Grounding System on a small boat

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lazza
lazza Solar Expert Posts: 336 ✭✭✭
HI, have been searching forum but couldnt come up with the answer.

What sort of grounding should you have on a boat? Should the -ve battery terminal be grounded as well as the AC PE wire and Neutral?

thanks
Larry

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  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
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    Re: Grounding System on a small boat
    lazza wrote: »
    HI, have been searching forum but couldnt come up with the answer.

    What sort of grounding should you have on a boat? Should the -ve battery terminal be grounded as well as the AC PE wire and Neutral?

    thanks
    Larry

    I think a floating ground would work on a boat. :D
    http://www.sailmail.com/grounds.htm
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
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    Re: Grounding System on a small boat

    You have to be a little more specific on exactly what you are hooking up. (Shore power, AC inverter, etc).

    One of the primary things you have to provide protection for is stepping on and off the boat where someone might have one hand on some conducting surface on boat and other hand grabbing something grounded on shore.

    All shore power feeds should only be from a GFCI protected lines.

    Small floating neutral inverters are tricky. Small modified sinewave inverters often do not allow their neutrals connected to ground.
  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
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    Re: Grounding System on a small boat

    Its an AC inverter that will be connected to the 2 main boat batteries of 180Ah each. The inverter is TSW and there is an option to ground the neutral in this case. It's actually mainly a fibreglass boat
  • BB.
    BB. Super Moderators, Administrators Posts: 33,502 admin
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    Re: Grounding System on a small boat

    Lazza,

    As you know, grounding is a big/complex issue... And doing it wrong can let out magic smoke and other problems.

    So--If the boat's AC power system is shore powered with a transfer switch to the local inverter... Really need to know the brand/model of inverter and how your transfer switch system is setup/works.

    First, the usual warnings:
    • Never directly connect the output of a standard off-grid inverter to AC mains.
    • Most TSW inverters can have an output wire grounded (for neutral). And most MSW inverters cannot. However, there is enough variability in design (and safety issues) that you need to read the manual for your particular inverter. A mistake, at the very least, will usually ruin your inverter.
    • Most shore power/utilities ground reference their neutral ground. And, it is usually a good idea not to ground reference the neutral again in your vehicle/boat. Note, in North America, we do ground reference the transformer at the pole, and again at the entrance to the home. It does have its issues--But that is the "way it is done" per code. Grounding a neutral again in the boat can raise issues with redundant current paths (neutral and green wire ground in parallel). If one wire is broken, it may not be obvious--and can cause a smaller wire to carry over it rated current in some cases.

    So, to your case. Grounding the Neutral is done so that all largish metal objects (plumbing, sinks, stoves, refrigerators, etc.) are always at the same potential as "earth". If you grab something in one hand and place the other on a sink (for example), you should not get shocked. Double insulated appliances (blenders, drills, mixers, etc.) do not have exposed metal that can become "electrofied" unless there are TWO (unlikely) FAILURES in the device's electrical wiring. So, some appliances only have the two wire connections instead of three.

    The normal way to add a second level of protection is to install a GFI (Ground Fault Interrupter) outlet or breaker on circuits that are near water/outside/etc. If there is a current flow from Hot to "elsewhere" (ground, etc.), then the GFI will open and prevent (hopefully) dangerous current flow.

    The other reasons for grounding include lightning protection (important on a boat) and to ensure that any short from "hot" to earth will trip a breaker (instead of making that metal object hot).

    So--back to your needs. If you have shore power, you should not make a hard neutral to earth bond. If you have a transfer switch, a two pole transfer switch will allow you to ground reference the inverter's output (assuming TSW or other inverter that supports earth reference). The transfer switch will switch both Hot and Neutral wiring. Hot from shore to inverter. And the Neutral--which will be ground reference (if shore) or ground referenced (if local inverter).

    In general, with a small inverter it is of questionable value if it is important to ground reference its AC output. With most MSW inverters, you cannot ground reference their output (i.e., share ground with battery "-" and ground reference an AC output). With TSW (and isolated MSW) inverters, you can either let the output float (which in itself, a pretty good safety feature--you cannot get a short with a single connection to AC "Hot") or grounding/making a "neutral" output (common ground inside the boat).

