Maximum permitted Zs values for circuits with TT earthing system

[soms]

Hello,

I am assisting my supervisor and getting some first hand experience of completing certification by filling in an electronic copy of an EIC, working from a handwritten circuit schedule with test results and my memories of the installation work.

I am at home so cannot ask him for advice at this moment.

This question also satisfies my broader desire to develop a thorough understanding of the correct requirements according to BS 7671.

So - I am filling in an EIC for an installation with a TT earthing system and
I am not sure what to record as the maximum permitted Zs value for each circuit.

The circuits are supplied from a consumer unit with dual 30 mA RCDs (BS EN 61008-1) and a selection of BS EN 60898 type B MCBs.

Where a TN earthing system is used, this is of course easy to do, by referring to the values in BS 7671 table 41.3.

I am struggling to understand what I should be entering as the maximum permitted values for a TT installation.
I note the following:

- The reason for calculating, measuring and checking Zs values is to ensure the correct operation of ADS.
- The maximum disconnection time for final circuits 32 A or below is 0.2 s (TT) instead of 0.4 s (TN installations).
- An RCD can be used for fault protection on a TT installation. The maximum earth fault loop impedance for an RCD being calculated from the residual current rating and a touch voltage of 50 V.


I will be honest, I find the BYB difficult to use at times when trying to determine an answer for myself.
With this matter for example, I have been reading through Chapter 41 and it's sending me in circles, with one regulation referring to another and not really arriving at a clear answer.

Any input would be much appreciated.
Thanks.

 

[telectrix]

where an RCD is used for fault protection, due to the high value of Ze ( and hence Zs), the max. Zs value is 1667 (for a 30mA RCD). R = V/I....... R = 50V/o.03A = 1667, 50V being considered a safe touch voltage.

 

[soms]

Thank you, I can see that makes sense. Re-reading regulation 411.5.3 I see it requires that the 0.2 s disconnection time (or 1 s for a distribution circuit) is met AND the resistance is low enough to achieve RCD operation at 50 V.

Although the required disconnection time for BS EN type RCDs at x1 rated tripping current is 300 milliseconds.
Of course the times achieved are often far better, in this case around 30 ms.

 

[happysteve]

I like your thinking, you've got all the relevant points I think. The RCD table at the start of Appendix 3 gives maximum disconnection times at 2x I∆n of 150ms. In practice, if Zs is anywhere near 1667Ω, you've probably got other things to worry about!

Table 41.5 ultimately gives you your answer, though.

 

[soms]

Thank you for the advice.
Much appreciated.