Main bonding conductor resistance

Which guidance are you looking at? Is that from 8th Edition of Guidance Note 3?

In essence, GN3 simply said "suitably low reading" as the basic criteria, which of course may depend on conductor length, cross-sectional area, and other factors.

The explanation which follows that basic statement, regarding conductor resistances and accuracy of instruments, has been updated in the 9th Edition 2022, which better explains the 0.05 Ω is meant to relate to (roughly) 15 m of 6.0 sq mm or 25 m of 10 sq mm, but should not be used as a limit ...  because of instrument accuracy issues, where the expected reading is less than 0.1 Ω, then the inspector should be looking for ≤ 0.1 Ω as an acceptable reading.

(See 1st para on Page 61 of the IET Guidance Note 3: Inspection & Testing, 9th Ed 2022.)

Thanks Graham

So if I am interpreting your explanation correctly, then the 0.05 Ohm comes from real world typical scenarios e.g. 15 m of 6.0 sq mm or 25 m of 10 sq mm, e.g. 25 x 0.00183 = 0.04575 (0.05 Ohm 2.d.p.).

Allow another 0.05 Ohm for instrument accuracy and you get ≤ 0.1 Ω being an acceptable reading.

I thought the 0.05 was merely a test of the actual joint resistance from bonding condutor to extraneous-conductive-part - no mention being made of the resistance of the conductor itself.

If not, what is the relevance of the conductor resistance and why is there no mention of it in BS7671?

10 sq.mm. being required for PME because of CCC; not resistance.

Way back when I was involved in such things, IIRC, - the 0.1 (or 0.05) values were meant to be just convenient rules of thumb - get that on your meter and you can tick the box without worry (which probably works for 80 or 90% of installations) - only if the meter reads higher need you start having to think about conductor length & c.s.a to see if your reading is reasonable.

  - Andy.