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.

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.