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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
    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.
  • 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.


  • 0
    The danger is that you are carrying out a test to live terminals, albeit IP2x / IPxxD, which in reality should not be a "danger" to those with suitable training / experience, but not something you'd want to expose ordinary persons to.


    In many CUs, the job can be made safer by using GS38  croc-clips to connect to a neutral bar (or perhaps 2 if you're doing a 3-wire test), so you're only holding one GS38 probe in a terminal.


    I'm aware that space is limited in some modern caravans - of course you're there doing the risk assessment and of course it's up to you on the day.


    I do, however, object to the "up/down" method being called an "N trick" - it's a procedure that's recommended in a British Standard Published Document, and in this particular case may well be safer than disabling an upstream RCD
  • 0
    Last night after work we went to a supermarket to collect our weeks food order for us, my dad and mother in law, so we were at the shed around the back and I got out of the car to go to stand by a supermarket delivery van to distance myself. The van was parked and plugged into the mains with a flex to keep the fridge and freezer running, I'm eyeing the box on the wall up and noting that there was not an obvious earth rod and wondered what the risks of leaning against the van would be.


    Having returned home, with a fish and chips, I went out again to take my Dad his food then returned a hour or so later to find my wife half way through a TV programme, so I put a JW YouTube video on that had popped up a couple of days ago, about tool isolation transformers. JW sketched out the earth free arrangement on the output that I understood to be the case, the videos kept rolling and the next one was an American guy discussing if bench top isolation transformers being sold were actually earth free and by now the general drift was towards avoiding blowing up an oscilloscope whilst bench testing, but he had rigged up experiment with an RCD upfront and downstream of the isolation transformer, but I cannot tell you what happened, because I went to sleep without finishing my bottle of beer.


    My understanding was that tool isolation transformers are really intended to supply one tool, preferably a double insulated tool without a long extension lead to work safely without an RCD, so do they take the earth through or not?


    Even if they do a RCD cannot work downstream of them.

     

  • 0
    I do, however, object to the "up/down" method being called an "N trick" .

    Apologies Graham, I had not intended to offend.

    "Tricks" in a broader sense
  • 0
    gkenyon:


    I do, however, object to the "up/down" method being called an "N trick" - it's a procedure that's recommended in a British Standard Published Document, and in this particular case may well be safer than disabling an upstream RCD




    I don't view it as a "trick" it does however require dismantling of the installation and it cannot be done with some portable RCD devices such as plug in RCD adapters. In a BS7909 installation everything should be bench tested before going to site, so a socket without RCD protection at the back of the test bench seems the obvious answer. For several years I have tested some BS7909 distribution boards for an events company, they PAT guy does all the leads etc, but fought shy of doing the distribution boards, so I have the issue of testing RCDs in mobile distribution boards at home, where all my sockets are RCD protected. But not this year, all the events have been cancelled.


    Andy B.


  • 0
    Paul Robinson:


    I have an acquaintance who asks why his IET 18th Edition RCD tester when plugs it in to the Local RCD 


    Paul , Swindon LN IET. 



     


    From the original post, I was specifically exploring how to do the RCD test by plugging the tester in, rather than dismantling the installation. 


    The answer is use the test button on the RCD apart from when annual servicing is being carried out, then the cover needs to come off the consumer unit.


    Andy Betteridge


  • 0
    Sparkingchip:



    I don't view it as a "trick" it does however require dismantling of the installation and it cannot be done with some portable RCD devices such as plug in RCD adapters.




    Yes it can be done very safely with PRCDs using plug-in test adaptors, provided there's a socket-outlet at the other end (so, plug-in RCD, or RCD plug on an extension lead).


    This technique should also work for testing at double SRCDs where the RCD element is shared between the two outlets ... but of course not for testing the SRCD itself: to test the SRCD you'd have to pop the green wire with a probe on the incoming N.


    A double socket-outlet is not always necessary, provided you choose two socket-outlets on the same circuit downstream of any RCD, it won't trip whilst you test the PRCD.


    Certainly this gets higher marks from a H&S advisor to the measurement at a CU approach.




     



    From the original post, I was specifically exploring how to do the RCD test by plugging the tester in, rather than dismantling the installation. The answer is use the test button on the RCD apart from when annual servicing is being carried out, then the cover needs to come off the consumer unit.




    Perhaps not... thinking about it, you could also use this PRCD test technique with a wander lead in place of the green lead to actually test the RCD in the caravan: Replace the PRCD in the diagram with the socket-outlet in the caravan, and plug the caravan, via a "connect my caravan to a socket-outlet in my PME installation at home" adaptor, in the other outlet of the double socket-outlet.

     

     


  • 0
    Best do this all at home and don't take an installation tester on holiday with you in your caravan ?


    I do actually think I have all the required leads and adapters to hand.


    Andy Betteridge.
  • 0
    By all means rename 'trick' to 'technique' if the use of the word concerns you - to me in this case  they are synonymous - 'tricks of the trade' clever ways of solving otherwise tricky = potentially difficult problems .

    However as a technique it is not as well understood / explained, and therefore not in as widespread use, as it could be.

    There are a few things surrounding earthing, RCDs and fault currents that catch the unwary - especially when folk raised on TNC-s  examples try and do things on a  TT site.

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