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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.
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    Andrew Jewsbury:




    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?


    Well, that's a question for the manufacturers, I guess. All that we do know, is that testing at a current of greater than 5IΔn for 40 ms may be hazardous, therefore not practicable.

     


    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.


    Perhaps ... there are a number of problems related to RCD testing, and it perhaps could be said that it's obvious that trying to do an "in-situ" test based on the parameters of what is effectively Table 3A in BS 7671 doesn't always lead to expected results. See below on the L-N test for the "test button".

     


    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.

     

    Given the fact that another recognised means of additional protection is supplementary local equipotential bonding, I don't quite follow your argument (although I'm not dismissing it entirely). I'm not sure anyone would say that someone touching just a line conductor of a circuit protected by a 30 mA RCD would be 100 % safe ... however, I do follow where you are coming from here.

    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?

    Perhaps not, as the RCD test is designed to be conducted downstream of the RCD, using the associated protective conductor. The RCD is not guaranteed to work with an earth return path by another means (although I'm happy to accept for additional protection, it's a possibility).


    Basically, as things stand (with BS 7671 Part 6 and the guidance in GN3 and OSG for testing), the designer should ensure Zs is low enough to accommodate the RCD at 5x for additional protection, and not simply Table 41.5 alone. When we are talking about automatic disconnection in times less than 0.3 s, this does present another issue, and definitely one for further discussion.


    We should certainly be looking at the performance of the installaition as part of the Verification, and not a particular protective device.
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    I think the references to RCD testing in Part 6 of the 18th are a dogs breakfast.


    In 643.8 the a shining example of ambiguity in the Note to the regulation. It needs to be understood that Notes do not create a regulation in themselves but are only guidance so not a "shall" or "shall not". You may have an RCD that is stickier than sticky the stick insect and it will operate at 5x Idelta n or higher or give it a jolt with 2A, 5A or more and it will operate and give you a very fast disconnection time. What does this prove? 


    As Graham K has pointed out that in Appendix 3  Table 3A you will find the the test currents and times for RCD product standards (BS EN 61008 and 61009). My view is that RCDs should meet this standard and that means they should trip in the stated  times set out in the standard. If an RCD does not trip at 1 x Idelta n then it is defective. This softer test is a better indicator of RCD health than the 5 x test and consumer safety.


    643.7 compounds the ambiguity as it applies to RCDs used for fire protection and fault protection. It seems to suggest that if you have carried out an earth continuity test which is satisfactory on installations that rely on an RCD for fault protection, and the RCD gives you a disconnection time that meets the appropriate time for the type of circuit and earthing system, you do not then need to carry out earth loop impedance testing so would put N/A in the column on the form. It does not indicate a test current but refers you Table 3A of Appendix 3. 


    All of the references to RCD testing require the test to be carrier out using an instrument that complies with BS EN 61557-6. Those instruments have functions that will test at both X1 and  X5 tripping currents at both halves of a full cycle. Most modern instruments  have an auto function that runs through the tests and stores the test results for examination post test.


    My view is, for both fire and fault protection, the RCD should be tested a X1 tripping current on both half cycles and the highest test result recorded. As you were probably taught on the C&G I&T courses you don't do a X 5 test for fault protection and you carefully avoided the trap exam question when asked what is the maximum test current for a 100mA RCD.


    The new Appendix 6 test forms only have one column for the RCD test result. My view is that you should enter the highest time out of the applied tests.


    The new Stroma software forms that will soon be launched will have have both X1 and X 5 columns on the test sheets.
  • 0
    John,


    In general, I don't really disagree with you, but ...

    If an RCD does not trip at 1 x Idelta n then it is defective. This softer test is a better indicator of RCD health than the 5 x test and consumer safety.






    This does not unfortunately take account of a small number of cases where the RCD passes a 1x test, but then goes on to fail a 5x test, although I agree this is not simply a case of "sticky RCD".


    In fact, I am undecided in some cases whether this phenomenon is a combination of other external influence and/or a property of how a particular test set operates under given conditions.


    Having said that I have seen this behavior on devices with different test sets at the same time doesn't help, as an influence on the supply might well have caused the problem, and of course we know of cases where RCDs returned to manufacturers having "failed" on-site have been tested in isolation and found to be operating perfectly well.


    But that in itself causes another question regarding RCDs used for Additional Protection, or fault protection where 0.2 s disconnection time is required ... i.e. will they actually operated as expected?


  • 0
    Or, alternatively, have we selected the correct type of RCD for a particular installation or application, is possibly another answer to this question.
  • 0

    Graham Kenyon:

    John,


    In general, I don't really disagree with you, but ...




    If an RCD does not trip at 1 x Idelta n then it is defective. This softer test is a better indicator of RCD health than the 5 x test and consumer safety.






