BS7671;2018 Minor works certificates.

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.

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.