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Old 04-05-2016, 07:37   #1
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Shore Power Grounding Question - No Inboard Engine

I'm installing shore power on Pelican. She has an existing DC system with solar. She has no inboard engine - just an outboard.

Question is where to tie the ground (green wire).

I've done a bunch of searching and reading and just about everything I've seen says to tie the AC safety ground (green wire) to the DC ground at the inboard engine (and hence to sea water through the prop shaft). Of course, Pelican does not have an inboard engine - so where to tie the AC and DC grounds? Or should they be tied together at all?

There is an (unused) dynaplate installed on the boat, but I don't think that is appropriate for grounding the electrical system.

I'm inclined to just leave the grounds separate - so the AC ground will be through the shore power. I would never leave the shore power plugged in when I was not on the boat. The only place the AC and DC systems might possibly interact is at the charger/inverter.

I'll add another question here if I may... If I leave the AC and DC grounds separate (floating), do I need a galvanic isolator? My impression from my reading is that I would not.

Thanks!
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Old 04-05-2016, 08:28   #2
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Re: Shore Power Grounding Question - No Inboard Engine

Quote:
Originally Posted by polaris2.11 View Post
I'm installing shore power on Pelican. She has an existing DC system with solar. She has no inboard engine - just an outboard.

Question is where to tie the ground (green wire).

I've done a bunch of searching and reading and just about everything I've seen says to tie the AC safety ground (green wire) to the DC ground at the inboard engine (and hence to sea water through the prop shaft). Of course, Pelican does not have an inboard engine - so where to tie the AC and DC grounds? Or should they be tied together at all?

There is an (unused) dynaplate installed on the boat, but I don't think that is appropriate for grounding the electrical system.

I'm inclined to just leave the grounds separate - so the AC ground will be through the shore power. I would never leave the shore power plugged in when I was not on the boat. The only place the AC and DC systems might possibly interact is at the charger/inverter.

I'll add another question here if I may... If I leave the AC and DC grounds separate (floating), do I need a galvanic isolator? My impression from my reading is that I would not.

Thanks!
Polaris,

In general, keep the grounds isolated on your boat AC and DC systems.

But it can become more complicated than that should you add an inverter, depending upon your battery charger, etc.

And don't take my word for it: get it from an authoritative source. The best in my opinion is Nigel Calder's Boat Owners Mechanical and Electrical Guide.

It is much easier than wading through ABYC recommendations...

Best wishes with your project, and don't get ahead of yourself without reading the correct way to wire things for your present and future circumstances...

Cheers!

Bill
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Old 04-05-2016, 11:37   #3
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Re: Shore Power Grounding Question - No Inboard Engine

Thanks Bill -

I have Don Casey (who does not address this directly), but have not yet consulted Calder. Thanks for the tip.

My take also is that it is best to keep the AC and DC grounds isolated - though they do come together, in some sense, in the charger/inverter.

As I mentioned, there is a charger/inverter in my system. How more complicated? The wiring diagrams from the mfg look very simple.

specs...
(Fleet Power 1000-12) with Xantrex Link 1000. I know this equipment is older and not necessarily well thought of, but would like to use them as they in new condition and I have them on hand.
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Old 04-05-2016, 12:34   #4
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Re: Shore Power Grounding Question - No Inboard Engine

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Originally Posted by polaris2.11 View Post
Thanks Bill -

I have Don Casey (who does not address this directly), but have not yet consulted Calder. Thanks for the tip.

My take also is that it is best to keep the AC and DC grounds isolated - though they do come together, in some sense, in the charger/inverter.

As I mentioned, there is a charger/inverter in my system. How more complicated? The wiring diagrams from the mfg look very simple.

specs...
(Fleet Power 1000-12) with Xantrex Link 1000. I know this equipment is older and not necessarily well thought of, but would like to use them as they in new condition and I have them on hand.
Lets start this conversation over. I wasn't clear and didn't give due diligence to separately discussing the AC and DC grounds vs. the AC Neutral wire bonded to Ground on shore, but NOT on the boat... Therefore, please ignore my earlier response and I will start over here. [We can chalk it up to allergy season and the required meds... I apologize...]

