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BS7671;2018 Minor works certificates.

Sparkingchip
Are you all organised and have a supply of the new certificates to hand?


What do you actually think of them?


Andy B.
  • 0
    Well. 1.IΔn would give the worst case for both fault conditions and additional protection even though 5.Δn was quoted for additional protection in the last edition. It now appears to be left open.


    Legh


  • Former Community Member
    0 Former Community Member
    I Bought some 2018 MWC forms from niceic before Xmas (no niceic logo on them) and I was a bit concerned that they might not comply now that the 18th. is in force but from what I've read here it seems they're ok to use. As i'm no longer a scheme member I will probably only be using MWC's now!
  • 0
    I can't see a problem with a rubber stamp indicating the new edition as most of the criteria are the same as previous editions.


    Legh
  • 0

    Philip Wood:

    In part 4 of the 'Minor Works Certificate' for RCD's it says you should use 'I∆n' and this should be the longest time it takes for the RCD to operate, or in other words the worst case scenario. 




    Is this in BS 7671, or another publisher's version? If BS 7671, please could you let me know the page?


    Regardless, this makes no sense given the test requirements (particularly, the test for additional protection only specifies the 5x test so why record a result for a different test???)

  • 0

    • TT system, the disconnection time of 0.2 s might not be achieved at IΔn and therefore the disconnection time should to relate to 5IΔn (if, as per the Note to 643.7.1 states, we take into account the operating characteristics of the device per Table 3A in Appendix 3) - that is unless your tester has a 2IΔn setting


    On the other hand it would be rather odd to specify a test that couldn't necessarily physically be performed in a compliant installation. For a 30mA RCD, BS 7671 specifies a max Zs of 1667 Ohms (table 41.5) (or thereabouts - actually is is permitted to be higher provided RA doesn't exceed that figure (but I'll ignore that for the moment), the 200Ω figure is only a suggestion) - thus the maximum earth fault current we can really rely on is 230V/1667Ω = 138mA - i.e. below the 150mA needed for a 5x test. (Or if the tester has some means of generating the extra current, the touch voltage would exceed 50V which would likely abort the test anyway).


    I guess this is all a results of the discussion about removing RCD testing from the 18th - it does seem peculiar though to record the result of a test if you're not recording an indication of which test was performed.


      - Andy.
  • 0

    Andrew Jewsbury:




    • TT system, the disconnection time of 0.2 s might not be achieved at IΔn and therefore the disconnection time should to relate to 5IΔn (if, as per the Note to 643.7.1 states, we take into account the operating characteristics of the device per Table 3A in Appendix 3) - that is unless your tester has a 2IΔn setting


    On the other hand it would be rather odd to specify a test that couldn't necessarily physically be performed in a compliant installation. For a 30mA RCD, BS 7671 specifies a max Zs of 1667 Ohms (table 41.5) (or thereabouts - actually is is permitted to be higher provided RA doesn't exceed that figure (but I'll ignore that for the moment), the 200Ω figure is only a suggestion) - thus the maximum earth fault current we can really rely on is 230V/1667Ω = 138mA - i.e. below the 150mA needed for a 5x test. (Or if the tester has some means of generating the extra current, the touch voltage would exceed 50V which would likely abort the test anyway).


    I guess this is all a results of the discussion about removing RCD testing from the 18th - it does seem peculiar though to record the result of a test if you're not recording an indication of which test was performed.


      - Andy.

     




    Andy,


    Whilst on the surface it seems you're correct, in reality, there are two back-to-back requirements in 411.5.3:

    (i) to meet the disconnection time of Table 41.1

    (ii) to meet Table 41.5


    If you meet (ii) but not (i), doesn't meet BS 7671 requirements for ADS. This is why Table 3A is included. Now, whilst the trip-time for 2IΔn will be easily met by a loop impedance meeting the requirement for Table 41.1 for TT systems, Most RCD testers do not test at 2IΔn, and therefore the only option is 5IΔn. I've definitely seen RCDs with longer trip times at 5IΔn than at IΔn, so IΔn is not always the "worst case" either!


    AND


    To complicate things even more, Regulations 411.3.3 and 411.3.4 may well apply to the circuit. These require RCDs for Additional Protection - in this case this would invoke the 5IΔn test. To add all these requirements together, we have a maximum loop impedance for a 30 mA RCD of 230*.95/0.15 Ω (where additional protection is needed), i.e. 1457 Ω and not 1667 Ω (although, consider the fact that the earth electrode resistance shouldn't exceed 200 Ω, you should be far less than the required maximum).


