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Old 18-03-2008, 06:28   #1
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Grounding question

I've wired two houses but I'm new to the DC stuff.

If I understand correctly, the there is a negative connection between the negative bar in the distribution panel, the battery, and the negative post on the engine. What did I miss?

Another question. In a catamaran in which the drive leg or the outboard(s) is/are completely lifted out of the water, isn't the ground to seawater lost? If so how should the ground be established with the leg out of the water?

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Old 18-03-2008, 09:52   #2
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All 12V fixtures and equipment should have their own DC negative wire which runs to the ground bus on the distribution panel.

In turn, the ground bus should be connected to the ground lug -- or the ground terminal block -- on the engine. The negative side of the battery bank should also be connected to the engine ground or terminal block.

The reason for this is that there should only be ONE DC ground point, connecting to the water.

If by raising the I/O you no longer have a connection to the water, it might be best to install a ground plate specifically for the DC ground. Note that this is NOT the same as a scintered bronze groundplate (or "DynaPlate") sometimes used for RF grounding.

Note also that any radio RF grounding and any lightning grounding should be independent of the DC ground.

Stan Honey did a very good article on boat grounding for West Marine some years ago. You can find it by Googling Stan Honey.

Marine Grounding Systems ~ by Stan Honey
This article was originally published in the October 15, 1996 issue of Practical Sailor.


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Old 18-03-2008, 11:50   #3
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Lightning will take ALL possible paths to get from ANY metal metal object above the waterline to the sea. To prevent "flashovers" during a lightning strike all metal objects aboard should be connected to a common seawater ground. This includes stays, shrouds, mast, poles, sail tracks, lifelines, stanchions, anchor(s), anchor chain(s), tanks, through-hulls, rf grounding plates, engines, etc. If you use low inductance connections to connect these items together you will by default have a very good rf ground system. Low inductance connections can be made using copper or stainless steal tape and braid. The stainless will last longer. Some common sense is required when making these connections. If an anchor is laying on a already grounded anchor roller then it too is grounded.

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Old 09-07-2008, 10:50   #4
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My first post, so be gentle.

I have been reading about grounding and I have found that there are a lot of recommendations out there. Here are three that I came up with. I did this with some knowledge of electronics and electrical engineering, including ground fault detection, longitudinal balance, return loss, transmission lines, etc. I also have built electronic test systems where grounds were a relative thing and the test systems had to be green wire protected.

So here we go, for better or worse. Please respond at will.

(1) Hull through fittings should be electrically isolated to avoid galvanyic corrosion. If it is true that lightning will find a path through any metal object, then a gounding wire attached to a metal hull through fitting will direct current to additional items which are connected by the ground circuit. This doesn't seem like a good idea. Why not let the fitting that got struck take the hit and spare the rest of the boat. Also, isn't the gound wire at sea ground? The hull fitting is already at sea ground.

(2) The lightning prection system should be the mast, forward stay, highest shrouds and keel. These should be isolated from everything else. I do not see the benefit of including the rear stay as I will be standing next to it and I don't want lightning to travel down a skinny wire next to my head.

(3) I can see no reason to ground the DC system. It is an isolated system runnning on batteries. Why connect it to the AC or the sea? This just leads to opportunities for galvanic erosion and adds complexity. We don't ground flashlights and lanterns? Our cars aren't grounded to the road.

The way I see it, here are three independent systems, Shore Power, Battery, and lighting protection. None of these should be connected.

I fear my frustration is coming through in this message. I don't mean to offend anyone but I can't seem to find any science behind the advice I find.

Steve M
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