BS7671;2018 Minor works certificates.

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

Playing Devil's advocate for a moment... on a TT system is there a requirement to test a 30mA unit at 1x at all? While I can of course see the common sense in testing at 30mA, 643.8 seems only to actually require:

• That ADS is proved - typically that means showing a ≤0.2s disconnection time - hence a residual current exceeding 2x - so a 5x test using an ordinary instrument.

• and for additional protection, only the 5x test for 40ms is specified (and even that's only in a NOTE).

Also is there any value in using a lower multiple on a higher setting on the test instrument - e.g. using a 300mA x 1 as a better approximation to a 2x test for a 100mA device than a 5x 100mA test (and reduce the reliance on a low Zs in the process). Or is that instrument abuse?


On a different tangent, should we be verifying the RCD's reaction to d.c. components in the residual current too these days?


    - Andy.

Playing Devil's advocate for a moment... on a TT system is there a requirement to test a 30mA unit at 1x at all? While I can of course see the common sense in testing at 30mA, 643.8 seems only to actually require:

• That ADS is proved - typically that means showing a ≤0.2s disconnection time - hence a residual current exceeding 2x - so a 5x test using an ordinary instrument.

• and for additional protection, only the 5x test for 40ms is specified (and even that's only in a NOTE).

Well, I don't disagree with that, although the question that I think we will still be asked by JP, is whether we are verifying the device (in which case all tests in accordance with BS EN 61557-6 would be necessary) or the correct application of the electrical installation (which is at least as much about loop impedance and product selection as whether the device itself works - after all, we don't test mcb's do we ...)

Also is there any value in using a lower multiple on a higher setting on the test instrument - e.g. using a 300mA x 1 as a better approximation to a 2x test for a 100mA device than a 5x 100mA test (and reduce the reliance on a low Zs in the process). Or is that instrument abuse?

Perhaps, although you've got to wonder about the tests required in Chapter 64 and BS EN 61557-6. For example, I have a 300 mA RCD in a TT system protecting a final circuit containing fixed equipment < 32 A, for which Additional Protection is not required. It has a disconnection time of 220 ms at x1 test (which is OK, but doesn't meet the 0.2 s disconnection time):

• Do I replace the RCD just because I can't verify the 0.2 s disconnection time?
  
  OR

• Do I let it go, write 220 in the test cert (but then have no evidence of verification of the required disconnection time as per Chapter 64 and Table 41.1)?

On a different tangent, should we be verifying the RCD's reaction to d.c. components in the residual current too these days?

Yes, where necessary (noting Type AC still acceptable for general use), but you set that on the instrument in any case.

Graham


You posted a very good table above. Could this not be fettled and honed and go in to Part 6 as it would take out all the ambiguity? You might want to submit it in the next DPC?


As for the test that produces a 220ms for a final circuit on a TT system requiring a 0.2s disconnection time that is for once clear in 643.7.1 as it says, " The effectiveness of the automatic disconnection of the supply by RCDs shall be verified using a suitable test equipment according to BS EN 61557-6 to confirm that the relevant requirements of Chapter 41 are met, taking account of the operating characteristics of the device". So a 220ms time would not meet the requirements of Chapter 41 although it would meet the product standard. The same would apply to RCDs to the older BS 4293 standard.

As for the test that produces a 220ms for a final circuit on a TT system requiring a 0.2s disconnection time that is for once clear in 643.7.1 as it says, " The effectiveness of the automatic disconnection of the supply by RCDs shall be verified using a suitable test equipment according to BS EN 61557-6 to confirm that the relevant requirements of Chapter 41 are met, taking account of the operating characteristics of the device". So a 220ms time would not meet the requirements of Chapter 41 although it would meet the product standard. The same would apply to RCDs to the older BS 4293 standard.

220ms at 1x doesn't prove that that requirement has been met, but neither does it prove that it hasn't been either.


You could re-do the test at say 2x or higher (say using the 1x setting on the next range up, which would normally give you around 3x) which hopefully gives you a result within 200ms. Or you could do a 5x test (using the supply N rather than PE if Zs is a problem - as that verifies the effectiveness of the RCD itself just the same) and perhaps get a result below 150ms.


I think we have a problem though in that recording the time without recording what the test involved makes it all rather meaningless to anyone subsequently reading the schedule.


Or perhaps we take the attitude that if the RCD is within spec. for the tests we perform then it can be deemed to be operating correctly, so it's reasonable to presume it'll also operate within specification for other residual currents. After all we don't attempt to test any other component of the installation to anything like this degree.


   - Andy.