    What's more concerning is where a larger RCD is used up-front, e.g. 300 mA RCD for main switch. To make this testable (with an RCD tester that only tests at 1x and 5x) requires a loop impedance of 145.7 Ω, and therefore your earth electrode resistance has to be significantly less than 200 Ω. And similarly, for a 500 mA up-front RCD, you'd get a maximum loop impedance of 87.4 Ω before this became "untestable" with many standard testers.



    I wholly accept that some could argue, if I meet the 0.2 s disconnection time with the 1x test, surely that's OK? Well, as I've said above, perhaps not as I've seen higher trip times with the 5x test than the 1x test (although the tester stops testing at 40 ms on the 5x test), so there's no guaranteeing the performance at 2x, and also there's the requirement for additional protection to consider in some cases (which as you've pointed out moves the 5x requirement from Chapter 41 to Part 6).




    It's a minefield ... but then, there's perhaps nothing different here than the situation in the 17th Edition. The requirements remain the same for TT disconnection and additional protection, and standard RCD testers still perform the way they did before 1 January 2019.

  • 0

    I've definitely seen RCDs with longer trip times at 5IΔn than at IΔn, so IΔn is not always the "worst case" either!



    In which case, can we have any confidence that a 1x (or 5x) test will give realistic assurance that the RCD will open in the required time when subject to a real earth fault - which could be many amps in magnitude?


    Most RCD testers do not test at 2IΔn, and therefore the only option is 5IΔn.



    Cue for a new range of "18th Ed" RCD testers with a 2x option? wink


    Perhaps more practically we should only be thinking in terms of 'sampling' the RCDs performance at a few points and if they're within spec then say it's reasonable to assume that the RCD is functioning as designed and so will operate within specification throughout its entire range. Perhaps single test (1x perhaps) would be sufficient. I gather that some manufacturers suggested that operating the T button and observing an 'immediate' trip could be sufficient.

    These require RCDs for Additional Protection - in this case this would invoke the 5IΔn test. To add all these requirements together, we have a maximum loop impedance for a 30 mA RCD of 230*.95/0.15 Ω (where additional protection is needed), i.e. 1457 Ω and not 1667 Ω



    Humm - not sure of the logic there - additional protection is usually aimed at situations of direct contact (nails through cables, flexes cut by lawnmowers, touching the contacts of lampholders) - where the c.p.c. and hence Zs aren't usually of relevance (likewise any calculations involving Uo or Cmin). In a functioning installation we're only really worried about 150mA going through the victim's body, where it goes after that isn't a concern. Certainly for a successful test we'd need a lowish loop impedance, but that requirement doesn't seem to be obviously written into BS 7671.


    Would you think it acceptable to perform a 5x test by placing the tester's earth probe on the supply N rather than PE? That would avoid problems due to a high Zs but still test the RCD's fundamental operation?


        - Andy.
  • 0
    We have had the discussion about RCD testing that is out of range on our testers on the old forum, I have also spoken to RCD manufacturers and test equipment manufacturers, in particular about testing 100 mA S-type time delayed RCDs installed in TT installations.


    The general consensus is that if such an RCD is within specification when tested at its rated current it will be okay when whacked with a bigger current.


    Testing a 100 mA RCD at five times its rated current is generally considered to be potentially dangerous in a domestic or similar installation as you are shoving a half an ampere of current through the earthing system with the meter set to deliver 500 milliamperes, a potentially lethal current.


     Andy Betteridge
  • 0
    Sadly, this general assumption that if it's OK when tested at 1x, it will work OK at 5x is something that I've not witnessed myself.


    However, a time-delay RCD would usually be for "distribution" circuits, and perhaps other circuits not covered by 411.3.2.2, and therefore 1 s disconnection time applies for TT systems, so the 1x test is what you want - i.e. usually you don't want to use 5x test for those in any case?


    I do think that there will be a few rounds of discussion on RCD testing to come. Certainly, it's no good checking an RCD works "to spec" ... that's not the objective of the test.


    The objective of the test is to check that either disconnection times to Table 41.1 are met, or, for RCDs providing Additional Protection, the RCD (typically 30 mA) disconnects within 40 ms at 5x - and the test should utilise the protective conductors.
  • 0
    Agreed, there can be a marked difference between x1 and x5 test results from a 30 mA RCD, but both of those tests can be completed with a standard RCD tester.


    Standard RCD testers don’t have the option to test at x2 which I have been told by RCD manufacturers and tester manufacturers does not matter, as you don’t need to do it anyway if you confirm a higher rated RCD is within spec at x1.


    We can only do what we can do regards testing RCDs, but we can write all the test results on a certificate, which may be five entries rather than one.


     Andy Betteridge

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