What is continuity testing and why is it required?

Continuity testing verifies that all conductors in a circuit are intact — that there are no open circuits, broken connections, or missing conductor runs. It is a mandatory test under BS 7671 Chapter 64 (Reg 643.2.1) for all new installations and forms part of the periodic inspection (EICR). The test confirms that line, neutral, and protective conductors are all connected end-to-end. For ring final circuits, it additionally verifies the integrity of the ring — confirming that the circuit forms a complete loop and has not been incorrectly wired as a spur.

How do you test a ring final circuit for continuity?

Ring final circuit continuity requires two tests. First, the end-to-end test: disconnect the ring at the consumer unit and measure the resistance of the complete loop of each conductor — line, neutral, and CPC. These end-to-end readings are recorded as r1, rn and r2. Second, the cross-connected test: reconnect the ring at the consumer unit but transpose the connections (cross the line of one leg with the neutral of the other, and vice versa). Then measure resistance from the line to neutral at each outlet. This gives the R1+Rn value at each point. For a true ring, the reading at each outlet should be approximately one quarter of the sum of the end-to-end line and neutral readings — (r1+rn)/4 — and GN3 states the readings at each socket outlet should be substantially the same.

What are r1, rn, and r2 values?

In the GN3 ring final circuit procedure the lowercase values are the end-to-end (open loop) readings taken at step 1: r1 is the resistance of the complete line conductor loop, rn the complete neutral loop, and r2 the complete CPC loop, each measured with the ring disconnected at the consumer unit. The uppercase values are what the cross-connected test gives you at each outlet: R1+Rn between line and neutral, and R1+R2 between line and earth. R1+R2 is the value recorded on the schedule of test results, and it is used to calculate Zs (total earth fault loop impedance) without live testing: Zs = Ze + (R1+R2), where Ze is the external earth fault loop impedance.

How do you test CPC continuity?

CPC continuity is tested by disconnecting one end of the CPC from the main earthing terminal (or the earth bar at the consumer unit) and connecting the instrument between the disconnected end and the other end of the CPC at the furthest point of the circuit. For a ring final circuit, the cross-connected test gives R1+R2 directly at each outlet. For a radial circuit, measure the resistance from the consumer unit earth bar to the earth terminal of the furthest accessory on the circuit. The value obtained (R2) must not be so high that Zs would exceed the maximum permitted value for the protective device.

What resistance value should continuity tests give?

Continuity testing does not have a single prescribed pass/fail resistance value. Regulation 643.2.1 requires the measurement but states no numeric maximum. The result must be low enough that the CPC will permit sufficient fault current to flow to operate the protective device within the required disconnection time. The R1+R2 value combined with Ze must give a Zs value within the maximum permitted for the protective device — the maximum earth fault loop impedance tables are BS 7671 Tables 41.2 to 41.5. For main and supplementary bonding conductors, the resistance should be very low — a working figure of 0.05Ω or less is commonly used in the trade for main protective bonding, though BS 7671 itself does not prescribe one.

Why must bonding conductors be disconnected at one end before testing?

Main and supplementary bonding conductors connect metallic parts of the installation to the main earthing terminal. If you test a bonding conductor while both ends are connected, the instrument current has a parallel path through the general mass of earth. This gives an unreliably low reading that does not reflect the resistance of the bonding conductor itself. Disconnect the bonding conductor at one end (typically at the metalwork or pipework clamp), measure the resistance of the conductor, then reconnect it. Always verify continuity is restored after reconnection.

What does a high continuity reading indicate?

A higher than expected continuity reading indicates increased resistance in the conductor path. Common causes include: a loose or corroded connection at an accessory or junction; undersized conductors (the resistance increases with smaller cross-sectional area and greater length); a broken or partially broken conductor; a spur connection that was incorrectly identified as part of a ring; incorrect polarity of connections causing extra resistance through additional terminations; or a high-resistance joint caused by overheating or mechanical damage.