    This does not unfortunately take account of a small number of cases where the RCD passes a 1x test, but then goes on to fail a 5x test, although I agree this is not simply a case of "sticky RCD".




    For my own peace of mind, at home, I would wish to be assured that an RCD does trip as intended. If passing at 5x means that one will also pass at 1x, then all well and good: only the one test is required. However, if not, then surely both tests are required.


    Of course, there is no reason to stop doing the 5 tests, it is just that only one time needs to be recorded in the EIC/EICR (plus the all important test button).

  • 0

    Chris Pearson:




    Graham Kenyon:

    John,


    In general, I don't really disagree with you, but ...




    If an RCD does not trip at 1 x Idelta n then it is defective. This softer test is a better indicator of RCD health than the 5 x test and consumer safety.






    This does not unfortunately take account of a small number of cases where the RCD passes a 1x test, but then goes on to fail a 5x test, although I agree this is not simply a case of "sticky RCD".




    For my own peace of mind, at home, I would wish to be assured that an RCD does trip as intended. If passing at 5x means that one will also pass at 1x, then all well and good: only the one test is required. However, if not, then surely both tests are required.


    Of course, there is no reason to stop doing the 5 tests, it is just that only one time needs to be recorded in the EIC/EICR (plus the all important test button).


     




    I'm still not satisfied. There are also the cases where the 1x time passes (i.e. < 300 ms) but does not provide evidence of compliance for 643.7.1(b) - in particular the disconnection time recorded > 200 ms in cases when Chapter 41 demands 0.2 s disconnection time.

    And the even greater number of cases where 643.8 is not verified by the 1x test - in particular disconnection time recorded > 40 ms (per the Note to the regulations)?


    Surely, this is the purpose of the verification stated in the Regulations, and if the evidence is not there, from the test as specified, then the particular requirements of Chapter 41 for ADS are not met - similar to a high loop impedance reading where OCPDs are relied upon to provide ADS?


    Presented with a set of test results that record the fact that the requirements have not been met, and certified by a competent person by their signature, what else could a court decide?


    Sure, if the 1x test is below 40 ms (as might be the case) then no-one would know the difference. But this doesn't help anyone decide what is really necessary.

    For a 30 mA RCD, there is rarely a reason to not carry out the 5x test, and record the result. I agree with larger residual current ratings, the 5x test is not desirable, but the way the 18th Edition requirements are written does not really afford us a means of providing evidence compliance (unless of course you're lucky enough to be able to achieve the required disconnection time on the 1x test).


    This is why I don't subscribe to the "only record 1x" ...

     

  • 0
    Graham


    Yes 30mA RCDs need testing at 5 x I delta n to verify they will trip at not more than 40ms but we need to verify at a lesser current than 150mA coursing through someones body they will also trip in a safe time. Reference to the IEC shock current curves would seem to suggest this? As for testing at more than 5 x I delta n at "higher than 5 times its rated residual operating current", as suggested in the Note to 643.8, I will ask again what does that prove? How about a test current of 5A that should operate any sticky 30mA RCD.


    For fire and fault protection 643.7.1 directs us to use an RCD tester to BS EN 61557-6 which may not be able to deliver test currents at 5 times higher rated RCDs. 643.7.1 directs us to Table 3A of Appendix 3. 


    So we need to know what the rating of the RCD is and the purpose for which it is installed?


    So Graham head above the trench time. For additional protection what current(s) would you apply to the RCD and what maximum trip time would you expect? The same question for a 300mA RCD, time and not time delayed, provided for fault or fire protection?
  • 0
    JP,


    A summary of my thinking where RCDs are used to provide ADS in TT systems (or in the case of 10 mA and 30 mA RCD, Additional Protection and ADS):

     
    RCD Residual current Rating

    Tests, results and notes

    10 mA

    Conduct both 1x and 5x tests

    Record the 5x result.
    Verifies both additional protection to 643.7 and ADS to 643.7.1 b) for 0.2 s and 1 s disconnection.

    I would be able to demonstrate compliance with Regulation 411.3.2.4 and 411.3.2.1.

    30 mA

    Conduct both 1x and 5x tests. Requires loop impedance less than 1364 Ω if you take into account full volt drop

    Record the 5x result.
    Verifies both additional protection to 643.7 and ADS to 643.7.1 b) for 0.2 s and 1 s disconnection.

    I would be able to demonstrate compliance with Regulation 411.3.2.4 and 411.3.2.1.

    Whilst the full Zs of 1667 Ω presented in Table 41.5 wouldn't provide sufficient current for a 5x test with lower voltages, earth electrode resistance should be limited to 200 Ω so no issues anticipated

    100 mA

    Conduct both 1x and 5x tests. requires loop impedance less than 409 Ω if you take into account full volt drop

    Record the 5x result.
    Verifies ADS to 643.7.1 b) for 0.2 s and 1 s disconnection.

