Continuity testing proves that the protective conductor of every circuit runs unbroken from the main earthing terminal to each exposed-conductive-part, and it is the first dead test in the sequence set out in Chapter 64 of BS 7671:2018+A4:2026. There are two accepted methods. The R1+R2 method links line and cpc at the distribution board and measures the combined resistance at every outlet, giving a figure you reuse for Zs verification. The wander lead, or R2, method measures the cpc alone from the main earthing terminal. For a 2.5 mm² twin and earth circuit with a 1.5 mm² cpc, expect about 19.51 mΩ per metre at 20 °C, so a 25 m radial reads roughly 0.49 Ω.
- Null the leads before every session and after changing probes; unnulled leads add 0.1 to 0.3 Ω to every reading.
- The R1+R2 method gives one figure that feeds straight into Zs = Ze + (R1+R2) × 1.2; the wander lead method only proves the cpc.
- Compare readings against the mΩ/m figures for the cable size and length. Readings that are too low are as suspicious as readings that are too high.
- Record the highest R1+R2 for each circuit, taken at the point furthest from the board.
- Parallel paths through bonding, metal containment and other cpcs are the commonest cause of a false pass.
What is continuity testing and why is it done first?
Continuity testing is the measurement of the resistance of a conductor to confirm it is electrically unbroken and properly terminated. The cpc is the path fault current takes to operate the protective device, so a broken or high-resistance cpc means an earth fault may not clear in time. Chapter 64 of BS 7671 lists continuity of protective conductors, including main and supplementary bonding, as the first dead test, because a circuit with a broken cpc will pass insulation resistance perfectly and still be dangerous.
The instrument is the continuity range of a multifunction tester (MFT), which BS EN 61557-4 and Guidance Note 3 require to deliver a short-circuit test current of at least 200 mA; a multimeter on its buzzer range is not acceptable because its test current is too small to show up a poor crimp or a corroded terminal.
How does the R1+R2 method work?
The R1+R2 method measures the line conductor (R1) and the circuit protective conductor (R2) in series; the instrument sees the loop formed by the two.
Step by step
- Safely isolate the circuit and prove dead.
- Disconnect the circuit's line conductor from its protective device and link it to the cpc at the board with a short lead and two crocodile clips.
- Null the test leads (see below).
- At each outlet, switch, luminaire or accessory, measure between the line terminal and the earth terminal.
- The highest reading, normally at the point electrically furthest from the board, is the R1+R2 for the circuit.
- Remove the link and reinstate the line conductor before moving on.
For lighting circuits the switch must be on when you measure at the luminaire; for two-way and intermediate switching, test in each switch position so you have measured through the longest strap.
The value has a second use. Because the loop you measured is the loop fault current will take, Zs at the far end equals Ze plus R1+R2, corrected for temperature. A single set of readings verifies continuity and, after multiplying by 1.2, confirms Zs against the maximum Zs values in BS 7671 without a live loop test at every point.
How does the wander lead (R2) method work?
The wander lead method measures the protective conductor alone. One lead is connected to the main earthing terminal or the earth bar, and a long trailing lead, typically 20 to 50 m on a reel, is taken to each exposed-conductive-part in turn. Null the instrument with the full wander lead in circuit. The method suits points with no line conductor to link to: main bonding to gas and water, supplementary bonding in a bathroom, metal trunking and conduit, and extraneous-conductive-parts. Guidance Note 3 accepts both methods for proving continuity, but R2 alone cannot be used to calculate Zs.
| R1+R2 method | Wander lead (R2) method | |
|---|---|---|
| What is measured | Line and cpc in series | cpc (or bonding conductor) alone |
| Link at the board | Line to cpc, circuit isolated | None; lead from MET |
| Gives Zs by calculation | Yes, Zs = Ze + (R1+R2) × 1.2 | No |
| Best for | Final circuits on an EIC or EICR | Bonding, containment, parts with no line conductor |
| Vulnerable to parallel paths | Less so, but still possible via other cpcs | Very, through pipework and structure |
What should the readings be?