TESTING GUIDE

Continuity Testing Guide: Ring Final Circuit & CPC Testing

The complete UK electrician's guide to continuity testing — ring final circuit end-to-end and cross-connected tests, r1, rn and r2, R1+Rn and R1+R2 values, CPC continuity, bonding conductor testing, and recording results on the schedule of test results.

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13 min readUpdated 2026-07-02Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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The short answer

What is an acceptable R1+R2 reading?

There is no single pass or fail figure for R1+R2 — the measured value must be low enough that Zs (Ze + R1+R2) stays within the maximum for the protective device in BS 7671 Tables 41.2 to 41.5. Compare your reading against the expected R1+R2 for the cable type and length; a much higher value points to a loose or high-resistance connection.

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Key takeaways

01Continuity testing verifies that all conductors are connected end-to-end with no open circuits, and that protective conductors (CPCs) provide an unbroken path back to the main earthing terminal.
02Ring final circuit testing requires two stages: an end-to-end resistance test of each conductor loop (r1, rn, r2), followed by a cross-connected test to derive R1+Rn (or R1+R2) at each socket outlet.
03For a ring final circuit, GN3 states the cross-connected readings taken at every socket outlet on the ring should be substantially the same, at approximately one quarter of the sum of the end-to-end values — R1+Rn ≈ (r1+rn)/4. A noticeably higher reading indicates a spur rather than a point on the ring, or an incorrect connection.
04CPC continuity must be tested on every circuit. The R1+R2 value measured at the furthest point — or, on a ring final circuit, the highest reading found around the circuit — is added to Ze to give Zs (total earth fault loop impedance) without applying live voltage.
05Bonding conductor continuity testing (main and supplementary bonding) requires the bonding conductor to be temporarily disconnected from the earthed metalwork at one end to avoid the instrument current flowing through the general mass of earth.

01 · Testing Guide

What Is Continuity Testing?

Continuity testing is the verification that all conductors in an electrical installation are intact and properly connected — that current can flow unimpeded from one end of a conductor to the other with no open circuits or high-resistance joints. It is a mandatory test under BS 7671:2018+A4:2026 Chapter 64 (Reg 643.2.1) and must be carried out on all circuits during initial verification of a new installation (EIC) and during every periodic inspection (EICR).

Continuity tests cover three separate requirements:

Ring final circuit continuity: Verifying that socket outlet circuits wired as a ring form a complete loop with no spurious spurs or incorrectly wired connections (Regulation 643.2.1).
CPC continuity: Confirming that every circuit has an unbroken protective conductor connecting all exposed-conductive-parts to the main earthing terminal (Regulation 643.2.1).
Equipotential bonding conductor continuity: Verifying that main equipotential bonding conductors and supplementary bonding conductors are intact and properly connected (Regulation 643.2.1).

Note that Reg 643.2.1 requires live conductor (line and neutral) resistance measurement specifically for ring final circuits. For radial circuits, only the CPC continuity measurement is required under this regulation — there is no BS 7671 requirement to measure the resistance of the line and neutral conductors of a radial circuit as a separate continuity test.

Continuity tests are performed with the installation de-energised using a low-resistance ohmmeter (often a combined multifunction test instrument). The instrument injects a test current and measures the voltage drop, deriving the resistance of the conductor path.

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02 · Testing Guide

Ring Final Circuit Test 1 — End-to-End Resistance

The first stage of ring final circuit testing measures the total loop resistance of each conductor, confirming that the ring is complete and giving reference values for the cross-connected test that follows.

Isolate the circuit: Switch off and isolate the circuit at the consumer unit. Prove dead at the consumer unit terminals.
Disconnect the ring at the consumer unit: Disconnect both ends of the line conductor from the MCB (or fuse), both ends of the neutral from the neutral bar, and both ends of the CPC from the earth bar. You now have two separate legs of the ring — leg A and leg B.
Measure end-to-end resistance of the line conductor: Join one end of leg A to one end of leg B (link the two line conductor ends together at the consumer unit). Connect the instrument between the remaining free ends of the line conductor loop. The reading is the total resistance of the line conductor around the full ring. Record this as r1 (line end-to-end).
Measure end-to-end resistance of the neutral conductor: Repeat the same process with the neutral conductors. Record as rn (neutral end-to-end).
Measure end-to-end resistance of the CPC: Repeat with the CPCs. Record as r2 (CPC end-to-end).

