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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. 



  • 0
    ebee:

    Can`t we cheat that one in a similar manner?

    At best it will only check the RCD itself is working under those conditions (Fake TNC-S) and not the actual conditions on site but hey ho better than not testing at all methinks


    The testing would only be "site conditions" if the unit is tested at the site it's being used at, which won't be the case for many touring caravan periodics (or "service checks"), and pre-delivery or pre-use inspections of mobile and transportable units.


    Also, as I said earlier, do we need to worry about the actual cpc being part of the test, particularly considering that 

    (a) we check earth continuity anyway and

    (b) for additional protection, the fault path doesn't necessarily include the cpc.


  • 0
    A mobile caravan owner or user cannot really test a RCD in anyway other than by pressing its built in test button.


    Using a Very basic loop polarity and RCD tester  may trip the upfront RCD in the fixed installation first, which won’t prove the caravan works. So long as one of the RCDs works you can sleep easy on your trip away, as you know at least one of the devices works.


    Testing the caravan RCD is going to involve dismantling the consumer unit, as Graham pointed trying to trick the RCDs by playing with the earthing arrangement is not straightforward for a big lump of metal that introduces alternative earth paths, so is not easy to isolate within the installation for testing.


    Going back to the “PAT” analogy when insulation testing a caravan because of reported RCD tripping I just unplug it and put the installation tester croc clips across the pins of the plug testing the caravan and its flex in one go, then move on from there.


    Don’t overthink things, the user just needs to press the test buttons on the RCDs and check both of them.


    Andy B.
  • 0
    gkenyon:

    Wouldn't this provide electrical separation to more than one item of equipment if used on a caravan or mobile/transportable unit?


    I'm not a big fan of that idea, especially as the RCD test will actually introduce a fault ... and I may be in contact with exposed-conductive-parts at the same time ... for example when resetting the RCD in a transportable unit with metal CU !


    I think I'd prefer to simply connect the caravan or unit to a socket-outlet with additional protection by 30 mA RCD in accordance with Regulation 411.3.3 or 708.415, and conduct the "up/down" test on the RCD in the caravan or mobile/transportable unit !


     Yes, you're right, there is not an easy cheat to avoid dismantling the caravan installation for some testing by at least opening up the consumer unit within the caravan, even for something as simple as testing the RCD inside the caravan with a RCD tester of any type without tripping an upfront site installation RCD.


    I have been trying to figure out the wiring arrangement within that Seaward PAT RCD isolation transformer test adapter and ended up back at square one with an CPC to neutral link inside it, the conclusion I came to is that with an isolation transformer is that you don't need RCD protection downstream of it and you have got to tweak the wiring arrangement to reintroduce an "earth" connection to fire the RCD off, so it's no longer an isolation transformer. 


    Andy Betteridge.


  • 0
    It would be like trying to use the two-pin shaver socket in your bathroom to test a portable plug-in RCD adapter when there's no where to connect the earth test lead, you aren't going to do it without tweaking the wiring.


    Andy Betteridge
  • 0
    See Fig A722 in Amendment 1:2020.
  • 0
    gkenyon:

    See Fig A722 in Amendment 1:2020.


    That's not in the Big Blue Book, I'll have to have a look online on the IET website, I am guessing we are heading off in the direction of how to use an isolation transformer for charging an electric vehicle.


    Andy betteridge.


  • 0
    gkenyon:

    See Fig A722 in Amendment 1:2020.



    BS7671:2018:Amd1


    That's the arrangement I pictured it in my mind to get the RCD to trip under test using an isolation transformer, it takes away risks associated with broken PEN conductors, but surely has to reintroduce some risk, but I suppose an EV is not double insulated like an electric razor and a hell of a lot bigger and there's not much of an option to solely rely on the isolation transformer.


    By now we could have took the RCD out of the caravan, bench tested it and put it back in again ?


  • 0
    Sparkingchip:



    By now we could have took the RCD out of the caravan, bench tested it and put it back in again ?




    Or just used the "up/down" method ?


  • 0
    Or indeed got everyone in the caravan to stay in and everyone outside to stay out, and done the test that may make it live - modern  test instruments automatically cut-off before the dangerous time/voltage product is reached,


    I'd be happy with 2 tests, one to fire the first RCD using the CPC, and then a second to fire the second RDC using the neutral trick and  just getting in and out quickly. A transformer is something else to go wrong, and temporarily installing one for the test duration is probably as likely to hinder as to help and not very practical.


  • 0
    I'm not feeling an urge to buy one of the Seaward PAT RCD testers.

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