DC/AC grounding - SailNet Community

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  #1  
Old 03-29-2013
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DC/AC grounding

Hello folks. I have recently acquired a sailboat with an outboard motor and plan to rewire her this spring. I have rewired a few of my previous boats that had inboard engines which makes the single ground point obvious. However, since my outboard is raised out of the water when not in use then where is the common ground point?

If I connect the AC ground to the DC ground (per ABYC) on the engine then it doesn't seem it would work if the engine was raised out of the water. Do I need to install a separate grounding plate on the outside of the hull to use as the single grounding point instead?

Thanks!
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Old 03-29-2013
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Re: DC/AC grounding

I have wondered about this exact scenario myself.

Paging Maine Sail...paging Dr. Maine Sail to the shore power ward...
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Re: DC/AC grounding

I'm in the same situation. Our boat had an inboard but we're currently using an outboard until we can get a diesel put in.

We have a ground plate, I was thinking about grounding to that, but I have no idea if that's safe or not.
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Re: DC/AC grounding

Only the secondary ground (Green wire) should be grounded aboard the boat. The primary ground goes back to the AC power source. This is different from house wiring, where the primary and secondary grounds are linked in the panel. You will need a conductor to the water; a ground plate will work.
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Old 04-03-2013
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Re: DC/AC grounding

Thanks for the replies. I suppose I could just ground to the outboard and keep it lowered in in the water when connected to shore power. I only connect to shore power if staying on the boat overnight. Otherwise I disconnect it when leaving the boat.
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Old 04-03-2013
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Re: DC/AC grounding

That is the intent of the rule and it makes sense you need to provide a path to ground if there is a short. You cannot always assume the shore ground is intact. So yes connecting to the engine in the water or a ground bar on the bottom will work.

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Re: DC/AC grounding

Quote:
Originally Posted by Seascape View Post
Thanks for the replies. I suppose I could just ground to the outboard and keep it lowered in in the water when connected to shore power. I only connect to shore power if staying on the boat overnight. Otherwise I disconnect it when leaving the boat.
You'd better keep a close eye on your zincs if you use the outboard then. Outboard engine zincs are tiny, and will get eaten up quick if one of your neighbors is leaking current into the water.
I recommend using a ground plate instead of your O/B.

To the OP who is using an O/B, but still has the dead inboard engine in place:

Continue using the inboard engine as your ground point. The engine doesn't run, but it's still a large hunk of metal in contact with the water, so it makes and adequate ground point for the bonding between AC and DC systems.
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Re: DC/AC grounding

99% of people with corrosion problems always think it is the other guy causing the problem but on 90% of the corrosion surveys I do the problem is located on the boat with the corrosion. I am not sure why everyone thinks it cannot be a problem on their boat? But I always say make sure your stuff is right before pointing fingers. And I agree I would rather see a ground plate installed as a permanent setup. Also another misconception is that AC causes most corrosion, this is wrong DC is to blame. If you think about this it makes sense as AC flows back and forth so it will not move metal atoms from one piece of metal to another while DC is a flow in one direction and can carry atoms in a steady flow removing from one mass and plating another. When you plug into shore power the problems occur from the ground wire allowing DC current to flow. Think of the boats in a slip as cells of a battery and the ground wire connecting them just like a battery. This is why a ground isolator helps, it blocks DC flow in the ground wire but will allow AC to flow if there is a short.

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Re: DC/AC grounding

Ok, the source of current leakage is irrelevant.

As you agree, a ground plate is the better solution because no matter who is leaking current into the water, his outboard engine lower unit and prop will get eaten up because they usually only have very small zincs on them.
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Old 04-03-2013
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Re: DC/AC grounding

I'm confused. Maybe the image I have of shore power connection is wrong. Isn't there an isolation transformer between shore power and the boat load? And aren't the grounds between the two (shore power and boat load) isolated as in not connected? If this is the case then how does DC current flow between boats?
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