Expected values: For a 2.5mm² line and neutral conductor (typical for a 32A ring final circuit in flat twin-and-earth cable), copper conductor resistance is 7.41 mΩ/m — 7.41Ω per km — at 20°C (GN3 Appendix B). A ring with 60m of cable in the loop would give an end-to-end r1 of approximately 0.44Ω. Values significantly higher than expected for the cable run length indicate a high-resistance joint; a reading significantly lower than expected suggests the circuit is not a true ring — see the figure-of-eight note below.

03 · Testing Guide

Ring Final Circuit Test 2 — Cross-Connected Test

The cross-connected test is the definitive ring final circuit check. It derives the R1+Rn value at every socket outlet on the ring, confirming that every outlet is genuinely on the ring and not a spur, and that connections are correct throughout.

Cross-connect at the consumer unit: Take one end of the line conductor (leg A) and connect it together with the opposite leg's neutral conductor (leg B neutral). Take the other line conductor end (leg B) and connect it with leg A neutral. This transposition means the instrument now sees the line of one ring leg in series with the neutral of the other.
Do the same for the CPC if testing R1+R2: Cross-connect the line and CPC ends of opposite legs at the consumer unit. This gives R1+R2 at each outlet when testing between line and earth.
Test at each outlet: At every socket outlet on the ring, connect the instrument between line and neutral terminals. Record the resistance. This is the R1+Rn value at that outlet.
Test R1+R2 at each outlet: Connect the instrument between line and earth at each outlet. This gives R1+R2, which combined with Ze gives Zs. GN3 requires the highest R1+R2 found on the circuit — usually at a socket wired as a spur — to be the value recorded on the schedule of test results and used for the Zs calculation.

For a correctly wired ring, the R1+Rn value at every outlet should be approximately equal to one quarter of the sum of r1 and rn (the end-to-end totals from Test 1) — R1+Rn ≈ (r1+rn)/4 — and GN3 states the readings taken at each socket outlet on the ring should be substantially the same. That consistency is the point of the test: a reading that climbs towards the middle of the ring and falls away again indicates the line and neutral have not been cross-connected to opposite legs. Outlets connected as spurs will show a higher reading than the rest of the ring.

The line-to-earth readings behave slightly differently. Where the CPC is a smaller cross-sectional area than the line conductor — as in standard flat twin-and-earth — the R1+R2 readings will not be identical around the ring, and GN3 tabulates the expected spread between the lowest and highest reading for common conductor pairings.

Figure-of-eight fault: GN3 identifies this as a specific diagnostic outcome of the three-step ring test. A figure-of-eight occurs when the ring cable is cut and rejoined incorrectly at an intermediate point, so the circuit appears continuous but the conductors are actually looped back on themselves. It shows up first in Test 1: the end-to-end reading comes out lower than the value you would expect from the cable length and cross-sectional area, because the measurement is no longer following the full route. GN3 then directs you to confirm the suspicion with the cross-connected line-to-neutral measurement and by visually inspecting the wiring for misconnections. This is also why line, neutral and CPC must be measured separately in Test 1 — measuring them together can mask the fault.

04 · Testing Guide

r1, rn, r2 and R1+R2 Explained

The case of the letter matters, and getting it the wrong way round is a common source of confusion. In the GN3 ring final circuit procedure, the lowercase values are the end-to-end (open loop) readings taken at step 1, with the ring split at the consumer unit. The uppercase values are the circuit values the cross-connected test produces at each outlet, and R1+R2 is what goes on the schedule of test results.

