A ring final circuit starts at the distribution board, loops round every socket-outlet and returns to the same protective device, so every socket is fed from both directions. Testing it is a three-step dead test set out in IET Guidance Note 3: measure the end-to-end resistance of each conductor (r1, rn and r2), cross-connect line and neutral and read at every socket, then cross-connect line and cpc and repeat. On a healthy 2.5/1.5 mm² twin and earth ring, r1 equals rn, r2 is about 1.67 times r1, and the final cross-connected reading at every socket is (r1+r2)/4. Regulation 433.1.204 of BS 7671:2018+A4:2026 allows a 2.5 mm² ring on a 32 A device only because both legs share the load.
- Three steps, all dead: end-to-end r1, rn, r2 at the board; cross-connect L and N and read at every socket; cross-connect L and cpc and read at every socket.
- Expected figures for 2.5/1.5 mm² T&E: r1 ≈ rn, r2 ≈ 1.67 × r1, cross-connected readings ≈ (r1+rn)/4 then (r1+r2)/4, the same at every socket.
- A socket reading higher than the rest is a spur; readings that climb towards one end mean a broken leg.
- The highest cross-connected L-E reading is the R1+R2 for the circuit and goes on the schedule.
- The test is required on both EICs and EICRs.
Why does a ring final circuit have to be tested this way?
A ring protected by a 32 A MCB in 2.5 mm² cable relies on current sharing. Clipped-direct 2.5 mm² T&E is rated at around 27 A (Table 4D5, reference method C), so a single leg cannot carry the full 32 A. If one leg is broken, the sockets beyond the break are fed as a radial from the other leg and a heavy load can overload the cable without the MCB ever tripping. A broken cpc is worse: the sockets still work, nothing trips, but the protective conductor is relying on a single path or none.
A simple continuity test to the furthest socket does not detect either fault, because a broken ring still gives continuity via the other leg. Only the three-step test proves that both legs of every conductor are intact and that there are no interconnections (a cable joining two points on the ring, creating a parallel section that can be overloaded).
The test is part of initial verification under Chapter 64 of BS 7671 and is repeated within the agreed limitations at each periodic inspection under Chapter 65. Results go in the r1, rn, r2 and R1+R2 columns of the schedule of test results; see Schedule of Test Results: How to Fill In Every Column.
Step 1: how do I measure r1, rn and r2?
Isolate the circuit, prove dead, and disconnect both legs of the ring from the MCB, neutral bar and earth bar so you have six conductor ends at the board. Identify which line, neutral and cpc belong to each leg, because you will pair them in steps 2 and 3. Null the test leads on the low-ohms range, then measure:
- r1: between the two line conductor ends
- rn: between the two neutral conductor ends
- r2: between the two cpc ends
Each reading is the resistance of that conductor all the way round the ring. An open-circuit reading on any of the three means the ring is broken in that conductor and there is no point continuing until it is found.
What should the readings look like?
| Conductor | 2.5/1.5 mm² T&E, typical | Relationship |
|---|---|---|
| r1 (line, 2.5 mm²) | 0.20 to 0.60 Ω for a domestic ring | Baseline |
| rn (neutral, 2.5 mm²) | Same as r1 | r1 ≈ rn (within about 0.05 Ω) |
| r2 (cpc, 1.5 mm²) | 1.67 × r1 | Ratio of 1.5 mm² to 2.5 mm² resistance |
The ratio comes from conductor resistance: 2.5 mm² copper is about 7.41 mΩ per metre and 1.5 mm² about 12.10 mΩ per metre at 20 °C (IET On-Site Guide Table I1), so r2/r1 is 1.63 in theory and 1.67 is the accepted rule of thumb. A 40 m ring would give r1 ≈ 0.30 Ω and r2 ≈ 0.48 Ω. For a 4/1.5 mm² ring the ratio is about 2.6; for a ring in singles with a 2.5 mm² cpc it is 1.0.
If r1 and rn differ by more than a few hundredths of an ohm, suspect a poor termination, a section in a different cable size, or legs from two different rings. If r2 is much higher than 1.67 × r1, look for a loose cpc terminal somewhere round the ring.
Step 2: how do I do the line-neutral cross-connection test?
