This discussion is locked.

You cannot post a reply to this discussion. If you have a question start a new discussion

BS7671;2018 Minor works certificates.

Sparkingchip over 5 years ago

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

What do you actually think of them?

Andy B.

Parents

0 gkenyon over 5 years ago
Andrew Jewsbury:

TT system, the disconnection time of 0.2 s might not be achieved at I_Δn and therefore the disconnection time should to relate to 5I_Δn (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 2I_Δn 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 R_A 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.
Cancel
Vote Up 0 Vote Down

Cancel

Reply

0 gkenyon over 5 years ago
Andrew Jewsbury:

TT system, the disconnection time of 0.2 s might not be achieved at I_Δn and therefore the disconnection time should to relate to 5I_Δn (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 2I_Δn 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 R_A 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.
Cancel
Vote Up 0 Vote Down

Cancel

Children

No Data

BS7671;2018 Minor works certificates.

Join us to get the best from IET EngX.

Community Rules & Guidelines