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RCD test on all circuits or just once for each RCD?

Discussion in 'Electrics UK' started by eveares, 29 Dec 2017.

  1. eveares

    eveares

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    Curious about how those on here handle testing RCD's when doing EICR's or MWC's?

    If you have several circuits protected by an RCD as is often the case, do you test the RCD on each circuit it protects or just once for each RCD what is ideally done with no loads at the distribution board.

    In other words, do you simply test that the RCD it's self functions correctly or do you also test it functions under earth fault conditions on every circuit (or even socket outlet) it protects.

    I.e. You have a very odd socket that is not earthed, but has the neutral and earth terminals joined together. On first glance, that socket may appear to be ok and pass it's Zs test.

    Assuming you're unaware of the fault and thus have not done any of the following;

    • A R1 + R2 test at the DB with the joint/link at the defective socket, what would hopefully show ∞Ω
    • A N-E IR test initiated at the defective socket what would show at or very close to 0Ω
    How do you catch the above fault without manual inspection or an RCD test if the Zs reading you get is within the range your expecting and similar to other sockets?


    Regards: Elliott.
     
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  3. Lectrician

    Lectrician

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    You only test the RCD once. Test it from any point after the RCD. Often plugged into a socket is easiest.

    There is a test sequence for a reason.
     
  4. ericmark

    ericmark

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    Seems you are mixing and matching, the R1 + R2 is tested with a low ohm meter with at least a 200 mA current where the RCD is tested with a RCD tester normally 6 tests for each RCD, so three tests one shows it does not trip, next it trips with rated current, and last it trips within rated time at 5 times rated current and each test done on pos and neg half cycle so 3 x 2 = 6 tests.

    I have found with the old mechanical RCD mounting in the consumer unit can cause some warping of the casing causing them to fail, even the supply wires can put some physical strain on the device, so it needs testing after it is installed. In the main using a socket which it supplies.
     
  5. John D v2.0

    John D v2.0

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    Good question! I Agreed with above, the RCD test is to make sure the RCD is itself is working correctly. The rest of the installation should be proved correct before it is energised.
    If you're thinking of eicr, you could earth loop and RCD test every accessory to be really thorough. But you're not really testing the RCD once you've done it once, you're testing the circuit wiring
     
  6. flameport

    flameport

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    A Zs test is one of the last ones to be done.

    If you haven't found a N-E short by then, you have missed most of the other tests, and therefore the entire test process up to that point is invalid.
     
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  7. eveares

    eveares

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    It was purely theoretical after seeing this and this thread. Also, I am well aware that RCD's are tested at 1X, 5X and 1/2X at 0° and 180° plus a ramp test at 0° and 180° and then lastly the test button on the RCD/RCBO.

    My point about a R1 + R2 test with a 200ma low ohm meter and a Insulation Resistance test was about tests that have not been carried out what would have flagged up an issue in the first place.

    That may be, but would you manually inspect every socket during a EICR. Assuming the defective socket is not the furthest (and thus the link point in a R1 + R2 test) and that the loop impedance of other correctley wired sockets is low to begin with, Is it likley such a fault would go unnoticed during a EICR and put down as a limitation that not every socket was opened up.
     
  8. Risteard

    Risteard

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    Ramp tests are not an official test. In fact they are completely pointless if you prove an RCD isn't tripping at half its rated residual operating current and is operating at its full rated residual operating current as that is the requirement.
     
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  9. flameport

    flameport

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    Not usually.

    However no inspection is necessary to identify a socket with no earth connection and N-E connected together.
    The very first test which is to confirm continuity of the CPC would identify such a thing - and once the problem was found, the socket would be opened and examined to establish the cause.
     
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  11. Taylortwocities

    Taylortwocities

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    Testing an RCD is normally done without any circuits attached. Otherwise any leakage current in the installation will skew your testing.
     
  12. eveares

    eveares

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    As I said:

    This is what I mean by the way:


    Of course you could check the continuity of the earth in question against a known earth, and risk tripping the RCD if you cause a large enough current shunt through the true earth.

    If it is a TT system, you would also discover an unusually low Zs for being it is a TT arrangement such as by a advanced socket tester with a built in loop check.
     
  13. JohnW2

    JohnW2

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    What exactly (if anything) are you suggesting that the CPC(s) would be connected to?

    Kind Regards, John
     
  14. eveares

    eveares

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    Nothing, as in some one has installed a socket with 2 core cable and used the neutral to provide a link to the earth terminal of the socket.
     
  15. JohnW2

    JohnW2

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    Hmmmm. Have you ever seen, or heard of, a case in which that had been done?

    Kind Regards, John
     
  16. eveares

    eveares

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    No, but I am sure someone's done before now.
     
  17. flameport

    flameport

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    Which would be the very first test when doing an EICR.

    Continuity tests are done with the power isolated.

    Not only unlikely - it's impossible if the required tests are done.

    Ultimately the only circumstances in which someone could do an EICR without identifying such a blatant defect would be if they were totally incompetent, completely missed most/all of the testing and then completed the report with purely fictional information before riding off into the sunset on their horse.

    For the other question - RCDs are tested at the origin, with other circuits switched off / disconnected. You are testing the device, not the circuit(s) it is connected to, as the circuits would already have been tested by that point.
    In some cases it may be convenient to connect the RCD tester via a socket outlet, but that should only be done with any loads disconnected / unplugged.

    Not necessary or even desirable.
     
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