or otherwise made contact with something it shouldn't? are you ready to bet your life on that?

The good thing about isolated electrical systems is they let you have a "free fault" without killing you. The problem with isolated electrical systems is that "free fault" is silent, with no good way to detect it.

This is why while there is room made in the shoreside electrical code for isolated electrical systems in things like Operating Rooms, there is also a requirement for increased inspections and testing to ensure that the isolated electrical system is, in fact, isolated.

This is particularly bad if you are running a floating system where the PE and Neutral are bonded, and you wind up with a fault between your hot conductor and the hull/water/whatever. Suddenly everything connected to your PE is going to be hot. Grab the wrong thing, you're potentially going to be in a world of hurt as that PE is far more likely to be touchable. If you're running with 3 independent conductors (PE, N, and L) you're better off, but still you can have a silent fault that can bite you later.

If, instead, you have a bonded electrical system, those kinds of faults will trip relatively simple things like an ELCI.

In effect, my logic boils down to the fact that if there is a fault in my system, I want to know about it as soon as it happens. I do not want to find out about it later.

Now, as far as the galvanic isolator and/or isolation transformer goes, you're right in regards to the transformer. That breaks the PE on purpose, and makes your own PE independent of the shore PE. I don't have one of those, so haven't put much thought into it. The Galvanic Isolator, though, effectively puts something like 4 diode drops on the conductor in either direction. Basically if there's less than a 3 or 4 volt potential between the PE on your boat, and the PE on shore, no current will flow. This blocks any galvanic currents that could come from all the underwater metal on your boat being electrically connected to your neighbour's boat. It still allows a high voltage fault to conduct through it.

Well, right now I will bet on being isolated but in 5 years I’m only as sure as you are that your bonding wire is still there. Those aren’t valid arguments, anything can happen.

So assume you are at anchor, using your genset or inverter. What does the seawater give you? Nothing. Your whole case is purely about shore power, nothing else. Only shore power is grounded, all other power sources aboard don’t have a return path via ground.

[s/v Jedi]

Originally Posted by Shanachie

I'm not an electrician, just a cautious person.


As I understand it, 110 volt electricity can leak into the 12-volt system through problems in equipment that uses both, either through shore power or possibly a generator or inverter.



No path to ground, you get electrocuted.

So the 120V hot gets connected to your DC positive. How is the seawater gonna save you?

Another example: your inverter hot connects to dc negative, one scenario you describe. How is this gonna bite you, how does the circuit gets completed? With one hand you touch the engine block and with the other hand? You open a breaker box and grab the neutral? It’s just not possible, there are magic tricks more likely to happen for real than this.

[hjohnson]

Originally Posted by s/v Jedi

Well, right now I will bet on being isolated but in 5 years I’m only as sure as you are that your bonding wire is still there. Those aren’t valid arguments, anything can happen.

So assume you are at anchor, using your genset or inverter. What does the seawater give you? Nothing. Your whole case is purely about shore power, nothing else. Only shore power is grounded, all other power sources aboard don’t have a return path via ground.

It's comparatively easy to test for a good solid ground connection. Plug tester, a couple of checks, and it's done. Plus I can easily inspect that the bonding is there, as I know precisely where it is. In my case, the neutral-earth bond is a monitored relay in the Victron. The PE (and Neutral) bond to DC and thus the water, is a 2/0 conductor between the chassis of the victron and the negative bus. That's not going to fail any time soon.

[s/v Jedi]

Originally Posted by hjohnson

It's comparatively easy to test for a good solid ground connection. Plug tester, a couple of checks, and it's done. Plus I can easily inspect that the bonding is there, as I know precisely where it is. In my case, the neutral-earth bond is a monitored relay in the Victron. The PE (and Neutral) bond to DC and thus the water, is a 2/0 conductor between the chassis of the victron and the negative bus. That's not going to fail any time soon.

Testing for isolation is just as easy as testing for conductivity, it is done with the same multimeter at the same setting.

The neutral to ground relay inside the inverter has nothing to do with this. It simply connects the inverter neutral output to the ground conductor. In the setup as done in most of the world you still have all that… you just don’t connect it to metal plates under the boat.

I can easily verify that I don’t have a grounding plate under my boat and one isn’t gonna grow there by itself anytime soon

[hjohnson]

Originally Posted by s/v Jedi

Testing for isolation is just as easy as testing for conductivity, it is done with the same multimeter at the same setting.

The neutral to ground relay inside the inverter has nothing to do with this. It simply connects the inverter neutral output to the ground conductor. In the setup as done in most of the world you still have all that… you just don’t connect it to metal plates under the boat.

I can easily verify that I don’t have a grounding plate under my boat and one isn’t gonna grow there by itself anytime soon

Ok, but now you’ve broken a seacock, or your raw water pump has been spraying on deck or whatver. Are you still safe? That’s the problem. It’s a heck of a lot easier, and no harm/no foul to underwater metal if you just run it grounded.

There is a very good reason as to why shoreside electrical codes are built this way.

[s/v Jedi]

Originally Posted by hjohnson

Ok, but now you’ve broken a seacock, or your raw water pump has been spraying on deck or whatver. Are you still safe? That’s the problem. It’s a heck of a lot easier, and no harm/no foul to underwater metal if you just run it grounded.

There is a very good reason as to why shoreside electrical codes are built this way.

No, there is no problem with seawater on deck or inside the boat. Even if there was, as the AC is not connected to it, there is no problem. You must envision that you always need to complete a circuit, through your body, before electrocution can take place.

The more stuff you connect AC power to, the more things can go wrong.