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.

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.