Continuity of protective conductors (R1+R2)
The first dead test. You prove the circuit protective conductor (cpc) is continuous all the way back to the earth bar, and you get the R1+R2 value you will need to check Zs later.
Why it matters
If the cpc is broken or loose, a fault to exposed metalwork can leave it live and the protective device won’t disconnect. R1+R2 is also the resistance of the line and cpc together, so it tells you whether the earth fault loop impedance (Zs) will be low enough: Zs = Ze + (R1+R2).
Equipment
- A low-resistance ohmmeter (the continuity range of a multifunction tester), test current at least 200 mA.
- Test leads to GS38, and a short link lead or crocodile clips.
- Null (zero) the leads before you start so their resistance isn’t added to your readings.
Method 1 — the link (R1+R2) method
- Safely isolateProve dead and lock off before you open anything.
- Link line to cpc at the boardConnect the circuit’s line conductor to its cpc at the distribution board with a short link.
- Test at every pointAt each accessory, measure between line and earth terminals. The highest reading — usually at the furthest point — is R1+R2 for the circuit.
- Remove the linkNever leave it in place. Record R1+R2 on the schedule of test results.
A bonus: because you test line to earth at every point through the link, this also confirms polarity at each accessory.
Method 2 — the wander lead (R2) method
Connect one test lead to the main earthing terminal with a long wander lead and touch the other on each exposed-conductive-part or earth terminal. This measures R2 only (plus the wander lead, which you null out). It’s useful for checking protective bonding and for metalwork on long runs.
Typical values
Resistance per metre of line + cpc at 20 °C (IET Guidance Note 3 / On-Site Guide). Multiply by the cable length to sanity-check a reading.
| Cable (line / cpc) | mΩ per metre |
|---|---|
| 1.0 / 1.0 mm² | 36.20 |
| 1.5 / 1.0 mm² | 30.20 |
| 2.5 / 1.5 mm² | 19.51 |
| 4.0 / 1.5 mm² | 16.71 |
| 6.0 / 2.5 mm² | 10.49 |
| 10 / 4.0 mm² | 6.44 |
Example: 20 m of 2.5/1.5 mm² twin and earth ≈ 20 × 19.51 mΩ = 0.39 Ω. A reading far higher than expected points to a loose termination or a break.
Common mistakes
- Forgetting to null the leads.
- Taking the reading at the nearest point instead of the furthest.
- Leaving the link in at the board.
- Parallel paths through metal conduit, trunking or bonding giving a falsely low reading.
iZapp’s 3D Testing Bench lets you do this test on a virtual board with real-looking kit: place the probes, read the tester and record the result on a practice schedule. Some attempts have a fault planted for you to find.
Frequently asked questions
What is R1+R2?
R1 is the resistance of the line conductor and R2 the resistance of the circuit protective conductor (cpc), measured together from the distribution board to the furthest point of the circuit.
What is a good R1+R2 reading?
There is no single pass value. It should be close to the expected value for the cable size and length (mΩ per metre × length), and low enough that Ze + (R1+R2) is within the maximum Zs for the protective device.
Is continuity a live or dead test?
Dead. The circuit must be safely isolated before you test continuity.
Last updated 2026-10-14. Training summary of BS 7671 and IET Guidance Note 3 practice, written by NAPIT-registered electricians. Always check the current edition and amendment of BS 7671 and your scheme’s requirements.