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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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  • 0
    Yes, the method uses the Neutral to test the RCD.


    Is that an issue?
    1. Where the RCD is used for additional protection, as per the requirement in 411.3.3 or 708.415 for the socket-outlet supplying the caravan and 721.415.1 for the caravan itself, the earth fault path may not involve a protective conductor at all (e.g. cut lead causes exposes live part which is touched by a person standing on the ground).

       

    • More importantly, PD IEC/TR 62350 Guidance for the correct use of residual current operated protected devices (RCDs) for household and similar use says that a test between L and PE may not be a suitable test ... especially in conditions that you might find when testing a caravan (loads operating elsewhere in the installation supplying the caravan, either at home, on site, or at the caravan maintenance workshop. I've certainly come across the issues stated in that standard when testing RCDs in Mobile & Transportable Units using the "L to PE" method



    I understand that there are differing views on this method where RCDs used for ADS - is it OK not to use the PE? Well, 643.7.1 does not preclude the use of the up/down method to check the device is operating to spec, and other means, e.g. Ze+R1+R2 or Zs, to determine the disconnection times are met taking into account the characteristics of the device?
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  • 0
    Yes, the method uses the Neutral to test the RCD.


    Is that an issue?
    1. Where the RCD is used for additional protection, as per the requirement in 411.3.3 or 708.415 for the socket-outlet supplying the caravan and 721.415.1 for the caravan itself, the earth fault path may not involve a protective conductor at all (e.g. cut lead causes exposes live part which is touched by a person standing on the ground).

       

    • More importantly, PD IEC/TR 62350 Guidance for the correct use of residual current operated protected devices (RCDs) for household and similar use says that a test between L and PE may not be a suitable test ... especially in conditions that you might find when testing a caravan (loads operating elsewhere in the installation supplying the caravan, either at home, on site, or at the caravan maintenance workshop. I've certainly come across the issues stated in that standard when testing RCDs in Mobile & Transportable Units using the "L to PE" method



    I understand that there are differing views on this method where RCDs used for ADS - is it OK not to use the PE? Well, 643.7.1 does not preclude the use of the up/down method to check the device is operating to spec, and other means, e.g. Ze+R1+R2 or Zs, to determine the disconnection times are met taking into account the characteristics of the device?
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