    I would be able to demonstrate compliance with Regulation 411.3.2.4 and 411.3.2.1.

    Whilst the full Zs of 500 Ω presented in Table 41.5 wouldn't provide sufficient current for a 5x test with lower voltages, earth electrode resistance should be limited to 200 Ω so no issues anticipated


    If you are asserting it's unsafe to test the RCD at 5x in domestic, then do and record the 1x, but now you only have verified disconnection at 1 s and compliance with 411.3.2.4, and therefore in domestic this RCD can only provide ADS for distribution circuits, and final circuits not covered by 411.3.2.2.

    300 mA

    Not practicable (or perhaps safe) to test at 5x, so we are limited here to testing at 1x.

    Record the 1x result.
    Verifies ADS to 643.7.1 b) for 1 s disconnection.

    I would be able to demonstrate compliance with Regulation 411.3.2.4, but not 411.3.2.1.

    This leads me to assert that the 300 mA RCD may only provide ADS for distribution circuits, and final circuits not covered by 411.3.2.2

    500 mA

    Not practicable (or perhaps safe) to test at 5x, so we are limited here to testing at 1x.

    Record the 1x result
    Verifies ADS to 643.7.1 b) for 1 s disconnection.

    I would be able to demonstrate compliance with Regulation 411.3.2.4, but not 411.3.2.1.

    This leads me to assert that the 500 mA RCD may only provide ADS for distribution circuits, and final circuits not covered by 411.3.2.2

    100 mA Type S

    Conduct both 1x and 5x tests. requires loop impedance less than 409 Ω if you take into account full volt drop

    Record the 5x result.
    Verifies ADS to 643.7.1 b) for 0.2 s and 1 s disconnection.

    I would be able to demonstrate compliance with Regulation 411.3.2.4 and 411.3.2.1.


    If you are asserting it's unsafe to test the RCD at 5x in domestic, then do and record the 1x, but now you only have verified disconnection at 1 s and compliance with 411.3.2.4, and therefore in domestic this RCD can only provide ADS for distribution circuits, and final circuits not covered by 411.3.2.2

    300 mA Type S

    Not practicable (or perhaps safe) to test at 5x, so we are limited here to testing at 1x.

    Record the 1x result.
    Verifies ADS to 643.7.1 b) for 1 s disconnection.

    I would be able to demonstrate compliance with Regulation 411.3.2.4, but not 411.3.2.1.

    This leads me to assert that the 300 mA Type S RCD may only provide ADS for distribution circuits, and final circuits not covered by 411.3.2.2

    500 mA Type S

    Not practicable (or perhaps safe) to test at 5x, so we are limited here to testing at 1x.

    Record the 1x result.
    Verifies ADS to 643.7.1 b) for 1 s disconnection.

    I would be able to demonstrate compliance with Regulation 411.3.2.4, but not 411.3.2.1.

    This leads me to assert that the 500 mA Type S RCD may only provide ADS for distribution circuits, and final circuits not covered by 411.3.2.2



     

  • 0

    Graham Kenyon:




    Chris Pearson:




    Graham Kenyon:

    John,


    In general, I don't really disagree with you, but ...




    If an RCD does not trip at 1 x Idelta n then it is defective. This softer test is a better indicator of RCD health than the 5 x test and consumer safety.






    This does not unfortunately take account of a small number of cases where the RCD passes a 1x test, but then goes on to fail a 5x test, although I agree this is not simply a case of "sticky RCD".




    For my own peace of mind, at home, I would wish to be assured that an RCD does trip as intended. If passing at 5x means that one will also pass at 1x, then all well and good: only the one test is required. However, if not, then surely both tests are required.


    Of course, there is no reason to stop doing the 5 tests, it is just that only one time needs to be recorded in the EIC/EICR (plus the all important test button).



    I'm still not satisfied.



    I think that I have been slightly mis-read here.


    I was referring to the five tests at 1/2 Ideltan, Ideltan (x 2), and 5 Ideltan (x2). Performing them and writing them in a notebook is no bother at all.

  • 0

    I think that I have been slightly mis-read here.


    I was referring to the five tests at 1/2 Ideltan, Ideltan (x 2), and 5 Ideltan (x2). Performing them and writing them in a notebook is no bother at all.





    Don't disagree with you. I think from the Table in my previous post, you will see where my thinking is coming from re 10 and 30 mA RCDs.


    And, more importantly, where JP is going with higher rated RCDs. The rub being, that a designer could select an RCD with rating 500 mA Type S to meet 411.3.2.1, and, providing the loop impedance were low enough to achieve the stated disconnection time (in this case, 200 Ohm would be good enough for the 200 ms disconnection time at 2x, or 1 A) , all would be well ...

    That is, until we come to our Initial Verification (or with existing installations Periodic Verification) in accordance with BS 7671:2018 - if we take 643.7.1 b) (2) at its verbatim word !

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