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Caravan RCD trip power connections

Paul R
Dear all , a holiday question


I have an acquaintance who asks why his IET 18th Edition RCD tester when plugs it in to the Local RCD (<30mA/300mS) , the primary supply RCD trips (that is further up the AC supply line) , rather than the caravan’s, so in effect nullifying the test.

Is this because the Primary Supply RCD  is monitoring a small standing current from other connections and the Local RCD needs to be tested by using the Local Earth at the unit ? ( which is not so easily accessible). 


Are there any other thoughts please ? 


Paul , Swindon LN IET. 



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    ebee:

    My reluctance to elaborate in open forum was because (ref the N trick) it should done under very careful conditions.

    Personally I was envisaging disconnecting the RCD from all local circuits and the local (caravan) earth/metalwork.

    That is, a purely 2 wire test with the incomming N linked to earth test lead on the meter (my meter only does 3 wire test).

    In other words the equivalent of a "bench test".

    Of course the other caution is all other persons would be excluded and I would be the only one about, locked doors etc etc.

    Of course MAPJ1s mention of a polarity test prior metered tests is an excellent idea.


    Like I said, I did not really want to mention it because if any precautions are missed it is potentially dangerous and I did not want to risk someone becomming startled by their own mortallity.


    What's the danger? It's not like we're connecting the installation's exposed metalwork to N - it's only the meter's earth lead that's connected to the supply N. Electrically isn't it similar to many RCD's test buttons - that connect outgoing L to incoming N (or vice versa) via a resistor?

       - Andy.


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  • 0
    ebee:

    My reluctance to elaborate in open forum was because (ref the N trick) it should done under very careful conditions.

    Personally I was envisaging disconnecting the RCD from all local circuits and the local (caravan) earth/metalwork.

    That is, a purely 2 wire test with the incomming N linked to earth test lead on the meter (my meter only does 3 wire test).

    In other words the equivalent of a "bench test".

    Of course the other caution is all other persons would be excluded and I would be the only one about, locked doors etc etc.

    Of course MAPJ1s mention of a polarity test prior metered tests is an excellent idea.


    Like I said, I did not really want to mention it because if any precautions are missed it is potentially dangerous and I did not want to risk someone becomming startled by their own mortallity.


    What's the danger? It's not like we're connecting the installation's exposed metalwork to N - it's only the meter's earth lead that's connected to the supply N. Electrically isn't it similar to many RCD's test buttons - that connect outgoing L to incoming N (or vice versa) via a resistor?

       - Andy.


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