Install galvanic isolater in DC to AC ground connection?

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Dec 23, 2014.

  1. sdowney717
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    sdowney717 Senior Member

    [​IMG]

    Note the figure #2.
    It seems to me that would be a better location.

    Where will galvanic currents originate, in AC power or DC power?
    Dont they mostly originate from the DC side, as in the anodes attached to the boat?

    If install as in #2, then the GI wont be exposed to high amp AC power shorts and possibly burn out.
     
  2. sdowney717
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    sdowney717 Senior Member

    For example the purpose is for this alone

    http://www.pkys.com/FAQ/galvanic.htm
    As long as your AC ground system is not in connection with your DC ground system EXCEPT at your bonding - connnecting wire that joins the AC ground to the DC ground, then a galvanic isolator installed in that wire is all that is needed to stop DC galvanic current flow. It just seems more robust and safer to keep the AC green ground connection totally intact and only galvanically isolate the DC side from the AC side in the bond wire. I plan to do that on my own boat unless someone can object with a significant reason otherwise.

    Doing that the GI would only see high current -voltage flows IF say a AC hot wire shorted to the engine block or other DC negative ground point. Which is less likely than shorting an AC hot wire to its own dedicated AC ground wire by way oaf a faulty appliance, wire, or plug.

    AND this, it would most likely be an AC branch circuit with a lesser rated circuit breaker say 15 to 20 amp rated that would have such a short to negative ground, so current flows will be less than the maximum breaker in the Ac system, 30 amps or higher. An unlikely short of the mains to ground of 30 amps or more would never have to flow through the GI, and would just trip the mains breaker and the ground circit would never burn up die to an overheated-burnt out GI.

    They have come up with fail safe GI for a lot of cash to make sure the ground is always intact, but I wonder.
     
  3. CDK
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    CDK retired engineer

    The GI circuit consists of a number of diodes pairs, connected head to tail. Each link allows approx. 700 mV differential voltage without a significant current flow.

    The sole purpose is to allow a differential voltage between ship's ground and shore ground because the latter is contaminated by all kinds of offset voltages originating from unbalanced loads like heavy motors, transformers and ground faults somewhere in the power grid. An offset of a few volts is quite common and although that is acceptable for a protective AC ground, it turns a hull into one big sacrificial electrode if DC ground is bonded to it.
     
  4. sdowney717
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    sdowney717 Senior Member

    While exploring my boat ground wiring, I found two places where extra grounds exist between the AC and DC systems separate from the wire that runs from negative ground within the helm to the AC ground block in the AC panel.

    One is the gen ground wire that runs back along with the AC gen output wire to the AC panel. The gen is also grounded to the DC ground and then the AC panel by another wire that joins to the engine blocks. My feeling is to disconnect that AC ground wire going to the AC panel and rely on the 6 gauge ground wire to the engine block - helm which then goes to AC panel for it's ground.

    The other is the OEM boat builder ran a ground from the copper water pipe to the negative DC ground. Somewhere along those pipes I think might exist another ground point back to AC panel ground, but I will need to trace back all the pipes to find out if it does and where and why.

    If I have only a single durable ground connection through that one wire from DC ground to AC ground, a galvanic isolater in that wire would work as per fig 2, as other ground points wont exist, it will all have to travel by way of that one wire.

    I have never had corrosion problems on my boat before I had a GI, seeing it is all bronze and SS underwater.

    more info on GI http://www.boatus.com/seaworthy/galvanic/default12_06.asp
     
  5. Mikeemc
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    Mikeemc Junior Member

    I've always made the AC floating ground to the shore circuit. But I always get different options. Mount AC appliance on rubber with no metal touching them. The AC hot water may cause stray leakage due to the element being emerged , AND keep an eye on that when it go's bad.
     
  6. rangebowdrie
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    rangebowdrie Junior Member

    There have been an incredible number of nice wooden boats that have suffered extensive damage from the de-lignification of the wood surrounding sea-cocks, shaft glands, etc., from the process of connecting together the AC and DC grounds along with a bonding curcuit.
    GI or not, you end up with a pea-soup of various potentials, all of which are seeking equality, (save the metal, destroy the wood).
    One of the ways to be safe, is for all of the AC units, refrigeration/batt charger/hot water heater, etc., is to use a GFI outlet as a feed-thru device, the hot wiring into the line connections, and the user device wired to the load side. No connection between AC and DC grounds, I'am fully aware that that violates ABYC,,, they don't care about wood destruction, use a GI if it makes you feel better.
    Many are those with long expertise in wooden boats that would say that with bronze fastenings and bronze underwater metal, a bonding system is both unneccessary, and in fact, looking for trouble concerning the wood.
    The issues of; 1, stray currant corrosion. 2, electrolisis. 3, AC ground safety for people. 4, lightning protection. Are seperate and individual issues. Most of what I've said concerns wood boats, metal has more issues, fiberglass less.
     
  7. CDK
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    CDK retired engineer

    The simple solution is to cut the shore ground near the GFI and rely on DC ground alone. That violates ABYC but is healthier for the hull, regardless of the material. Check the GFI once a month by pressing the test button.
     
  8. Carioca
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    Carioca Junior Member

    And for the alternative of installing an isolation tranformer on the boat, thereby isolating the AC shore supply from the boat´s AC wiring ?

    In this particular case would the restriction to tie the DC ground and AC ground on the boat be waived, or still hold ?

