Question 1

Can I calculate R1+R2 from a live Zs reading?

Accepted Answer

No. A live Zs measurement includes parallel earth paths through bonding conductors and other circuits, which lowers the reading. R1+R2 must come from a dead test using a low-resistance ohmmeter to give an accurate value for the specific circuit under test.

Question 2

My ring circuit R1+R2 readings vary across socket outlets — is the ring broken?

Accepted Answer

Not necessarily. After the initial cross-connection readings, the figure-of-eight test produces values that vary across sockets due to the parallel resistance effect. The highest reading recorded at any socket is the circuit's R1+R2. A broken ring would show readings that are roughly double the expected value at one end.

Question 3

What R1+R2 value should a 20m run of 2.5/1.5mm² cable give?

Accepted Answer

At 20°C the combined resistance is 19.51 mΩ/m. For a 20m run: 20 × 19.51 = 390.2 mΩ, or approximately 0.39Ω. In practice, add a small amount for termination resistance and ensure you have nulled your test leads before measuring.

Question 4

Do I apply the 80% rule to R1+R2?

Accepted Answer

No. The 80% rule applies to maximum Zs values listed in BS 7671 Tables 41.2–41.4, which already account for conductor temperature rise during fault conditions. R1+R2 measured at ambient temperature is used directly in the Zs calculation: Zs = Ze + (R1+R2).

Question 5

How do I estimate cable length from an R1+R2 measurement?

Accepted Answer

Divide the measured R1+R2 (in mΩ) by the per-metre value for the cable type. For example, if you measure 0.29Ω (290 mΩ) on 2.5/1.5mm² cable, the estimated length is 290 / 19.51 ≈ 14.9m. This is useful for verifying cable routes or identifying faults on long runs.

Question 6

What is R1+R2 and why is it important?

Accepted Answer

R1+R2 is the combined resistance of line conductor (R1) and CPC (R2) at the circuit's furthest point. It feeds into Zs = Ze + (R1+R2). If too high, the circuit may not disconnect fast enough per Regulation 612.2.

Question 7

What are the R1+R2 values per metre for common cable sizes?

Accepted Answer

At 20°C: 1.5/1.0mm² = 30.20 mΩ/m, 2.5/1.5mm² = 19.51 mΩ/m, 4.0/1.5mm² = 16.71 mΩ/m, 6.0/2.5mm² = 10.49 mΩ/m, 10.0/4.0mm² = 6.44 mΩ/m, 16.0/6.0mm² = 4.23 mΩ/m from IET On-Site Guide Table B1.

Question 8

How do I test R1+R2 on a ring final circuit?

Accepted Answer

Three steps: measure end-to-end line ring (r1) and CPC ring (r2) separately; cross-connect (figure-of-eight) at one end; measure at every socket. Reading should be approximately (r1+r2)/4. Higher reading at one point indicates a break.

Question 9

Why must R1+R2 be measured as a dead test?

Accepted Answer

A live Zs measurement includes parallel paths through bonding, gas/water pipes, and steelwork, which artificially lower readings. Dead test gives true circuit impedance per Regulation 612.2.

Cable (Line/CPC mm²)	R1+R2 (mΩ/m)
1.5 / 1.0	30.2
2.5 / 1.5	19.51
4.0 / 1.5	16.71
6.0 / 2.5	10.49
10.0 / 4.0	6.44
16.0 / 6.0	4.23

R1+R2 Calculator

How This Calculator Works

Quick Reference — R1+R2 Per Metre at 20°C

Practical Notes

Frequently Asked Questions