Connect the line of leg A to the neutral of leg B, and the neutral of leg A to the line of leg B, so the line and neutral loops form one figure-of-eight. Measure between the two cross-connected pairs at the board first: the reading should be about (r1+rn)/4, which for equal conductors is r1/2.
Then measure between line and neutral at every socket-outlet on the circuit. At every socket on the ring the reading should be substantially the same as the board reading, roughly (r1+rn)/4, varying by only a few hundredths of an ohm. Wherever you are on a cross-connected ring, the two paths back to the board add up to the same total.
What the anomalies mean:
- One socket reads higher by a fixed amount: it is on a spur. The extra resistance is the line plus neutral of the spur cable. Note it in the remarks.
- Readings climb steadily round the ring in one direction: the cross-connection is wrong (leg A line paired with leg A neutral) or one leg is open. Check your pairing at the board first.
- A reading much lower than expected at one socket: an interconnection bridging two parts of the ring.
- Readings roughly equal to r1 at most sockets rather than half of it: the conductors are not cross-connected.
Step 3: how do I do the line-cpc cross-connection test and find R1+R2?
Separate the neutrals, then connect the line of leg A to the cpc of leg B and the line of leg B to the cpc of leg A. Measure at the board between the pairs: expected value (r1+r2)/4.
Go round every socket again and measure between line and earth. The readings should again be constant, approximately (r1+r2)/4. For the 40 m example above, (0.30 + 0.48)/4 = 0.195 Ω, so every socket should read around 0.19 to 0.20 Ω.
The highest reading in this step is the R1+R2 for the circuit and is the value entered on the schedule of test results. It will normally be at a spur (the spur's line and cpc add to it) or at the socket electrically furthest from the board. Adding this R1+R2 to Ze gives the calculated Zs for the circuit, which you can compare with the live loop test later (see Maximum Zs Values BS 7671: Tables, 80% Rule and Ze).
A socket that reads open in step 3 but fine in step 2 has no cpc connected at that outlet: a C2 on an EICR and a fail on an EIC.
How do I interpret anomalies on a ring final circuit?
| Finding | Meaning | Typical outcome |
|---|---|---|
| Open circuit on r1, rn or r2 | Conductor broken somewhere in the ring | Locate and repair; C2 on an EICR (ring not continuous) |
| r1 ≠ rn by more than ~0.05 Ω | Loose termination, mixed cable sizes, or legs from different circuits | Investigate at the board first |
| One socket high in steps 2 and 3 | Unfused spur | Acceptable if one spur per ring socket serving one single or twin outlet |
| Two or more sockets on one spur | Non-compliant unfused spur arrangement | C3 typically, unless overloaded |
| Readings climb round the ring | Leg open, cross-connection wrong | Re-check pairing; if genuine, locate the break |
| Reading much lower at one point | Interconnection or bridged conductors | Locate and remove; parallel sections risk overload |
| Steps 1 and 2 fine, step 3 open at a socket | cpc not connected at that outlet | C2; connect |
Interconnections are the hard one, because everything works and the end-to-end readings look plausible. The give-away is the cross-connected reading dropping at part of the ring; a kitchen extension joined into the existing ring at two points is the usual culprit.
What are the 32 A limits for a ring final circuit?
Regulation 433.1.204 sets out the conditions under which the normal overload rule (Iz ≥ In) is relaxed for a ring: 2.5 mm² copper (or 1.5 mm² mineral insulated), protected by a device rated at 30 or 32 A, with the cable's current-carrying capacity Iz at least 20 A, and the load shared around the ring rather than concentrated on one section. Appendix 15 of BS 7671 gives the informative guidance most designers work to: a ring generally serves a floor area of up to 100 m², an unlimited number of socket-outlets, one unfused spur per socket or junction on the ring, each unfused spur feeding one single or one twin socket-outlet or one item of fixed equipment, and fused spurs serving as many outlets as the 13 A fuse allows. Fixed loads above 2 kW, such as immersion heaters and cookers, should have their own circuit.
A ring on a 40 A device, a 2.5 mm² ring run through thermal insulation (reference methods 101 to 103, where Iz drops below 20 A), or several sockets on one unfused spur, fails one of these conditions and needs an observation.
How Certio helps
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