Hi all,

As the title states....

How does one calculate it?

Thank you

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Regards
Carl.

I don't see how you can, not properly.

(You can prove a degree of safety using R2 alone - the old 'alternative method' - ensures that the voltage between MET and point doesn't exceed 50V if the fault current isn't sufficient to achieve ADS - still exists to some extent in reg 411.3.2.6 with the c.p.c. acting as a supp bond - but not the usual approach these days).

I suppose you could use R2 to have a guess at cable length and lookup the corresponding R1, but you'd normally have an idea of cable length anyway so could just lookup (per m) R1+R2, and that doesn't really prove that R1 is free of high resistance joints.

- Andy.

You could calculate it, subject to a few assumptions as Andy highlighted - one quick way (in an all insulated installation) would be to multiply R2 by the ratio of R1:R2 to give you a design R1 and R2 from which the addition of a known or assumed Zs and correction for temperature would give you a design value under stated or conventional conditions.

From a measured R2 in an instalation with metallic containment or multiple circuits enetring common conductive enclosures you have no chance without knowing the arrangements of R1 (or possibly of Rn, again assuming you don't have reduced neutrals present)

Regards

OMS

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Failure is always an option

and assumes that R1 is the same length as R2 - which for some circuits - e.g. lighting (due to switch drops) - it usually isn't.
- Andy.

Carl

I think in the 16th edition yellow book and pre there was a table for max R2 corresponding to CPD it's obviously not in the latest edition
I'm not sure if the latest edition schedule of test results gives the alternative R1+R2 or R2 method being recorded
Someone with a copy of the book will put us right on that
The table was probably removed when the powers that be realized it didn't work generally using modern installation practice

MrP raining at the moment, going home in 18days

Hi all,

Thanks a lot for the replies, it was just a thought and proves impractical in reality

Hope you had a great w/e

-------------------------
Regards
Carl.

As said above if live conductor is the same csa and length as the CPC you could just double the R2 to give you roughly r1+r2. If it's a 2.5 twin and earth you could divide the R2 value by 1.67 to give you an approx R1 value. Then add together again. Then add the Ze. I suppose it would only really be practical for straightforward radial circuits (not lighting). Easier to just r1+r2 though. As said you're only really supposed to calculate Zs if you have measured r1+r2.

You can still use R2 only in the latest schedule of test results. Not often used though.

Not as a method for calculating Zs, you could use the R2 column to demonstrate "continuity of protective conductors". You would need to test for polarity and you would still need to calculate or measure earth fault loop impedance (Zs).

Also in the brown book, reg 413-02-12 and Table 41C

yup - that was the 'old alternative method' I mentioned.
- Andy.