R1+R2 Testing on Ring Final Circuits: The BS 7671 Method Explained
A practical reference for verifying ring final circuit integrity to Regulation 643.2.1 of BS 7671:2018+A2:2022, with verified resistance values, expected readings and EICR coding implications.
What BS 7671 Actually Requires
The dead test for continuity on a ring final circuit is governed by Regulation 643.2.1 in BS 7671:2018+A2:2022. The regulation requires two distinct continuity measurements, and the second of them is unique to ring final circuits.
Regulation 643.2.1
“The continuity of conductors and connections to exposed-conductive-parts and extraneous-conductive-parts, if any, shall be verified by a measurement of resistance of: (i) protective conductors, including protective bonding conductors, and (ii) in the case of ring final circuits, line conductors.”
Sub-paragraph (ii) is the reason ring final circuits get a different test from every other circuit you inspect. Without measuring the line conductors as well as the cpc, you cannot prove that the ring is actually a ring. A circuit that has been broken at one socket and then fed back through the spur loop, or wired as a figure-of-eight, can still appear to function. Only the three-step R1+R2 procedure exposes that.
The Standard Ring Final Circuit (Appendix 15)
Appendix 15 of BS 7671 is informative, not normative, but it sets out the design that nearly every domestic ring you will ever test follows. It applies to circuits arranged in accordance with Regulation 433.1 (overload protection of conductors). The standard 32 A ring described in Fig 15A is built from these elements:
- Live conductors of 2.5 mm² forming the ring, with a 1.5 mm² circuit protective conductor also forming the ring (BS 6004 flat twin and earth cable)
- 30 A or 32 A overcurrent protective device at the origin
- BS 1363 socket-outlets, or fused connection units to BS 1363-4 with a maximum fuse of 13 A
- Unfused spurs in 2.5 mm² serving a single socket-outlet only, fed from a socket-outlet on the ring or directly from the distribution board
- Fused spurs (FCU) where the number of socket-outlets supplied is dependent on the load and diversity
- Historical floor-area guidance of 100 m² per ring, retained as informative
Appendix 15 explicitly does not cover protection against electric shock (Chapter 41), protection against thermal effects (Chapter 42), protection against overcurrent (Chapter 43) or selection and erection of equipment (Part 5). Those still apply to every ring you certify. The R1+R2 test is one of the inputs that proves you have met the disconnection time requirements of Chapter 41 by way of Regulation 643.7.3.
Why R1+R2 Matters: The Path to Zs
R1+R2 is the resistance of the line conductor plus the cpc from the origin of the circuit to the point of utilisation, measured cold. It feeds the standard earth fault loop impedance equation:
Zs = Ze + (R1 + R2)
Ze is the external loop impedance up to the origin. The (R1 + R2) component is the part the inspector calculates from a dead test. Once Zs is known, it is checked against the maximum permitted values in Tables 41.2 to 41.4 for the disconnection times required by Regulation 411.3.2.2 (0.4 s for final circuits up to 63 A in TN systems supplying socket-outlets or mobile equipment; 5 s for distribution circuits and certain fixed connections).
Skip the R1+R2 test on a ring and you are left with only the live Zs reading, which proves nothing about the integrity of the ring itself. A broken leg can still produce a passable Zs at one socket while the rest of the ring sits on a single conductor that is dangerously overloaded.
The Three-Step Test Method
The three-step procedure described in IET Guidance Note 3 implements Regulation 643.2.1 for ring final circuits. It is the only method that proves the ring is continuous, identifies spurs as spurs, and yields the worst-case R1+R2 for the Zs calculation in one set of measurements.
Step 1
End-to-end resistance of each conductor
Identify both legs of the ring at the consumer unit. Disconnect line, neutral and cpc at the protective device. Measure the end-to-end resistance of each conductor in turn: r1 (line), rn (neutral) and r2 (cpc). For a 2.5 mm² / 1.5 mm² T&E ring, r1 should be approximately equal to rn, and r2 should be approximately 1.67 x r1. A reading at the upper limit of the instrument range indicates an open circuit, which means the ring is broken.
Step 2
Cross-connect line and neutral, measure at every socket
At the consumer unit, link the line of leg A to the neutral of leg B, and the line of leg B to the neutral of leg A. Measure line-to-neutral resistance at every accessory on the ring. Each ring socket-outlet should read approximately (r1 + rn) / 4, and every reading on a correctly wired ring should be substantially the same. Any socket-outlet wired as a spur will read higher because the spur conductors are added in series. Step 2 is what distinguishes ring topology from spurs and exposes interconnections.