First, the shore ground [green] wire should be bonded to the boat's DC ground, which should also be bonded to the water the boat is floating in. [e.g., via prop, metal through hull, underwater bonding plate, etc.] The path to ground is the same, and if the shore ground were to fault, your ground to water would be a back-up. [This is where a galvanic isolator is useful to help prevent stray currents leading to premature anode consumption...]

It is the AC NEUTRAL [white] wire on the boat wiring side that should NOT be connected to either the AC or Boat ground wires. This is different from what is done in our homes where the white and green wires are bonded together.

If you wire shore power to pass through a typical inverter, it will keep the AC neutral and ground separated when shore power is hot.

I hope this helps clarify and I apologize for any confusion caused by my earlier post.

Like I said in that post, don't take my word for it. Consult the authorities.

Cheers!

Bill
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Old 04-05-2016, 12:55   #5
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Re: Shore Power Grounding Question - No Inboard Engine

Understood and thanks again. I understand the difference between the neutral (white) wire and the green wire. We're talking green wire here and I know white and green should be kept separate on a boat.

Your advice is consistent with what I have been reading, but didn't know that a through hull would be an adequate ground.

I do have a bronze through hull I could use. I also have a dynaplate - currently not connected to anything, that might work, but I seem to recall reading somewhere that was not advisable.

If the green and black are bonded together and to the water, I understand that I need to install a galvanic isolator.

I will consult Calder. I am also consulting with a local marine electrician.

All that said, I wonder about the specific risks with just leaving the AC (green) and DC (black) grounds isolated. Such an approach would seem to simplify things and I don't have a good grasp on the downside, other than that it might not be as safe somehow. Not to cheat, just wanna know why.

e
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Old 04-05-2016, 13:24   #6
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Re: Shore Power Grounding Question - No Inboard Engine

Hi Polaris,

I'll embed some comments within your message, below...

You are doing the right thing by making sure before proceeding...

Calder also has a good section on testing grounds, etc. with a VOM...

Cheers!

Bill

Quote:
Originally Posted by polaris2.11 View Post
Understood and thanks again. I understand the difference between the neutral (white) wire and the green wire. We're talking green wire here and I know white and green should be kept separate on a boat.

Your advice is consistent with what I have been reading, but didn't know that a through hull would be an adequate ground.

If the bronze is in direct contact with the water it would be... [i.e., not painted...]

I do have a bronze through hull I could use. I also have a dynaplate - currently not connected to anything, that might work, but I seem to recall reading somewhere that was not advisable.

I suspect you could use the dynaplate, but not if it is in use by your SSB/HAM radio... [Or you may want to save that for that purpose in the future...]

If the green and black are bonded together and to the water, I understand that I need to install a galvanic isolator.

You could get by without a galvanic isolator or isolation transformer, but risk the occasional stray current anode corrosion potential...

I will consult Calder. I am also consulting with a local marine electrician.

All that said, I wonder about the specific risks with just leaving the AC (green) and DC (black) grounds isolated. Such an approach would seem to simplify things and I don't have a good grasp on the downside, other than that it might not be as safe somehow. Not to cheat, just wanna know why.

My understanding is primarily so if an AC appliance on the boat [e.g., faulty battery charger, etc.] has an AC leak to the boat ground, and that ground is not connected to the shore ground, that leak goes into the water via the DC ground, and also into all the DC grounded equipment onboard. That makes for a potential hazard onboard and for anyone in the water nearby...

A secondary reason has to do with lightening protection and keeping the AC and DC grounds at the same voltage potential. I don't know how critical that is in the scheme of things...

I think there are also some additional reasons, but I don't recall the specifics off the top of my head...


e
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Old 04-05-2016, 14:34   #7
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Re: Shore Power Grounding Question - No Inboard Engine

Thanks again Bill. I had pretty much the same (though more vague) understanding as you wrt the downsides of leaving the green and black wires isolated from each other and the water.

That said, it seems the risk of a short to the case of the battery charger is pretty small and there really isn't anywhere else the AC and DC systems might meet on my boat - and does anyone really know what lightning might or might not do? I thought the risk to swimmers would be LESS if nothing was connected to the water.

It just seems a little weird to me to bond them together... after all it is AC and DC... and I recall reading that there are downsides (like the corrosion issue) to tying the green and black wires together - hence the need for galvanic isolators, etc.

