Phase imbalance tripping RCD

Discussion in 'OnBoard Electronics & Controls' started by ar2000d, Nov 2, 2011.

  1. ar2000d
    Joined: Nov 2011
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    Location: United Kingdom

    ar2000d New Member


    Our 42m gaff rigged ketch has just arrived in Liverpool and when connecting to the on-shore power the shore-side RCD keeps tripping. Monitoring the amp-meters for the generator each phase is drawing a different amperage.

    We are not using the neutral on the shore power, so have a transformer to step it down to single phase.

    My conclusion is that the imbalance of power across the phases is causing the RCD to trip, as it thinks there is an earth fault. Would this be a good assumption or might there be another explanation?

    I also think the problem might be with the transformer because is it getting very hot to the point of melting insulation, and the RCD only trips after about 20 mins. Would anyone have any further thoughts on this?


  2. gwboats
    Joined: Sep 2005
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    Location: UK

    gwboats Naval Architect

    Shore Supply


    First thing is disconnect shore supply and leave it off! If you are getting that much heat something is terribly wrong. Contact a local electrician (preferably one with marine experience) to sort you out.
    My thoughts would be:
    What are your boats voltages & frequencies- USA, UK, continental?
    To get single phase from three phase you need to tap across one phase and neutral not across phases.
    You would normally only need an isolating transformer on shore supply unless you want to go universal input/output on voltages and frequencies in which case a transformer alone is not enough.
    Hope this helps.
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Transformers can't step down power to single phase.
    The transformer may be for 60Hz.
    The transformer may have a ground on the housing and you have it disconnected at the source.
    Transformers are rated to a maximum temperature, you need to use a thermometer to make sure it is within limits.
  4. Poida
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    Poida Senior Member

    Gonzo, learn something about electricity!
  5. goboatingnow
    Joined: Oct 2008
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    goboatingnow Junior Member

    ar200d, without a bit more info its hard to work out whats wrong.

    in the UK the most common form of single phase is to take one of the three phases and neutral , giving a harmonised 230Vac, the phase to phase can also be used but generates a much higher voltage ie 1.7 times phase to neutral. How is your three phase transformer wired Y-delta or delta -Y.

    I suspect the single phase voltage is too high. How are the RCDs wired.

  6. Uldis
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    Uldis Junior Member

    If RCD is tripping after 20 minutes, this means that it is because of overcurrent - not leakage current. Leakage current trips RCD instantly.

  7. goboatingnow
    Joined: Oct 2008
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    Location: Ireland

    goboatingnow Junior Member

    Three phase RCDs have all phases and Neutral running through the current transformer. So imbalances shouldnt cause trips.

    Are you by any chance just using a single phase transformer across two phases of a three phase, This is a very dodgy way to do it and will generate transformer overheating unless the traffo is rated to handle the 1.7 times voltage and still produce 230Vac.

    If the on shore RCD is tripping and its a proper three phase RCD, it may be a a RCBO, ie it has overcurrent limits, I suspect its not a phase imbalance, its the transformer is drawing too much current as its not designed for the way you have it connected.

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