Step 3
Cross-connect line and cpc, measure at every socket
Repeat the cross-connection, this time linking line of leg A to cpc of leg B, and line of leg B to cpc of leg A. Measure line-to-cpc resistance at every socket-outlet. The reading at each socket is the R1+R2 for that point of utilisation. Where the cpc is a smaller csa than the line conductor, the readings will vary slightly around the ring. The highest reading is the worst-case R1+R2 used in the Zs calculation, and is what gets recorded on the Schedule of Test Results.
Expected Readings Around the Ring (Step 3)
Where the line conductor and cpc are the same csa, every Step 3 reading on a healthy ring will be approximately (r1 + r2) / 4 and substantially constant. Where the cpc is smaller than the line conductor (the standard 2.5/1.5 case), readings vary predictably as the test point moves around the ring. The variation is reproduced from IET Guidance Note 3 Table 2.8.
| Cable (line / cpc) | Lowest reading | Highest reading | Spread |
|---|---|---|---|
| 2.5 mm² / 1.5 mm² | 0.94 x (r1 + r2) / 4 | (r1 + r2) / 4 | 6 % |
| 4.0 mm² / 1.5 mm² | 0.80 x (r1 + r2) / 4 | (r1 + r2) / 4 | 20 % |
| 2.5 mm² / 1.0 mm² | 0.82 x (r1 + r2) / 4 | (r1 + r2) / 4 | 18 % |
The highlighted (r1 + r2) / 4 column is the value to record. It is also the worst-case R1+R2 used to verify Zs against Tables 41.2 to 41.4. A spur off any socket-outlet will read above the highest tabulated value because the spur conductors are added in series with the parallel ring path.
Verified Conductor Resistance Values
The expected r1, rn and r2 values for Step 1 are derived by multiplying the cable run length by the resistance per metre at 20 °C. The values below are reproduced from Table B1 of IET Guidance Note 3, applied to BS 6004 flat twin and earth. They are the canonical figures used to predict the readings on a healthy ring.
| Line / cpc (mm²) | Line only (mΩ/m) | cpc only (mΩ/m) | (R1 + R2) (mΩ/m) |
|---|---|---|---|
| 1.0 / 1.0 | 18.10 | 18.10 | 36.20 |
| 1.5 / 1.0 | 12.10 | 18.10 | 30.20 |
| 1.5 / 1.5 | 12.10 | 12.10 | 24.20 |
| 2.5 / 1.5 (standard ring) | 7.41 | 12.10 | 19.51 |
| 2.5 / 2.5 | 7.41 | 7.41 | 14.82 |
| 4.0 / 1.5 | 4.61 | 12.10 | 16.71 |
| 6.0 / 2.5 | 3.08 | 7.41 | 10.49 |
Values are quoted at 20 °C. If the conductors are at a different temperature when tested, multiply by the ambient correction factor: 0.92 at 0 °C, 0.94 at 5 °C, 0.96 at 10 °C, 0.98 at 15 °C, 1.00 at 20 °C, 1.04 at 30 °C, 1.08 at 40 °C. The general formula is F = 1 + 0.004 x (test temperature – 20 °C), the simplified copper / aluminium resistance coefficient from BS EN 60228. A loft-mounted consumer unit on a winter morning at 5 °C will deliver readings about 6 % below the 20 °C figure, which is enough to throw off a marginal Zs calculation if not corrected.
Worked Example: 35 m Standard Ring
A 35 m total run of 2.5 mm² / 1.5 mm² T&E forming a 32 A ring at 20 °C should produce the following expected readings.
- Step 1 line: r1 = 35 x 7.41 = 259 mΩ (0.26 Ω)
- Step 1 neutral: rn = 35 x 7.41 = 259 mΩ (0.26 Ω)
- Step 1 cpc: r2 = 35 x 12.10 = 424 mΩ (0.42 Ω), which is 1.63 x r1, in line with the IET-quoted 1.67 ratio
- Step 2 expected at every ring socket: (r1 + rn) / 4 = (0.26 + 0.26) / 4 = 0.13 Ω
- Step 3 expected at the midpoint: (r1 + r2) / 4 = (0.26 + 0.42) / 4 = 0.17 Ω
- Step 3 lowest reading on the ring: 0.94 x 0.17 = 0.16 Ω
- Worst-case R1+R2 to record: 0.17 Ω
If your installation has a TN-S Ze of 0.35 Ω, the resulting Zs = 0.35 + 0.17 = 0.52 Ω. That sits comfortably below the 1.37 Ω maximum for a Type B 32 A circuit-breaker in Table 41.3. If a measured Zs at any socket reads materially higher than this calculation predicts, that points to a high-resistance joint or a partially broken ring rather than a service problem.