For what it is worth, the only metal Pelican normally has below the water line is the one bronze through hull (the other is marelon), the dynaplate, and the stainless gudgeons/pintels for the transom/skeg hung rudder.

I know there must be other experience and expertise out there... anyone else wanna chime in? Second (and third) opinions encouraged.
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Old 04-05-2016, 22:29   #8
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Re: Shore Power Grounding Question - No Inboard Engine

if you don't have a main DC ground bus then I would just connect to the battery neg. ideally you have a ground bus bar. and from here the batteries, engine neg, bonding cables, ac ground etc all join.

I never use the engine as a ground point. it should have one wire only going to the main ground bus.
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Old 05-05-2016, 05:25   #9
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Re: Shore Power Grounding Question - No Inboard Engine

smac -

I do have a DC neg bus bar. My DC system is well put together, except that there was never a bond to the sea. This boat has never had an inboard or shore power before. I'm just adding shore power now.

I found copies of Calder and a similar book by Charlie wing nearby, and sunk some time last night into reading up. I instantly went from too little info to too much!

I can't say I was able to wrap my head around all of the whys and wherefores in one evening's cram session, but my takeaway is that yes, I really do need to bond my AC and DC groundING (not current carrying) conductors - as well as any other under water metal bits - together and to the sea - and I even understand how to do this thanks to y'all and Mr. Calder and Wing. It is not complicated to DO, it just seems complicated to understand WHY you need to - but I guess I am past that hangup now.

So, as I mentioned, there is only the dynaplate and the one bronze through hull (apart from the rudder gudgeons/pintles). I can bond those things together with a non current carrying bus using 6 AWG green wire and then bond my non current carrying AC and DC green and black wires to that.

Oh, and, optimally, there will be a galvanic isolator, or better, an isolation transformer to further protect nearby people and boats - including me and mine. I can't say I fully comprehend this either, but I guess the peace of mind is worth a few hundred bucks and doesn't add that much complication.

Sound right?

So, thanks again to Bill and smac...

...and the invitation is still open to further enlighthen me. If there is a simpler way to explain things than Wing and Calder, I'd love to hear it. I'll be digging back into those wonderful tomes as well....
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Old 05-05-2016, 09:18   #10
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Re: Shore Power Grounding Question - No Inboard Engine

Im my present marina, and in most of the places I've visited, the 110 socket provided to the boat is a "split" circuit - that is a 220 circuit with a neutral ground.

When one side of the circuit is more heavily loaded than the other, the neutral ground "drifts" away from the safety ground (green). Often this is sufficient to cause the ground fault interrupter (if installed) at the marina to trip.

I remove the ground pin from the 3-wire connector when I plug in, thus lifting the boat's neutral from safety without tripping the GFI. I re-establish safety ground by connecting the earth ground on the boat (engine, keel bolt, whatever you use to establish earth) to the green side of the a/c circuits.

Since you don't have control of the neutral drift problem at the marina, you either do this or lack a/c power on your boat - assuming the marina won't re-wire for you :-)
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Old 05-05-2016, 23:17   #11
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Re: Shore Power Grounding Question - No Inboard Engine

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Originally Posted by polaris2.11 View Post

...and the invitation is still open to further enlighten me. If there is a simpler way to explain things than Wing and Calder, I'd love to hear it. I'll be digging back into those wonderful tomes as well....
When you travel from marina to marina and plug in shore power you cannot rely on the shore ground. It is usually there but not always - and the AC items on the boat will work without the ground so there is not any indication of ground connection.

ABYC wants the AC ground (green) connected to DC negative so if there is a fault in the absence of a shore ground the current has a path to earth.

The problem with this connection is that when plugged into shore power you are connected through the ground wire to every boat on the same circuit. Their wiring problems and possible lack of zincs become yours, and your issues become theirs.

A galvanic isolator is a series of 2 diodes in each direction. It blocks about 1.2 volts of DC current on the ground wire in both directions. This is the current that causes corrosion of bonded underwater metal - shaft, seacocks, rudder attachments, etc. In the case of an AC fault the galvanic isolator passes it to shore.

