12 vdc ground isolation switch

Discussion in 'OnBoard Electronics & Controls' started by RoyB, Mar 31, 2007.

  1. RoyB
    Joined: Oct 2005
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    RoyB Junior Member

    New boat we just got has something I haven't seen before.
    There is a switch between the battery bank ground and the system ground. This in addition to a 1/2/both switch on the positive.

    The only thing I can attribute this to is someone's effort to isolate the engine and saildrive from electrical ground as a way to avoid galvanic corrosion that might result from use of the shore power charger.

    Is there any other reason why a ground interrupting switch would be installed?
     
  2. KnottyBuoyz
    Joined: Jul 2006
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    Location: Iroquois, Ontario

    KnottyBuoyz Provocateur & Raconteur

  3. TerryKing
    Joined: Feb 2007
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    Location: Topsham, Vermont

    TerryKing On The Water SOON

    Connections??

    RoyB, are you saying that this disconnect completely disconnects the negative of the battery from ANYTHING? Or just disconnects it from vessel (ground), while still leaving it connected to the engine block??
     
  4. RoyB
    Joined: Oct 2005
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    RoyB Junior Member

    The switch disconnects the negative of the battery bank from everything (including the engine block) except for the shore powered charger DC ground
     
  5. TerryKing
    Joined: Feb 2007
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    TerryKing On The Water SOON

    Why the switch??

    RoyB this then seems like an approach to isolate the boat's DC ground and the Shore Power AC ground WHEN charging the battery from Shore Power.

    The only reason I can see for doing this is if the charger itself has a connection between the Shore Power AC (green) Ground and the charge DC output negative terminal.

    As I Think I Understand It:

    - A shore-powered battery charger usually has the DC output isolated from the Shore power Line-Neutral-Ground (typically by having a transformer in the circuit that provides the isolation).

    - Typically the entire AC power system that is ONBOARD has it's Line-Neutral-Ground all (carefully!) isolated from the boat's DC ground.

    - When Shore Power is connected correctly, there are two isolated "Grounds" on the boat:
    -- Shore Power AC Ground goes back thru the shore power cable to the shore power distribution system. The shore power system (like a home system) has a single point (normally at the service entrance / meter) where the Neutral is connected to the Ground and to a ground rod driven into the earth.
    -- Boat DC ground (the boat "Common Ground" point) is connected to the in-the-water metallic parts. In seawater that is a fairly low resistance 'ground' relative to "Earth".

    BUT: What happens when you cast off, disconnect Shore Power, and turn on your battery-powered Inverter that supplies AC??
    - THEN you need to connect the boats AC ground to the boat's DC Ground!

    I understand there are various automatic and manual switching systems used on different boats.. I'm at my level of incompetency on that.

    Back at Your Switch! I THINK it is necessary to isolate the battery charger DC Output from the boat Ground because of the characteristics of that particular battery charger. Maybe.

    It's also distinctly possible that it's not needed but someone thought it was!

    Let us know how you finally figure this all out....
     
  6. RoyB
    Joined: Oct 2005
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    RoyB Junior Member

    Thanks Terry - I think your second explanation is more likely in this case.

    I'll be doing some rewiring soon, the boat has other little gems to remedy such as...

    1) the HF radio is wired to a dedicated circuit breaker. The circuit breaker is wedged in between the battery and it's box rather than being mounted anywhere.

    2) there are two "mystery wires" connected to the positive of one battery with no fusing or CB visible

    3) the battery monitor is wired in such a way that it can only see discharge from things connected at the main CB panel, not charging or loads that bypass the panel.

    4) Someone added about 2 thin layers of 'spaghetti' on top of a pretty neat and well thought out factory job behind the panel.

    5) There doesn't appear to be a GFI in the AC circuit

    etc, etc, etc
     
  7. TerryKing
    Joined: Feb 2007
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    Location: Topsham, Vermont

    TerryKing On The Water SOON

    RoyB, it would be interesting to check the battery charger separately. I'd disconnect it completely. There are (should be?) 3 connections in: Line, Neutral, (Green) Ground. And two connections out: (DC +) and (DC -) .

    I'd use an ohmmeter / multimeter and check resistance between (DC -) and each of the inputs. After an initial 'bounce' (from charging some bypass capacitor, possibly) there should be over, say, 100,000 ohms resistance.

    To go even further and check AC leakage, connect a 1000 ohm resistor from (DC-) to (AC ground), plug in the charger (no batteries or anything connected) and measure AC Volts across the resistor. Turn the power switch (if it has one) on and off. I'm not sure just what is acceptable here, but I would expect it to be less than a couple of volts. Hmmmm.. Maybe we can find some reference for that. This is the "AC leakage current" that would flow from the Shore Power "Line" to the boat ground if the battery charger was run with that great switch ON...

    I have to find a good simple diagram of a boat wired with both AC and DC power, and how the isolated grounds work, for the Wiki...
     

  8. Frosty

    Frosty Previous Member

    I have pos and neg switches on my dc.

    I do not have ac ground. I do not trust the marinas ground system. I use but 2 wires only. most of the domestic products I buy have 2 pin plugs anyway.

    I do not,--nor have I,-- had any problems.

    I may get a shock if some metal cased apliance goes wrong buy I sleep in comfort knowing my next door neighbour is not using my 5000 dollar propellors as a ground.
     
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