Try It: Predict Every Reading Before You Get On Site
Enter the cable type, ring length, ambient temperature, Ze and protective device. The tool computes r1, r2, the expected Step 2 reading at every socket, the recorded R1+R2 (Step 3 at midpoint), the resulting Zs, and a pass / marginal / fail verdict against Table 41.3. Use it to sanity-check meter readings on site or to model a circuit at the design stage.
Common Mistakes and EICR Coding Implications
The mistakes below are the ones that recur on remedial visits and follow-up EICRs. Each carries a defensible coding outcome under BS 7671 best practice.
1. Recording R1+R2 from the consumer unit only
Measuring line-to-cpc at the origin gives r1 + r2 in series, not (r1 + r2) / 4. Used in the Zs calculation it inflates loop impedance fourfold and may incorrectly fail an otherwise compliant circuit. This is a Schedule of Test Results admin error rather than a defect, but it leads to erroneous reporting. Investigate and correct before issuing the certificate.
2. Skipping Step 2 and only doing Step 3
Step 3 alone cannot distinguish a ring from a long radial. Without Step 2, an installation wired as a figure-of-eight or with one leg interconnected mid-ring can pass continuity yet fail to provide the parallel current paths the cable rating depends on. Coding: C2 (potentially dangerous) where overload protection is no longer satisfied, on the basis that 433.1.204 is not met.
3. Mistaking spurs for high-resistance joints
A socket-outlet wired as a legitimate unfused spur will give a higher Step 2 and Step 3 reading than the surrounding ring sockets. Coding it as a defect rather than identifying it as a spur is wrong. If the topology cannot be confirmed without further inspection, code FI (further investigation required) and resolve before completing the certificate.
4. Failing to test every socket-outlet on the ring
Sampling a few sockets misses interconnections that fall between tested points. The procedure requires Step 2 and Step 3 to be applied at every accessory on the circuit. Limited sample testing falls short of Reg 643.2.1. Coding: C3 (improvement recommended) where the test record is incomplete; C2 if a real fault is later found that the missed sockets would have exposed.
5. Not correcting for ambient temperature
A consumer unit at 5 °C will read about 6 % below the 20 °C figure. Inspectors who quote uncorrected R1+R2 against tables that assume 20 °C may either falsely pass or falsely fail a marginal circuit. Apply the Table B2 multiplier (or the F = 1 + 0.004 x (T – 20) formula) before recording the value. This is a record-keeping point, not a coding defect, but it changes whether other measurements are correctly assessed.
6. Cross-connection error at the consumer unit
Linking line-to-line and neutral-to-neutral instead of line-to-neutral cross-connection produces a near-zero or wildly inconsistent set of readings. Recognise the symptom and re-make the connection before recording. Persistent inconsistency after correctly cross-connecting indicates a real fault in the ring (a poor termination, a damaged conductor or an unintended interconnection), which should be coded C2 if it leaves overload protection unverified.
7. Substituting a live Zs measurement for the dead R1+R2 test
A live earth fault loop impedance reading at one socket cannot prove ring topology. It will pass even if the ring is broken at a single conductor, because current still flows via the remaining conductor. The dead R1+R2 procedure under Reg 643.2.1 is the only test that exposes a broken ring. Replacing it with a live Zs reading on initial verification or an EICR leaves Reg 643.2.1 not met. Coding: C3 where the live test still passes; FI to confirm the ring is intact; C2 where the ring is found to be broken and overload protection is no longer effective.
8. Not investigating an obviously broken ring
A Step 1 reading near the upper limit of the instrument range, or a Step 2 / Step 3 reading at the consumer unit roughly four times the expected value at the midpoint of an unbroken ring, indicates the ring is open. The circuit is now operating as a long radial loaded above the design current of a single conductor. Coding: C2 (potentially dangerous) under Reg 433.1.204; rectify and re-test. Reg 411.3.2.4 (and 419.3 where supplementary equipotential bonding is the basis of compliance) means broken cpc continuity in particular cannot be left unresolved.
Recording R1+R2 on the Schedule of Test Results
For ring final circuits the Schedule of Test Results column for R1+R2 takes the worst-case reading from Step 3, that is, the highest line-to-cpc value measured at any socket-outlet on the ring during the cross-connected test. That value, corrected for ambient temperature where appropriate, is then used to verify Zs against the relevant maximum value from Tables 41.2 to 41.4 (or, where appropriate, Table 41.5 for the 5 s disconnection time). For radial circuits, R1+R2 is measured at the furthest point of utilisation, which is normally the most distant socket-outlet or accessory on the circuit.
The continuity of the cpc on a ring final circuit is therefore proved twice: once in Step 1 as r2 end-to-end, and again in Step 3 at every accessory. That is what Reg 643.2.1 (i) and (ii) jointly require, and that is what makes the ring final circuit test the most complete continuity test in domestic certification.