An isolation transformer goes much farther by eliminating any physical connection to shore power.

A galvanic isolator is a very good idea. In a fiberglass boat as simple as yours an isolation transformer is not necessary.

Does your inverter have a chassis ground? If it does and it is connected to DC negative as required you already have a connection between DC negative and AC ground, as the inverter chassis is connected internally to AC ground.

Hope this helps.
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Old 06-05-2016, 05:57   #12
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Re: Shore Power Grounding Question - No Inboard Engine

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When you travel from marina to marina and plug in shore power you cannot rely on the shore ground. It is usually there but not always - and the AC items on the boat will work without the ground so there is not any indication of ground connection.

ABYC wants the AC ground (green) connected to DC negative so if there is a fault in the absence of a shore ground the current has a path to earth.

The problem with this connection is that when plugged into shore power you are connected through the ground wire to every boat on the same circuit. Their wiring problems and possible lack of zincs become yours, and your issues become theirs.

A galvanic isolator is a series of 2 diodes in each direction. It blocks about 1.2 volts of DC current on the ground wire in both directions. This is the current that causes corrosion of bonded underwater metal - shaft, seacocks, rudder attachments, etc. In the case of an AC fault the galvanic isolator passes it to shore.

An isolation transformer goes much farther by eliminating any physical connection to shore power.

A galvanic isolator is a very good idea. In a fiberglass boat as simple as yours an isolation transformer is not necessary.

Does your inverter have a chassis ground? If it does and it is connected to DC negative as required you already have a connection between DC negative and AC ground, as the inverter chassis is connected internally to AC ground.

Hope this helps.
The decision between an isolation transformer vs. a galvanic isolator has nothing to do with type or size of boat. It most always comes down to $$.

A galvanic isolator requires monitoring to make sure it's working and they can fail.

An isolation transformer creates a new power source that is completely isolated from any/all grounding issues commonly found with shorepower. The shorepower green safety stops at the IT and never touches the boat wiring.
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Old 06-05-2016, 06:21   #13
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Re: Shore Power Grounding Question - No Inboard Engine

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Originally Posted by captstu View Post
Im my present marina, and in most of the places I've visited, the 110 socket provided to the boat is a "split" circuit - that is a 220 circuit with a neutral ground.

When one side of the circuit is more heavily loaded than the other, the neutral ground "drifts" away from the safety ground (green). Often this is sufficient to cause the ground fault interrupter (if installed) at the marina to trip.

I remove the ground pin from the 3-wire connector when I plug in, thus lifting the boat's neutral from safety without tripping the GFI. I re-establish safety ground by connecting the earth ground on the boat (engine, keel bolt, whatever you use to establish earth) to the green side of the a/c circuits.

Since you don't have control of the neutral drift problem at the marina, you either do this or lack a/c power on your boat - assuming the marina won't re-wire for you :-)
I believe you may have other issues if you are tripping the dockside ELCI/RCD at a marina.

No electrician would wire a ELCI/RCD ahead of a split-phase shared neutral circuit. The second two appliances are plugged into both phases, the ELCI/RCD would trip.

Neutral drift is a sign of poor neutral/ground connection and/or extremely long wire runs. Regardless, neutral drift will not trip properly installed ELCI/RCD.

A panel mounted ELCI/RCD measures the difference in current flow between neutral and hot and trips when that difference is (typically) 30ma. If this is happening, you have a fault that is finding the secondary return to the source causing the imbalance in current flow.
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Old 06-05-2016, 07:47   #14
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Re: Shore Power Grounding Question - No Inboard Engine

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The decision between an isolation transformer vs. a galvanic isolator has nothing to do with type or size of boat. It most always comes down to $$.

A galvanic isolator requires monitoring to make sure it's working and they can fail.
I think on a boat with an outboard tipped out of the water and one metal through hull as the op has a galvanic isolator is sufficient.
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Old 06-05-2016, 08:07   #15
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Re: Shore Power Grounding Question - No Inboard Engine

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I think on a boat with an outboard tipped out of the water and one metal through hull as the op has a galvanic isolator is sufficient.
I agree with your point if talking about galvanic isolation only. The human safety factor of an IT is the primary benefit (not having to rely on Marina for safety ground).
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