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How to read Table B1, and the expected r2 of a bonding conductorWhich row for an r2 test?

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How to read Table B1 — the bit that goes wrong

Guidance Note 3 · Appendix B · Table B1, and the formula that turns it into a resistance. The table gives conductor resistance in mΩ per metre, and it has two conductor columns. Which row you take depends on how many conductors your two leads are on.

Your testConductorsWhich row
r2 — a cpc or a bonding conductorOneYour size in column one, second column blank
R1 + R2 — a line and cpc pairTwoThe row with both columns filled
There are several rows with 10 mm² in the first column. For an r2 test you want the one that has nothing written in the second column — the single conductor on its own. That row gives 1.83 mΩ/m.
The stars mean twin and earth. Every size with a star in the first column is a twin-and-earth size. The stars stop at 16 mm² because that is the largest twin and earth made — there is nothing starred above it.
✋Remove parallel paths first. Disconnect the bonding conductor before you test it, so parallel paths cannot flatter your reading. Parallel paths are only left in place for Zs, Zdb and prospective fault current — there you want them.
Two leads, one conductor → one column. Two leads, two conductors → both columns. r₂ — a cpc or a bonding conductor column 1 blank 10 mm² → 1.83 mΩ/m one conductor, both leads on it bonding conductors, cpcs, r₂ R₁ + R₂ — a line and cpc pair column 1 column 2 2.5 / 1.5 → 19.51 mΩ/m two conductors, one lead on each 7.41 + 12.10 — the table adds them for you ✋ Taking a pair row for a single-conductor test is the classic mistake — it roughly doubles your expected value.
Ask one question before you look: how many conductors are my two leads sitting on? One means a blank second column. Two means both are filled.

Calculation 1: the expected r2 of a bonding conductor

Bonding conductors in a property are generally 10 mm² (sized from Electrician’s Guide to the Building Regs · Tables 3.3.2a/b · p.59), and say it runs 7 metres from the MET to the gas service pipe.

10 mm² bonding conductor, 7 m, MET to gas service pipe

It is an r2 test -> one conductor -> Table B1, 10 mm², second column blank

mΩ/m for 10 mm² on its own = 1.83

R = (1.83 × 7) ÷ 1000
  = 12.81 ÷ 1000
  = 0.013 Ω

That is what your meter should show when you do that test. And it is a good result, because continuity wants the resistance as low as possible.

Why divide by 1000? The table is in milliohms per metre. You want ohms, so the thousand comes out at the end. Every one of these calculations ends the same way.

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