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Earthing and bonding portable generators?

Discussion in 'Electrical Engineering' started by Boxie-Boy, Aug 24, 2006.

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  1. Boxie-Boy

    Boxie-Boy Guest

    He is a question I've been pondering over for a while.
    Consider connecting a 3PH 200V 400Hz 60KVA aircraft power unit to a load
    bank for testing. The inputs to the load bank is 3ph +N (no earth wire,
    Inputs to aircraft do not have an earth)

    The generator is not connected to earth, however the tow bar and wheels
    will touch ground.

    Load bank neutral is not connected to chassis

    The star point of the generator is however is connected to the chassis.

    The Generator has no RCD protection, only a thermal overload, set to
    200Amps approx.

    The load bank manufacturer recommend connecting the load bank chassis to
    a good earth point, we have been using a solid steel building iron
    girder which is earthed to the incoming mains earth.

    So, what happens if the load bank develops a line to chassis earth
    fault? Will the GPU trip out. What potential will be seen on the load
    bank chassis, will the building framework become live?

    Someone in our works recons that because the building is earthed, the
    Earth will drag the supply down to Zero, i dont agree.

    I think that current must flow back to the generator with sufficient
    magnitude to trip it out, this I believe cannot happen because the earth
    fault path is limited by the high earth loop impedance (mA will flow)
    therefore the generator will not trip out. So, will the building frame
    be a 200V potential. I also think that if the load bank chassis is
    connected to the incoming neutral then high fault current will flow and
    trips will operate.

    Well long question, and thoughts or views?

  2. J. B. Wood

    J. B. Wood Guest


    Hello, and the question of grounding of portable generators has been
    discussed at length in this ng. Try a Google "groups" search. Sincerely,

    John Wood (Code 5550) e-mail:
    Naval Research Laboratory
    4555 Overlook Avenue, SW
    Washington, DC 20375-5337
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