Install galvanic isolater in DC to AC ground connection?

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

  1. Carioca
    Joined: Aug 2005
    Posts: 82
    Likes: 2, Points: 0, Legacy Rep: 26
    Location: BRAZIL

    Carioca Junior Member

    First of all, Happy New Year !

    Depending on the salinity of sea water, a ground plate on the boat ends up connected to shore ground, so yes, you are spot on, the primary GFI will trip.

    On the other hand, isnĀ“t the whole purpose of using an isolating transfomer just the opposite, that is, to establish an AC supply on board that has no bearing on the shore supply ?

    Or perhaps that last phrase should be reworded as 'little or no bearing ' ?

    As I am sure you are aware, isolating transformers are widely used to power up medical electronics equipment, and small ones embedded in the remote sensor probes themselves, so as to mitigate the shock hazard to patients.
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    A Happy New Year to you too!

    As you must have understood by now, I'm no fan of transformers, at least not for the purpose of connecting shore power to a boat. They add little or nothing to safety on board unless they have the same quality and construction as the one approved for medical equipment plus added features to cope with a marine environment. The price tag will be much higher than what the average boat owner expects.

    But the whole concept of a clumsy iron core, impregnated paper and copper windings is (almost) a thing of the past. A switch mode supply, operating at high frequency, completely isolates the ship from the power grid. It has the additional advantage of providing also capacitive and inductive isolation and allows a wide range of input voltages, frequencies and even DC, while the ship's AC system gets clean and stable power.
    Such a device provides at least 5 KV isolation between primary and secondary and needs no ground wire at the input.
     
  3. rangebowdrie
    Joined: Nov 2009
    Posts: 48
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    Location: Oregon

    rangebowdrie Junior Member

    In a quality marine grade isolation transformer, like a Charles, there are two diff ground connections,, they serve diff purposes and must not be mixed up.
    On the primary side, there is a ground connection that is only connected to the shield between the primary/secondary windings, that connection goes back to the shore power cable, thence to the dock system.
    The other ground is the case ground, and it is connected to the ground buss for the boats internal AC. There is NO connection between the boats internal AC grounds and the shore system.
    The IT needs an ELCI breaker between it and the shore power inlet, and the outlets on the boat need GFI protection.
    When an isolation transformer is used, it is considered a "new" power source, and as such, on the boat, the buss for the neutral/white, and the buss for the ground/green wires are jumpered together.
    With an IT you do not need a galvanic isolator inserted between the boats DC ground buss and the AC ground buss.
     

  4. sdowney717
    Joined: Nov 2010
    Posts: 955
    Likes: 53, Points: 28, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    I am beginning to think a GI is mostly for the benefit of slip-mates who have inferior underwater less noble metals compared to those that are on my bronze boat.
    If I have a DC current leak to ground, an aluminum boat near mine connected to the shore power is going to be sacrificing itself and protecting mine.

    My boat from 1970 for over 35 years had no GI installed and never had any trouble.
     
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