AC/DC Grounding W/out Engine or Plate

Discussion in 'Electrical Systems' started by Mylestec, Mar 24, 2009.

  1. Mylestec
    Joined: Mar 2009
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    Location: Miami

    Mylestec Junior Member

    While there appear to be at least a few threads regarding the general grounding topic I haven't found one that deals with my specific situation.

    In the process of re-finishing my 25' sailboat I have discovered there is no effective grounding device(s). The boat is equipped with a pull start outboard engine with no connection to the electrical system. At present time the battery bank is trickle charged with a normal non-marine type charger while the boat is in the water (I realize I just painted a nice big target on my forehead). Now, I'd like to correct this grave danger but I can't seem to locate any reference material depicting a boat without a engine or submerged grounding plate.

    I'd really appreciate some guidance on this as I'm not sure how I should proceed.

    Many thanks to you all in advance for your assistance!

    -brandon
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    If your only contact with shore power is a charger for the battery, the electrical system floats. The charger may have a ground wire if it has a metal casing, but the output is floating.
    A reason to ground the DC wiring could be protection against nearby thunderstorms or other static electricity. To achieve that, you must have a metal surface in the water. Doesn't the keel have a metal part?
    If not, mounting a ground plate somewhere below the waterline is the only option.
     
  3. Mylestec
    Joined: Mar 2009
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    Location: Miami

    Mylestec Junior Member

    CDK;

    Let me make sure I understand you: Assuming there is no ground to water, then AC/DC systems, bonded together through the Shore AC ground are considered safe...?? In that regard I'd connect my negative DC bus to the green shore power bus... is this correct?

    I believe the keel is a bolt-on fiberglass encased lead ballast piece. I'm not sure if the bolt connections make contact with the lead ballast (since that portion would have only about 1/2" of glass and mat separating it from the water). Although that still would not make for an effective AC or DC ground but may make for life saving lightning protection (something that could be useful in Miami!) On the other hand could blow my keel off and sink me if that much current was put through the keel bolts and had to escape through fiberglass into the water.....

    Thanks for your help.

    -brandon
     
  4. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    No, it isn't. I said the DC system floats. The charger has a transformer with 2500 V isolation voltage. Connecting the DC negative to shore ground is not considered to be good practice because the shore ground has an unknown origin and may not be at the same potential as the water you're in. In your case with just an outboard and no metal parts in the water a voltage difference is not important, but in the majority of other cases there is a prop, shaft or rudder that starts to dissolve when connected to shore ground. Even a few 100 millivolts may cause electrolysis.

    The grounding of the DC system, mast, sea-rail etc. is not meant to guard the boat against a direct hit from lightning, but against induced voltages from discharges around you. These are short spikes that easily destroy a radio or depth sounder. Also, with a thunderstorm nearby, you can feel electricity whenever you touch a metal object. Good ground wiring with an electrode prevents that.
     
  5. floridawriter
    Joined: Feb 2007
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    Location: Florida

    floridawriter Junior Member

    As long as you do two things: test for reversed polarity in the ac circuit and connect the trickle charger to an extension cord with a ground fault interrupt. A better solution and one that will help your batteries last longer is to ditch the trickle charger and use a small half amp solar charger like this one:
    http://www.defender.com/product.jsp?path=-1|328|49497|852986&id=986996
     

  6. Mylestec
    Joined: Mar 2009
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    Location: Miami

    Mylestec Junior Member

    Thanks for your help David... I need the charger for higher amp charging needs to replenish the bank between uses (not just a trickle charger)... and without any other means of charging the batteries I'm not sure that a half amp panel would alleviate my shore powered charging needs.

    I have been looking into either building my own 90 watt panel or buying one from a local seller (approx $300 for one 90watt). Of course I'd then need a charge controller, but may be able to significantly reduce my need for shore powered charging.... but alas I'm already significantly over my original project budget... so for now I'll use the ground fault method you proposed.

    Cheers!

    -brandon
     
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