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Old 30-04-2014, 17:42   #1
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Grounding

Power question. I have a big 3000w inverter and 420 amp hour battery's. When I am at the dock the boat is grounded through the shore power cord. When I'm away from the dock, where does the ground current go? Should I be dropping something on the water (fresh water) to better ground? In a residential situation there is a copper rod driven into the ground and connected to the panel box.
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Old 30-04-2014, 18:17   #2
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Re: Grounding

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Originally Posted by amanphoto View Post
Power question. I have a big 3000w inverter and 420 amp hour battery's. When I am at the dock the boat is grounded through the shore power cord. When I'm away from the dock, where does the ground current go? Should I be dropping something on the water (fresh water) to better ground? In a residential situation there is a copper rod driven into the ground and connected to the panel box.
What do you need a ground for? You are out there all on your own, self contained. An SSB and perhaps lightning protection would be your only need for a ground. Then of course you have your Zink..
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Old 30-04-2014, 18:20   #3
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Re: Grounding

There should never be _any_ current on ground. It's there for safety. So it pretty much doesn't matter electrically.

Now bonding to thru-hulls and connecting to the 12v system is a different question. (Which has multiple opinions).
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Old 30-04-2014, 18:22   #4
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Re: Grounding

No. There is no such thing as "ground current" in the context of the inverter output. The inverter will most probably have what is referred to as a "floating earth". There are arguments to running an earth wire to, well, the Earth but the cons would at least match the pros, if not exceed them IMO.
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Old 30-04-2014, 18:53   #5
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Re: Grounding

My boat has gfi's. Don't they measure the resistance between neutral and ground to trip them? I just want to make sure I'm not leaking power through my outdrive and desolving it.
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Old 30-04-2014, 19:05   #6
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Re: Grounding

You don't need to throw a ground rod over the side. A very sensitive voltmeter will tell you if there is potential at your outdrive.
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Old 30-04-2014, 19:13   #7
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Re: Grounding

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Originally Posted by amanphoto View Post
My boat has gfi's. Don't they measure the resistance between neutral and ground to trip them? I just want to make sure I'm not leaking power through my outdrive and desolving it.
Ground fault interrupters are the tail wagging the dog. They are needed BECAUSE your normal household circuit HAS an inherent unsafe condition in that one of the live wires of your circuit (the neutral) is connected to earth out of necessity. If your boat is constructed of metal or some other conducting material, then there is an argument for doing the same onboard, otherwise a floating earth is just as safe.

I should note that you can think of a "floating earth" as a double insulated circuit. Many appliances are double insulated, having no earth wire connected to their circuits or chassis. I think many would agree that double insulated appliances are safer than the equivalent earthed versions.
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Old 30-04-2014, 19:14   #8
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Re: Grounding

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You don't need to throw a ground rod over the side. A very sensitive voltmeter will tell you if there is potential at your outdrive.
Aluminum boat guys use, I think, something like a silver iode or some such, to make good contact with the water, not the meter lead itself.
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Old 30-04-2014, 21:21   #9
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Re: Grounding

If you have an all chain anchor rode, just connect it to the inboard end of the chain and the anchor will provide a suitable ground back along the seabed.
Can't use while underway of course...





If you don't like this idea, then take the advice of the more helpful posters above






Just in case someone actually wants to do this - don't - it's silly .
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Old 01-05-2014, 02:06   #10
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Re: Grounding

gfi's measure current between hot and N. they work with no ground.

the ground returns back to the inverter where it is connected to N at the inverter. same as a house is more or less.
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Old 01-05-2014, 02:48   #11
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Re: Grounding

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gfi's measure current between hot and N. they work with no ground.

the ground returns back to the inverter where it is connected to N at the inverter. same as a house is more or less.

GFI's measure the differential current between A & N. The differential comes from current from A finding another path to ground. This means they don't physically need to connect to earth to detect faults.

I find it hard to believe, but I could very well be wrong, that an inverter would have an internal connection between the earth pin and a live wire (there is no real "neutral" output terminal in the sense of that of a poly phase AC neutral) unless it was also fitted with a built in RCD device. To do so would make the inverter borderline dangerous imo.
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Old 03-05-2014, 16:04   #12
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Re: Grounding

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I find it hard to believe, but I could very well be wrong, that an inverter would have an internal connection between the earth pin and a live wire (there is no real "neutral" output terminal in the sense of that of a poly phase AC neutral) unless it was also fitted with a built in RCD device. To do so would make the inverter borderline dangerous imo.

this is a requirement for north America for marine inverters. it's the same as gennys. they are connected ground to N at the gen.

every source of power.

transfer switches only allow which ever source is being used to have that connection. inverters open and close that connection internally themselves based on weather they are inverting or passing through shore power. because if on shore power the N to G connection is already at the dock. which is the source of power
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Old 04-05-2014, 19:31   #13
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Re: Grounding

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this is a requirement for north America for marine inverters. it's the same as gennys. they are connected ground to N at the gen.

every source of power.

transfer switches only allow which ever source is being used to have that connection. inverters open and close that connection internally themselves based on weather they are inverting or passing through shore power. because if on shore power the N to G connection is already at the dock. which is the source of power
Well I can't speak for inverters, but I worked for a US company on this side of the globe as an engineer at a time we were being made to do the same thing with alternator generators. We ended up in the ridiculous situation of having to make machines less safe then they previously were because the regulators were, allegedly, unable to get their heads around the concept of "floating earth". To be fair, if you take a single phase generator and shove a whopping great earth stake into the ground beside it that is attached to what now becomes the pseudo neutral, and then test that the stake's electrical resistance is at or below mandated maximums then yes, it is safer. If, on the other hand, you do what most people do with portable equipment and jus' forget about it then you end up with a less safe system. In any event, as manufacturers, we had to include an RCD device into the machine as part of the deal as we were effectively connecting one of the live wires to the chassis and earth of the machines.
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Old 04-05-2014, 21:32   #14
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Re: Grounding

On metal boats you could run the ground to the hull. For plastic boats you have to wet your finger and stick it in the a.
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