    Sorry for the long and confusing post. Questions? Avoid me?

    -Bill :blush:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
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    Re: Grounding System on a small boat
    As you know, grounding is a big/complex issue...
    BB The marine inverters that I have worked on usually have a relay that switches from keel ground to shore power ground automatically when the boat is plugged into a marine shore power receptacle. Many of the Trace and Heart inverters had this grounding system set up as standard practice to keep boats from sinking due to electrolysis which can eat away and metal through hulls and propeller stuffing boxes when not properly grounded.
  • CDN_VT
    CDN_VT Solar Expert Posts: 492 ✭✭✭
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    Re: Grounding System on a small boat

    My take
    I got Small & fibreglass with two 180Ah Battery's.

    Power & sail have different grounding systems, Rigging can have it's own on sail and some boats use a special tracking system so they don't become a harbour ground.
    There are NO GFI's feeding our shore power , plus my neighbour slip-mate has two 50 amp feeds(Power boat & hog),Shore cable is 28mm thick ..

    Is there an onboard battery maintainer / charger ? Inboard or outboards , Do you have shore power ? If so how is used plus all the rest.

    If we get back that we are looking at a 19 foot outboard trailer unit or a 29 foot deep vee cruiser , they would have totally different items to consider.

    Now lets really stir the pot with adding a mast or two and rigging.

    Mr Larry , we would need much more info .
    Plus is this vessel insured ??Cuz mine must be inspected to pass for insurance purposes ..

    MikeO , I believe your thinking of the inverter/charger type of switching grounds.

    VT
  • lazza
    lazza Solar Expert Posts: 336 ✭✭✭
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    Re: Grounding System on a small boat

    Ok, I think we can simplify things a bit here:

    So as far as I understand:

    The inverter chassis needs to be connected to ground. This creates a direct connection from the PE wire in the AC output cabling to the common ground bus. The reason for this is to prevent a shock if the current carrying wires AC wires happen to come into contact with the metal frame of electrical equipment (connected to the PE wire)- thus if someone happens to touch the now "hot" metal frame, they wont get a shock because the current has an easier path through the PE ground wire, through the inverter ground connetion, to the common ground bus. The short to ground caused by this failure (hot wire touching a metal frame), should also trip a GFCI.

    However all this will only happen if the neutral is grounded too.
    So if I choose a to go for a floating neutral, what's the point of the grounding, the PE wire and the GFCI?

    Another question... what causes electrolytic corrosion? what should be done to prevent it?
  • CDN_VT
    CDN_VT Solar Expert Posts: 492 ✭✭✭
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    Re: Grounding System on a small boat

    Im going with a run-a-boat , small day boat. (inboard or outboard ??,,)
    Connect all PV,inverter ,CC to the boats negative. (can be battery or if the engine is inboard , a good cleaned block area, Use copper to steel or stainless to aluminum with aluminum paste (nylocal type of aluminum better connection paste) Keeping the metals that join happy is as important as any grounds .electrolysis eats with time.

    I would float the neutral being 220 to a GFI and then wire any other plugs off that GFI .. Use the GFI ground to the same common ground as the above mentioned .

    No shore power , this is just like a off grid..

    Your second question , use google or duckduck.


    VT
  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
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    Re: Grounding System on a small boat
    Another question... what causes electrolytic corrosion? what should be done to prevent it?
    This is mostly caused by improper grounding. If plugged into shore power, and the AC ground is the keel plate on your boat, there is a ground loop where a voltage potential is created that makes metal parts on the boat act as an sacrificial anode. Your copper shaft bearings and struts on an inboard, or any metal through-hull fittings start to basically melt. When they disintegrate enough, the boat sinks. The boats grounding to the keel should be switched to the shore power ground to prevent this. If your boat is a fully fiberglass hull with no grounded metal fitting though the hull, or the outboard engine is not grounded by raising it out of the water, you will have no problems.