R1+R2 Testing: How to Test Continuity on a Ring Final Circuit
The three-step ring final continuity test explained: end-to-end r1, rn and r2, cross-connect readings, the divide-by-4 rule, and calculating R1+R2 from Zs.
#R1+R2 Testing: How to Test Continuity on a Ring Final Circuit
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.
#What R1+R2 is
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:
Zs = Ze + (R1+R2)
It's recorded on the schedule of test results on an EIC or EICR, and it's the figure that — added to Ze — proves the circuit meets its maximum Zs for disconnection. On a radial, you measure it directly. On a ring final, you get it from the three-step test, which proves the ring is actually a ring at the same time.
#Before you start
Dead test: circuit isolated, proven dead, and disconnected at the board. Use a low-resistance ohmmeter (the continuity range on your MFT), null the leads, and have the end-to-end conductors of the ring identified — both legs of line, neutral and CPC free at the board.
Expected resistances for copper at 20°C, per the On-Site Guide Table I1 values:
| Conductor CSA | Resistance (mΩ/m at 20°C) |
|---|---|
| 1.0mm² | 18.1 |
| 1.5mm² | 12.1 |
| 2.5mm² | 7.41 |
| 4.0mm² | 4.61 |
| 6.0mm² | 3.08 |
So a typical 40m ring in 2.5/1.5mm² twin and earth should give end-to-end readings of about 40 × 7.41 / 1000 = 0.30Ω for line and neutral, and 40 × 12.1 / 1000 = 0.48Ω for the CPC.
#Step 1: end-to-end resistance of each conductor
Measure the resistance of each conductor loop end-to-end at the board:
- r1 — line to line
- rn — neutral to neutral
- r2 — CPC to CPC
Three sanity checks before moving on:
- r1 and rn should be substantially equal — same size conductor, same route. A difference of more than about 10% means something is wrong (crossed conductors, a poor termination, or you're not on the legs you think you're on). Stop and find out why.
- r2 ≈ 1.67 × r1 for 2.5/1.5mm² cable — the CPC is smaller, so its resistance is higher in the ratio of the conductor sizes. An r2 way above that points at a poor connection in the CPC path.
- The magnitude should match the cable length — 0.30Ω of 2.5mm² is about 40m of ring. A reading double what the house could plausibly contain suggests a break with the meter reading through some other path.
#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, and the maths collapses to a near-constant value at every outlet:
Expected L-N reading at each socket ≈ (r1 + rn) / 4
For our example ring: (0.30 + 0.30) / 4 = 0.15Ω at every socket. The readings should be substantially the same everywhere — that flatness is the proof the ring is continuous and correctly cross-connected. If readings rise steadily towards the far end of the circuit, the cross-connection has been made the wrong way (you've paired outgoing line with outgoing neutral) and you're measuring around an open loop — swap the pairing and start again.
#Step 3: cross-connect line and CPC
Repeat the exercise with line and CPC: outgoing line to returning CPC, returning line to outgoing CPC. Test line to earth at every socket:
Expected reading at each socket ≈ (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 — that's the value for the schedule of test results and the Zs calculation. It normally occurs at the socket electrically furthest from the board.
The ring final expected readings calculator turns your measured r1, rn and r2 into both expected socket values instantly, and flags an r1/rn mismatch over 10%.
#What bad readings mean
| Symptom | Likely cause |
|---|---|
| Readings rise steadily along the circuit | Ring broken, or cross-connection made incorrectly — you're testing round an open loop |
| One socket reads noticeably high | Loose or dirty termination at that point — or it's on a spur (see below) |
| A group of sockets reads low | Interconnection ("bridge") between the legs of the ring — the ring is effectively two smaller rings |
| r1 ≠ rn by more than 10% | Crossed conductors at a point, poor termination, or mixed conductor sizes |
| Open circuit on one conductor end-to-end | Broken ring — find the break before anything else; the circuit must not be energised as a ring |
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.
#R1+R2 on a radial circuit
No cross-connecting needed. Link line and CPC together at the board, then measure between line and earth at every point on the circuit. The highest reading — normally the furthest point — is the circuit's R1+R2. You can predict what it should be from the cable size and route length with the R1+R2 calculator: a 20m radial in 2.5/1.5mm² should read about (7.41 + 12.1) × 20 / 1000 = 0.39Ω.
#Deriving R1+R2 from Zs and Ze
Since Zs = Ze + (R1+R2), it follows that:
R1+R2 = Zs − Ze
Measure Zs live at the furthest point, subtract the Ze measured at the origin, and you have a derived R1+R2. It's a legitimate cross-check and sometimes the practical option on an EICR where circuits can't be isolated for dead testing. But it's a substitute, not an equal: a live loop test can read low through parallel earth paths (bonded pipework is the classic), it won't reveal a broken ring the way the three-step test does, and the two measurements carry combined uncertainty. For initial verification of a ring final, the full three-step test is the method — the derivation is for verification work where dead testing isn't reasonably practicable, and it should be noted as such.
#Temperature correction
The Table I1 values and your test readings are at roughly 20°C; conductors under load run hotter and copper resistance rises with temperature. The standard approximation is a factor of 1.2 to bring a cold R1+R2 up to operating temperature for a hot Zs calculation against full design limits. Alternatively, compare cold measured Zs against 80%-adjusted device limits — one correction or the other, never both.
Test methods and values here are for guidance only. Always verify against the current edition of BS 7671, the IET On-Site Guide and Guidance Note 3 for the actual installation.
#Do it faster with TradePlanr
The free R1+R2 calculator predicts what a healthy circuit should read from cable size and length, and the ring final calculator does the divide-by-four arithmetic from your end-to-end measurements — both free, no sign-up. TradePlanr is a job management app for electricians: quotes, scheduling, invoicing and certificates alongside the full BS 7671 calculator set, at a flat £9.99 a month.
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