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Old 29-09-2009, 12:37   #1
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Bonding AC & DC Ground

So again I'm inexperienced in this area

When I purchased my new C&C 27 1975 MK III. The surveyor said in his report the AC & DC grounds need to be bonded.

when i questioned him, he said the neutral of the AC should be connected to the Neutral of the DC

So when I looked at my AC breaker panel I had a bar with all the Neutrals connected, then I had some green wires (ground connected to a bar). I then had a wire connecting my AC green wires to my DC negative bus.

So I connected another green wire on the back of my AC breaker from the green (ground bar) to the Negative bar. Essentially now my AC & DC neutrals are both connected through the (ground connector bar)

now I plugged the boat back into my house outlet and my house breaker blew

I don't understand. Why.... it was ok before and connecting the one wire should not have caused that?
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Old 29-09-2009, 12:51   #2
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On the AC side: neutral and ground aren't always the same. They should be close though. I would check the outlet from the house and make sure the hot and neutral lines aren't reversed, if they are you just grounded the hot and that is why the breaker tripped.
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Old 29-09-2009, 13:00   #3
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The house should be fine, since I've used the outlet several times with no problem.

Had my boat plugged in prior to adding wire and it was fine
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Old 29-09-2009, 13:30   #4
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In wet (marine) environments, the circuit should be protected by a device called a Ground Fault Circuit Interrupter (GFCI) or just GFI. This device essentially separates the ground from the neutral. Ground is ground, neutral is not necessarily grounded. It will blow the breaker if such a device is present. I would check that.
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Old 29-09-2009, 14:16   #5
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This device essentially separates the ground from the neutral.
whaaa.. no never not at all.


Firstly OP the surveyor meant DC negative and mains earth, not neutral. The neutral may be at or close to ground but usually there can be a difference and this can cause problems

The reason for connecting these is to ensure that the protective effect of the earth wire that is common on AC devices is entended to DC devices. ie if a live AC wire touch a DC component then the protective earth will trigger the AC breaker. ( hopefully).

All this has nothing to do with GFIs (or more correctly residual current disconnect devices).
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Old 29-09-2009, 15:07   #6
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the following link

Boats, Yachts: Tips on Electrical System Use and Maintenance

suggests AC and DC negative should be seperate and not connected or bonded to each other

my house plug is not a GFI and so why is my breaker (house blowing) now that I have essentially connected the AC negative to the AC bonding system? Which in turn is connected with DC Negative Bus
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Old 29-09-2009, 15:37   #7
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I also suspect the polarity of your AC supply is reversed.
It's not a matter of "negative" versus positive with AC but that the Neutral lead is (or should be) at earth potential. That's why it's called the Neutral lead. The other one is the Hot lead which, if the polarities are reversed, is now a short circuit to ground.
That's why the breaker blows.

If you have a multimeter which will read AC volts, connect one lead to ground such as a thruhull or other metal which you know is actually grounded, and then connect the other lead alternately to each of the shorepower leads. This will tell you which one is indeed HOT.

Steve B.
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Old 29-09-2009, 15:59   #8
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Don't confuse the neutral and the ground of the shore power. The neutral wire is usually white, the gorund is green. The ground of the shore power (AC current) on the boat should be connected to the ground of the battery (DC power) on the boat. This is recommended in the ABYC standards, which is a consensus of the best minds in the business, and is what your surveyor is referring to.
Brian
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Old 30-09-2009, 04:57   #9
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Thanks. Perhaps I'm starting to get it

Though My surveyor specifically stated "he did not have continuity between AC & DC negative"

It turns out my extension cord did have reverse polarity. I did not understand how in 20 years I've not had an issue using various things with it.

I already had a green wire from DC negative bus to the AC ground. and continuity between the two.

However I did not have continuity between AC & DC negatives as my surveyor mentioned, so assumed I needed to connect the two.

though I've read a bit of various comments on the web and it is suggested I don't connect these.

It is difficult to find the detail of ABYC because you need to be a member.
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Old 30-09-2009, 05:28   #10
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I wouldn't want the neutral connected to DC ground for several reasons.

1. If you pull into a marina that has the leads reversed at the dock then you now have full main power on the outisde of all your DC devices (radios, windlass, etc...)

2. There could be a corrosion effect as the current returns through the neutral it could induce a current through the water from your engine block.

3. GFCI breakers are designed to trip when the current through the hot leg is different than the current through the neutral leg. The difference is assumed to be going through the ground or the user in either case a safety problem and it trips. When you tie your ground and neutral you have split the current between the two and you will always have more current on the hot than the neutral.
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Old 30-09-2009, 06:43   #11
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Amytom

But my understanding is the ABYC requires the DC ground (negative) to be connected to the AC ground.
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Old 30-09-2009, 06:54   #12
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The two grounds yes. The neutral no. On DC you have a pos and a neg and the electrons flow from neg to pos completeing the circuit. In AC you have a hot a neutral and a ground. The hot (usually black) has the alternating power applied, the neutral (white) is the return line, and the ground (green) is the safety. Somewhere back down the line (usually at the meter or main panel) the neautral and the ground are the same but after the breakers, GFCI's, and surge suppressors their not.

It can get more complicated if we go into multiple phases, delta wound versus y wound transformers, and high tension transmission lines but for the purposes of your house and the dock think of it as described above.
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Old 30-09-2009, 07:05   #13
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The AC Neutral (white) is NOT grounded on the boat.

There are exceptions for separately derived sysyems, which are not under discussion here.
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Old 30-09-2009, 07:18   #14
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The surveyor must either have missed the connection between the green AC ground and the negative DC bus, or not known his stuff, because what he asked for you already had.
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Old 30-09-2009, 17:02   #15
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Originally Posted by outdoor View Post
the following link Boats, Yachts: Tips on Electrical System Use and Maintenance suggests AC and DC negative should be seperate and not connected or bonded to each other
1. I don't think the author of that essay says not to connect AC green wire (North America) and DC ground (correct me if I am wrong). I think he says not to connect AC green wire to the boat's "bonding" system, which is not the same as the DC ground circuit. As he says, there are 4 different ground, grounding, or bonding systems to keep in mind.

2. On another point, the author writes "There is only one point where the DC side is grounded, and that is at the battery. It, too, is a "free floating" system in which nothing is ever grounded to any metallic part of the vessel, most especially not the bonding system" yet is it not true that most smaller boats run a large ground cable to the engine (usually a starter bolt) from the DC- post of the engine start battery? That means there is continuity between DC- and the ocean thorugh the prop shaft. Nigel Calder explains how to mount the alternator and other engine parts so they are electrically isolated from the engine block to create a floating ground in the boat (not connected to the ocean), taking the block out of the system, but not many smaller boats go to that length, right?
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