    Mine is a woody which has just gone through a major topsides and underwater refurbishment and the mere thought of de-lignifying the new wood seems like a nightmare, in light of the man-hours and money spent !

    BTW, a reasonably sized AC isolation transformer will put you out of pocket for no more than 500-700 dollars
     
  9. sdowney717
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    sdowney717 Senior Member

    I think with an isolation transformer, on the boat side of all the wiring, the DC to AC bond wire must still exist. Think if an AC hot wire touched the engine block, if the DC to AC bond wire cant drain the power blowing open the breaker, the engine will be hot and shock you.

    The only advantage of the isolation transformer is you break all solid wire contact to shore-grid power, so no DC galvanic currents will ever flow.

    I disable-broke all the bond wires to to the underwater metals. Except for the fuel tank to shaft log to engine block wire. I think draining charges from the metal tank is good for refueling, you might make a spark, is what I have read. I could eliminate shaft log and go direct to engine block from fuel tanks.
     
  10. rangebowdrie
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    rangebowdrie Junior Member

    Many are the boats, where everything is bonded; appliances/mast steps/chainplates/sea-cocks/skin fittings/fuel & water tanks, ad infinitum, and all of these objects have diff "electrons at rest potentials". Then this whole mass is connected up to the DC ground buss and the engine/shaft, with a big zinc on the shaft near the prop.
    After a few months/years, the owner starts wondering:
    1, why won't the paint stay on the mast step.
    2, why are the stainless water tanks corroding at the welds.
    3, how come I'am getting pin-holes in the alumunum fuel tanks.
    4, why does the zinc "go away" so fast.
    5, whats all this white fuzz on the sea-cock backing pads.

    So,,someone tells him,"you need more zinc(s)", so he does that, and the problem gets worse.
     
  11. CDK
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    CDK retired engineer

    No that won't happen. The GFI compares current coming in and going out.
    Your AC hot wire touching the engine block causes some current to flow through the block into the water, the GFI disconnects the AC immediately.
     
  12. sdowney717
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    sdowney717 Senior Member

    Yes true if you have a GFI. I do have a 20 amp GFI for the microwave on a dedicated circuit and a GFI for all the outlets that can be plugged into. But no GFI for any other circuit. So if a bad plugged in extension cord touched the motor, it would kill the power.

    So far I like GFI on the boat.
    One is Square-D QO GFI breaker in the QO distribution box. I keep both incoming neutrals separate for each side not joined.
    The other is Leviton outlet and all outlets wired after are GFI. I keep the fridge plugged into a non GFI outlet, that is my only exception for outlets.

    What I discovered is that single 20 amp rated GFI can protect 2 long strings of outlets port and starboard without nuisance tripping. One side has nine outlets, the other side has seven outlets. I have 2 outlets running back to the stern where it could get wet. For those they are plugged into outlet boxes like extension cords in a dry place, so they could be unplugged if they were tripping the GFI.

    I would buy more GFI breakers but they cost a lot. I even thought about installing two dual pole 30 amp GFI in the GE mains disconnect breaker box but the cost is a lot and people tell me they might nuisance trip if the entire boat's AC was GFI to detect that 5 milliamp level. And I dont know if they would work properly with breaking a hot and a neutral like your supposed to do with 2 pole breakers on boats for the mains breakers.

    So the un GFI circuits are all like hardwired, 2 cabin heaters, water heater, 3 burner Princess stove, refrigerator, Cruiseair heat pump.
     
  13. CDK
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    CDK retired engineer

    A transformer isolates the phase and return wires between shore and ship, but does not isolate the (protective) ground between them.
    It is just the ground lead that endangers the hull.

    You could keep the AC ground away from the DC circuit. Not very difficult in a wooden boat, although moist wood can still conduct electricity.
     
  14. Carioca
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    Carioca Junior Member

    For sake of argument :

    1. A single-phase and a neutral wire from shoreside connected to the isolating transformer primary, with a dedicated GFI relay overlooking it; a back-up breaker to throw open the primary circuit in case of a primary phase-to-neutral /ground short / fault, should the far side (shore) relay not function.

    2. Shoreside ground wire not connected to transformer, aside from necessary connections to the protection relays in item 1.

    3. Transformer secondary protected in a similar way (item 1), with the primary/secondary screening (if supplied) and transformer housing constituting the boat´s AC ground.

    4. Boat´s AC ground of item 3 tied to the boat´s DC ground plate.


    Should a primary phase/neutral wire touch the transformer case, the AC boat ground will end up being connected with the shore supply....this could entail danger to persons in the water

    Would this event be detected and acted upon by the transormer´s secondary protection ?

    If undetected, would it be conceivable to have opto-couplers installed somewhere to detect it and then trip the breaker/relay on the boat´s supply (shore) side ?
     

  15. CDK
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    CDK retired engineer

    In such a setup the secondary protection circuit does not function at all because the transformer output is floating i.e. not referenced to ground. For a GFI to work it must be able to detect a difference in current flowing through the power lines.

    Should one of the primary wires touch the transformer housing that is connected to the boat's ground, the primary GFI would trip immediately, so there would be no danger for swimmers.

    Imagine the GFI as a solenoid or transformer with two separate coils, cross connected to AC live and return. Any magnetic field generated by the current through coil 1 is cancelled out by the field from coil 2 as long as the currents are equal. Only when some current gets lost in the circuit because it is diverted by a person touching a bare wire or an isolation leak, there is a residual magnetic field to trip the GFI.
    A modern GFI has some electronics that can detect leakage currents of just a few milliamps.
     
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