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Stinkpotter
Master Marine Consultant

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Djibouti
9080 Posts
Initially Posted - 07/11/2016 :  15:12:59  Show Profile
This is a recurring topic, and Don Casey just put a brief article in Sail Mag. Note the caution at the end regarding shore power vs. the 12V system. The same principle relates to the potential for starting a fire, although 12V can do that quite well, as I learned on another guy's boat 10 miles out.

There's more in Don's book, Sailboat Electrics Simplified--one of several useful references on the topic.
Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge
Edited by - Stinkpotter on 07/11/2016 15:17:25

Voyager
Master Marine Consultant

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USA
5376 Posts
Response Posted - 07/11/2016 :  15:53:11  Show Profile
Hmmmm..... The article is generally well-informed and well written except one major faux pas: it mentions a 12VDC ground and shows a ground symbol on the schematic. IMHO I do not know what a DC ground would be on a boat - certainly not the negative lead of the battery, nor a lightning ground plate on the exterior of the hull.
I will not connect any part of my shore power circuit to any part of my boat. I consider ALL 120VAC systems exactly as I would an "extension cord" running from the dock's three-wire shore power outlet to my boat.
I agree that a GFCI should be used to prevent electrocution hazard on any 120 VAC system.
But introducing an electrical connection between a 120VAC and the boat's 12VDC or lightning grounding systems (if any) is a needless invitation to disaster. </IMHO>
Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT
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Steve Milby
Past Commodore

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USA
5902 Posts
Response Posted - 07/11/2016 :  16:41:30  Show Profile
I admit to not completely understanding it all, but the West Advisor appears to agree with Casey. This is the first I've heard of it. See item #4 in this linked article. http://www.westmarine.com/WestAdvisor/Ten-Deadly-Conditions
Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore
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Stinkpotter
Master Marine Consultant

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Djibouti
9080 Posts
Response Posted - 07/11/2016 :  21:02:37  Show Profile
Bruce: I missed that, and can't claim to be an authority, but I agree that the 12V system "ground" is actually just the negative return to the battery, while the AC ground is a third conductor to an actual external ground, that only has any current in the event of a fault. Whatever the WM Advisor is saying about the relationship between DC and AC "grounds" in the context of an inverter might be correct--I'm not sure--but it leads to my recurring theme that this stuff calls for a LICENSED MARINE ELECTRICIAN following ABYC standards. Some things, like plumbing, are OK for DIY... DIY electrical work burns things down, and a fire on a boat can be way more exciting than one in your house, where you can dial 911 and go outside.
Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge
Edited by - Stinkpotter on 07/11/2016 21:23:17
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Voyager
Master Marine Consultant

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USA
5376 Posts
Response Posted - 07/11/2016 :  21:43:49  Show Profile
Dave
I think section of the article referred to by Steve Milby here explains it.
The point #4. 'No “Green Wire” or poor-quality connection between DC negative and AC safety ground' explains why the DC negative should be bonded to the AC "green wire".

Here's why. If your boat has both AC wires and DC wires in close proximity to one another and if your inboard engine block as well as your prop and metallic skeg are "grounded" to DC negative, it is possible that the AC current in the AC wiring can inductively generate sizable voltages in the DC wiring right next to it, similar to the way a transformer works.

Because the induced current is now in contact with the water around your prop, it can produce potentially lethal AC currents in the marina. While you may scoff at the amount of induction, think of this:
A 30A AC load like an air conditioner can be inductively coupled to the DC wiring. If 1/100th of it = 0.3A or 300mA goes into the water, that current is sufficient to stop your heart.

If the green wire from the AC circuit, therefore, is connected to the DC negative, this "short circuits" the induced current.

That's at least what this item seems to be saying.

I'm with you Dave, hire a licensed and certified marine electrician.
Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT
Edited by - Voyager on 07/11/2016 21:46:26
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.