Expected values come from the resistance of copper per metre. The figures below are the standard IET On-Site Guide values for copper conductors at 20 °C, in milliohms per metre, and the combined R1+R2 for the common twin and earth pairings.
| Conductor csa (mm²) | Resistance (mΩ/m) at 20 °C |
|---|---|
| 1.0 | 18.10 |
| 1.5 | 12.10 |
| 2.5 | 7.41 |
| 4.0 | 4.61 |
| 6.0 | 3.08 |
| 10.0 | 1.83 |
| 16.0 | 1.15 |
| Twin and earth (line / cpc) | R1+R2 (mΩ/m) at 20 °C | 20 m run | 30 m run |
|---|---|---|---|
| 1.0 / 1.0 | 36.20 | 0.72 Ω | 1.09 Ω |
| 1.5 / 1.0 | 30.20 | 0.60 Ω | 0.91 Ω |
| 2.5 / 1.5 | 19.51 | 0.39 Ω | 0.59 Ω |
| 4.0 / 1.5 | 16.71 | 0.33 Ω | 0.50 Ω |
| 6.0 / 2.5 | 10.49 | 0.21 Ω | 0.31 Ω |
| 10.0 / 4.0 | 6.44 | 0.13 Ω | 0.19 Ω |
| 16.0 / 6.0 | 4.23 | 0.08 Ω | 0.13 Ω |
These are 20 °C values. Use the table as a sanity check rather than a pass mark: BS 7671 does not set a maximum R1+R2, it limits the resulting Zs. A reading two or three times higher than expected on a short run points to a loose terminal, a damaged conductor or a poor joint. A reading far too low, say 0.05 Ω on a 30 m lighting circuit that ought to read close to 1 Ω, almost certainly means you are reading through a parallel path rather than the cpc you think you are testing.
For ring final circuits the continuity test is a three-stage procedure using r1, rn and r2, covered in the guide on ring final circuit testing; the R1+R2 for a ring is (r1 + r2) ÷ 4.
Why multiply by 1.2 for Zs?
Under earth fault conditions the conductors are assumed to be at their maximum operating temperature, 70 °C for thermoplastic cable, where copper resistance is about 20 per cent higher than at 20 °C. So the design check is:
Zs (verified) = Ze + (R1+R2 measured) × 1.2
The result is compared with the maximum Zs for the device in Tables 41.2 to 41.4 of BS 7671. The alternative, when Zs is measured directly with a loop tester, is to compare against 0.8 times the tabulated maximum, as the On-Site Guide tables do. Do not apply both to the same figure. The earth fault loop impedance guide covers the live test.
How do you null the test leads?
Nulling, also called zeroing or lead compensation, removes the resistance of the leads from the reading. Short the probes firmly together, press null or zero, and confirm the display reads 0.00 Ω. Most MFTs hold the null until the range changes or the instrument is switched off, but some clear it whenever the dial moves, so check. Repeat the null whenever you change leads, add a wander lead or swap clips for probes. A pair of standard leads is typically 0.1 to 0.3 Ω, and a 50 m wander lead can be 0.5 Ω or more. If every reading on a modern board sits around 0.25 Ω regardless of circuit length, the leads were probably not nulled.
How are continuity results recorded?
On the schedule of test results enter the highest R1+R2 in ohms for each radial, and r1, rn, r2 and the derived R1+R2 for each ring. Where the wander lead method was used, enter the value in the R2 column and leave R1+R2 blank; the Appendix 6 model schedule has both columns so either method can be shown honestly.
Main and supplementary bonding continuity is recorded on the certificate itself. Guidance Note 3 suggests a main bonding conductor should read no more than about 0.05 Ω to the pipe it bonds; several ohms means a corroded clamp or a cable cut during other work. Do not record a test you did not do: an inaccessible accessory is a limitation, not a tick.
What are the common mistakes?
| Mistake | What happens | How to avoid it |
|---|---|---|
| Not nulling the leads | Every reading is 0.1 to 0.3 Ω high; short circuits look poor | Null at the start and after any lead change |
| Parallel paths | Reading far lower than the cable length allows; a broken cpc hides behind pipework or containment | Estimate the expected value first; disconnect bonding for R2 tests where practicable; test with the cpc removed from the earth bar if in doubt |
| Testing with bonding connected on an R2 test | Gas and water pipes return a low value and mask a missing cpc | Isolate bonding for the test and record it as a limitation if it cannot be done |
| Switch off on a lighting circuit | Open circuit at the luminaire | Put switches on; check every position on two-way circuits |
| Measuring the nearest point only | Recorded R1+R2 is not the maximum, so calculated Zs is optimistic | Test at the furthest point; record the highest value |
| Leaving the link in place | Line and cpc connected when the circuit is re-energised | Make removing the link a fixed step before the next test |
Parallel paths deserve the most attention. With metal back boxes, steel conduit and bonded pipework there are many routes back to the earth bar, and a cpc broken at a junction box can still read low through a back box screwed to a bonded pipe. Know roughly what the reading ought to be before you take it: if you expect 0.6 Ω and see 0.08 Ω, investigate rather than write it down.
How Certio helps
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