Electrical

How to Test R1+R2 on a Ring Main

A broken ring final circuit still works — every socket has power and nothing trips, while one leg quietly carries current it was never sized for; the three-step continuity test catches exactly that and produces R1+R2 for the certificate.

Updated 20 Aug 20269 min read

A broken ring final circuit still works. Every socket has power, nothing trips, the customer is happy — and one leg of 2.5mm² is quietly carrying current it was never sized for. The three-step ring continuity test exists to catch exactly that, and R1+R2 is the number it produces for the certificate.

This guide walks through the full test, what the readings should be, what bad readings mean, and when you can derive R1+R2 by calculation instead.

Key takeaways

R1 is the line conductor resistance, R2 the CPC resistance; together R1+R2 is the circuit's contribution to Zs.
Step 1: measure end-to-end r1, rn, r2 — r1 and rn should be within ~10%, r2 ≈ 1.67 × r1 for 2.5/1.5mm².
Step 2: cross-connect line and neutral — every socket should read ≈ (r1 + rn) / 4.
Step 3: cross-connect line and CPC — the highest socket reading ≈ (r1 + r2) / 4 is the circuit's R1+R2.
R1+R2 can be derived as Zs − Ze, but that's a cross-check, not equal to the full three-step test.

What R1+R2 is and how to prepare

R1 is the resistance of the circuit's line conductor; R2 is the resistance of its circuit protective conductor. Measured together during dead testing, R1+R2 is the circuit's contribution to the earth fault loop, since Zs = Ze + (R1+R2). It's recorded on the schedule of test results, and added to Ze it proves the circuit meets its maximum Zs for disconnection.

Before you start, dead test: circuit isolated, proven dead, disconnected at the board. Use a low-resistance ohmmeter, null the leads, and identify both legs of line, neutral and CPC free at the board. Expected copper resistances at 20°C (On-Site Guide Table I1): 1.0mm² = 18.1 mΩ/m; 1.5mm² = 12.1; 2.5mm² = 7.41; 4.0mm² = 4.61; 6.0mm² = 3.08. So a 40m ring in 2.5/1.5mm² should give about 0.30Ω line and neutral (40 × 7.41 / 1000) and 0.48Ω CPC (40 × 12.1 / 1000).

Step 1: end-to-end resistance of each conductor

Measure each conductor loop end-to-end at the board: r1 (line to line), rn (neutral to neutral) and r2 (CPC to CPC).

Three sanity checks before moving on. First, r1 and rn should be substantially equal — same size, same route — a difference over about 10% means something is wrong (crossed conductors, poor termination, or you're not on the legs you think). Second, r2 ≈ 1.67 × r1 for 2.5/1.5mm² cable, because the CPC is smaller; an r2 well above that points at a poor CPC connection. Third, the magnitude should match the cable length — 0.30Ω of 2.5mm² is about 40m of ring.

Step 2: cross-connect line and neutral

At the board, connect the outgoing line to the returning neutral, and the returning line to the outgoing neutral — the "figure of eight". Now test between line and neutral at every socket on the ring. On a healthy ring, every socket sits on two parallel paths whose resistances always sum to the full loop, so the reading collapses to a near-constant value everywhere: expected L-N reading ≈ (r1 + rn) / 4.

For the example ring: (0.30 + 0.30) / 4 = 0.15Ω at every socket. The readings should be substantially the same everywhere — that flatness proves the ring is continuous and correctly cross-connected. If readings rise steadily towards the far end, the cross-connection has been made the wrong way (outgoing line paired with outgoing neutral) and you're measuring around an open loop — swap the pairing and start again.

Flat is good

The proof of a healthy ring is that every socket reads the same. A steady rise along the circuit means an open loop or a wrong cross-connection.

Step 3: cross-connect line and CPC

Repeat with line and CPC: outgoing line to returning CPC, returning line to outgoing CPC. Test line to earth at every socket: expected reading ≈ (r1 + r2) / 4. Example: (0.30 + 0.48) / 4 = 0.195Ω ≈ 0.20Ω. The highest reading obtained at any socket is the circuit's R1+R2 — the value for the schedule of test results and the Zs calculation. It normally occurs at the socket electrically furthest from the board.

Spurs are the expected exception: a socket on a spur reads higher than the ring sockets by the R1+R2 of the spur leg itself. That's normal — note it, and remember the spur's reading may be the circuit's highest R1+R2.

Bad readings, radials and derivation

Reading the symptoms: readings that rise steadily along the circuit mean the ring is broken or the cross-connection is wrong. One socket noticeably high is a loose or dirty termination, or a spur. A group of sockets low means an interconnection bridging the legs. r1 ≠ rn by more than 10% means crossed conductors or mixed sizes. An open circuit end-to-end means a broken ring — find the break before energising as a ring.

On a radial there's no cross-connecting: link line and CPC at the board, measure line to earth at every point, and the highest reading is R1+R2. A 20m radial in 2.5/1.5mm² should read about (7.41 + 12.1) × 20 / 1000 = 0.39Ω. You can also derive R1+R2 = Zs − Ze from a live loop test, but it's a substitute, not an equal — a live test can read low through parallel earth paths and won't reveal a broken ring, so it's for verification work where dead testing isn't reasonably practicable, and should be noted as such.

Temperature correction

Table I1 values and test readings are at ~20°C. Apply a factor of 1.2 to bring a cold R1+R2 up to operating temperature — or compare cold Zs against 80% limits. One correction or the other, never both.

Frequently asked questions

What is R1+R2 on a ring final circuit?

It's the combined resistance of the line conductor and CPC, and it's the circuit's contribution to Zs (Zs = Ze + R1+R2). On a ring it's obtained from step 3 of the continuity test — the highest (r1 + r2) / 4 reading at any socket, normally the furthest one.

Why divide by 4?

Once the ring is cross-connected, every socket sits on two parallel paths that always sum to the full end-to-end loop. The parallel combination of two equal-summing paths gives a quarter of the total, so each socket reads about (r1 + r2) / 4 — the same value everywhere on a healthy ring.

Can I get R1+R2 from Zs instead of dead testing?

You can derive R1+R2 = Zs − Ze from a live loop test, but it's a cross-check, not a replacement. A live test can read low through parallel earth paths and won't reveal a broken ring. For initial verification of a ring, do the full three-step dead test.

From guidance to action

R1+R2 calculatorPredict a healthy circuit's R1+R2 from cable size and length.Ring final continuity calculatorTurn measured r1, rn and r2 into expected socket readings.