6.2 Continuity of Ring Final Circuits (r1, rn, r2)
Key Takeaways
A ring final circuit needs a special three-step test because its two parallel paths mean a broken ring can still show continuity via the other half
Step 1 measures end-to-end r1, rn, r2 at the DB; r1 ≈ rn, and r2 ≈ 1.67 × r1 when the CPC is 1.5 mm² against 2.5 mm² phase
Step 2 (P–N crossover) gives (r1+rn)/4 at every socket; step 3 (P–CPC crossover) gives (r1+r2)/4 at every socket, which is the R1+R2 recorded for the ring
The /4 value arises because the cross-connection forms a figure-of-eight with two equal parallel halves, so each accessory sees one quarter of the total loop
A socket that reads markedly higher than the others in step 2 or 3 is a spur off the ring or has a high-resistance joint
Why Ring Final Circuits Need a Special Test
A ring final circuit (the standard UK domestic socket circuit, BS 7671 Appendix 15) is wired as a loop: the phase, neutral, and CPC conductors each run from the distribution board, through every socket, and back to the distribution board. Each conductor therefore forms a closed ring with two parallel paths from the board to any given socket.
This parallel-path geometry defeats an ordinary end-to-end continuity test. If the ring is broken at one point, current can still reach every socket via the other half of the ring — so a simple end-to-end test at the board would still show continuity and miss the break. BS 7671 therefore requires a three-step ring continuity test, described in GN3 10th Edition and examined heavily on the 2391-52.
Step 1: End-to-End Resistance of Each Conductor (r1, rn, r2)
At the distribution board, identify and label the three conductor pairs. A ring has two ends for each conductor returned to the board:
- Phase (line): two ends, labelled r1 — call them P1 and P2.
- Neutral: two ends, labelled rn — N1 and N2.
- CPC: two ends, labelled r2 — CPC1 and CPC2.
With the circuit isolated, measure the end-to-end resistance of each conductor by connecting the ohmmeter across its two ends at the board. This confirms each conductor is unbroken around the ring.
| Conductor | End-to-end value | Expected relationship |
|---|---|---|
| Phase r1 | e.g. 0.40 Ω | — |
| Neutral rn | e.g. 0.40 Ω | rn ≈ r1 (same CSA) |
| CPC r2 | e.g. 0.67 Ω | r2 ≈ 1.67 × r1 if CPC is 1.5 mm² and phase is 2.5 mm² |
The phase and neutral are normally the same cross-sectional area (2.5 mm² thermoplastic), so r1 ≈ rn. The CPC is often 1.5 mm² — 60% of the phase CSA — so its resistance is higher by the ratio 2.5 / 1.5 = 1.67. If r2 is roughly 1.67 times r1, the CPC is the right size and unbroken. If r1 and rn differ significantly, investigate before proceeding — one conductor may be a different length or have a poor joint.
Step 2: Phase–Neutral Crossover (P1–N2 and P2–N1)
Cross-connect the phase and neutral ends across the ring: link P1 to N2 and P2 to N1 at the distribution board. This creates a figure-of-eight loop. Now test between phase and neutral at every socket on the ring.
The reading at every socket should be approximately:
(r1 + rn) / 4
and the readings should be roughly equal at every socket.
Why /4?
The cross-connection puts the two conductor rings in parallel as a figure-of-eight. From any socket, the test current can travel to the board by two equal paths — one clockwise, one anticlockwise — each of half the total conductor length. The resistance seen at the socket is therefore one quarter of the sum of the two conductor resistances: (r1 + rn) / 4. If every socket gives the same value, the ring is correctly formed phase-to-neutral.
Worked Example (Steps 1 and 2)
- r1 = 0.40 Ω, rn = 0.40 Ω, r2 = 0.67 Ω
- Step 2 expected reading at every socket = (0.40 + 0.40) / 4 = 0.20 Ω
A socket that gives a markedly higher reading in step 2 is not on the ring — it is a spur, or there is a high-resistance joint. Spurs are identified this way: the ring sockets all read about 0.20 Ω, and the spur reads higher because it adds an extra radial run on top of the ring value.
Step 3: Phase–CPC Crossover (P1–CPC2 and P2–CPC1)
Cross-connect the phase and CPC ends: link P1 to CPC2 and P2 to CPC1 at the distribution board. Test between phase and earth at every socket.
The reading at every socket should be approximately:
(r1 + r2) / 4
and again roughly equal at every socket. This value is the R1+R2 for the ring final circuit and is what you record on the schedule of test results — not the end-to-end r1 or r2 from step 1.
Worked Example (Step 3)
- r1 = 0.40 Ω, r2 = 0.67 Ω
- Step 3 expected reading at every socket = (0.40 + 0.67) / 4 = 0.2675 ≈ 0.27 Ω
- Record R1+R2 = 0.27 Ω on the schedule.
Then Zs = Ze + (R1+R2) = Ze + 0.27 Ω, compared to the tabulated maximum for the protective device (typically a 32 A Type B MCB, or a 30 mA RCD on a TT system).
Summary of the Three Steps
| Step | Action | Connect at DB | Test at each socket | Expected reading |
|---|---|---|---|---|
| 1 | End-to-end each conductor | Across own two ends | — | r1, rn, r2 (r1≈rn; r2≈1.67×r1) |
| 2 | P–N crossover | P1–N2, P2–N1 | Phase to Neutral | (r1+rn)/4 |
| 3 | P–CPC crossover | P1–CPC2, P2–CPC1 | Phase to Earth | (r1+r2)/4 = R1+R2 |
Exam-Critical Points
- Record (r1+r2)/4 as R1+R2, not the end-to-end r1 or r2. The end-to-end value is a check that each conductor is unbroken; the crossover value is the figure used for Zs.
- The figure-of-eight is why you divide by 4 — two equal parallel halves each carry half the current, so each accessory sees one quarter of the total loop. This derivation is exam-critical.
- A socket reading higher than the others in step 2 or 3 indicates a spur or a fault — investigate before recording the circuit's R1+R2.
- Always null the test leads before step 1; lead resistance is significant at these values and propagates into every subsequent reading if not removed.
- The three step-1 end-to-end values confirm each conductor is unbroken; the two crossover steps confirm the ring is correctly formed and give the recorded R1+R2.
Why does a ring final circuit require a special continuity test rather than a simple end-to-end measurement?
Ring circuits use a higher voltage rating than radial circuits and must be tested at 1000 V
The CPC in a ring is always a smaller CSA than the phase and cannot be tested with a standard ohmmeter
The two parallel paths mean a broken ring can still show continuity via the other half, so an end-to-end test could miss a break
Ring final circuits must be tested with the supply energised and a load applied
In step 1 of the ring continuity test, the end-to-end resistance of the phase conductor r1 is 0.40 Ω. The CPC is 1.5 mm² and the phase is 2.5 mm². What is the expected end-to-end resistance of the CPC, r2?
0.24 Ω
0.67 Ω
0.40 Ω
1.07 Ω
In step 3 of the ring continuity test, what value is recorded as R1+R2 for the ring final circuit?
The end-to-end resistance r2 from step 1
The end-to-end resistance r1 from step 1
(r1 + rn) / 4
(r1 + r2) / 4
During step 2 (phase–neutral crossover), one socket reads 0.45 Ω while every other socket reads approximately 0.20 Ω. What is the most likely explanation?
The socket is a spur off the ring, not part of the ring itself
The ring is correctly formed and the reading is within acceptable tolerance
The test lead was not nulled and is adding 0.25 Ω uniformly to every reading
The protective device at the distribution board is the wrong rating
Sections you finish are checked off in the contents.