r1: End-to-end resistance of the complete line conductor loop of the ring, measured in Test 1 with both ends free at the consumer unit.
rn: End-to-end resistance of the complete neutral loop. For standard flat twin-and-earth cable, where line and neutral are the same size, r1 and rn should come out substantially equal.
r2: End-to-end resistance of the complete CPC loop. In flat twin-and-earth the CPC is a smaller size than the line and neutral, so r2 is correspondingly higher.
R1+Rn: The line-to-neutral reading at each socket outlet during the cross-connected test. For a correctly wired ring it is approximately (r1+rn)/4 and is substantially the same at every outlet.
R1+R2: The line-to-earth reading at each socket outlet, and the most important value on the test sheet. Combined with Ze it gives Zs without applying live voltage: Zs = Ze + (R1 + R2) — see our earth fault loop impedance calculation guide for the full method. This calculated Zs must not exceed the maximum Zs for the protective device. GN3 requires the highest R1+R2 found on the circuit to be the one recorded and used.

Note on CPC size: In flat twin-and-earth cable, the CPC is typically smaller than the line and neutral conductors (1.0mm² CPC in 1.5mm² T&E, 1.5mm² CPC in 2.5mm² T&E). This means r2 is higher than r1 and rn, and the R1+R2 value will be higher than the R1+Rn value at the same point on the ring. This is normal and expected. GN3 also notes that with an unequal CPC the R1+R2 readings will not be identical at every outlet, and tabulates the expected spread around the ring.

05 · Testing Guide

CPC Continuity Testing

Circuit protective conductor (CPC) continuity must be verified for every circuit in the installation — not just ring final circuits. This test confirms that there is an unbroken earth path from every exposed-conductive-part back to the main earthing terminal.

Radial circuit method: Disconnect the CPC at the consumer unit earth bar. Use a long wander lead to connect from the disconnected CPC end at the consumer unit to the test instrument's one terminal. Connect the other terminal to the earth terminal at the furthest accessory on the circuit. The reading is R2 for that circuit. GN3 requires the measurement to be repeated at each utilisation point, switch and termination, not taken once.
Alternative method using instrument leads: Where a long wander lead is not available, the instrument can be "zeroed" with the leads joined, then the leads extended to reach the distant point. Some instruments allow lead resistance to be nulled out of the reading. Always null or compensate for lead resistance, particularly on short runs where lead resistance may be significant.
What constitutes a CPC?: On modern installations, the CPC is usually the bare or green-and-yellow conductor in the cable sheath. On older installations, conduit, trunking, or armour may serve as the CPC. Test the actual protective conductor, not just a parallel path that may be present.

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06 · Testing Guide

Bonding Conductor Continuity Testing

Main equipotential bonding conductors connect extraneous-conductive-parts (gas, water, and oil pipework; structural metalwork entering or passing through the building) to the main earthing terminal. Supplementary bonding connects simultaneously accessible parts in special locations (such as bathrooms). Both must be verified for continuity.

Disconnect at one end before testing: This is essential. If both ends of the bonding conductor remain connected, the test current flows through the general mass of earth and the general bonding network rather than through the conductor itself. The result is unreliably low and meaningless. Disconnect the bonding conductor at the metalwork clamp (not at the main earthing terminal, as this is more difficult to access safely).
Expected resistance values: Main equipotential bonding conductors are short and of large cross-sectional area. Under BS 7671 Reg 544.1.1 / Table 54.8, where the supply PEN conductor is 35mm² or less (the normal domestic TN-C-S case), the minimum copper main protective bonding conductor is 10mm². The resistance should be very low — typically 0.05Ω or less. Higher values indicate a poor connection at a clamp, a corroded conductor, or a join in the bonding conductor.
Reconnect and verify: After testing, reconnect the bonding conductor and test again with both ends connected to confirm continuity is restored. Never leave bonding conductors disconnected.

07 · Testing Guide

Test Method and Instrument Use

Continuity testing is carried out with a low-resistance ohmmeter. GN3 recommends a test source with a no-load voltage between 4V and 24V and a short-circuit current of not less than 200mA, and states that an instrument conforming to BS EN IEC 61557-4 will meet those requirements. This distinguishes a proper continuity test from a simple resistance measurement — the higher current helps identify high-resistance joints that may pass at low test currents but fail under load.

Null the lead resistance: Before any continuity test, short the instrument leads together and record or null the lead resistance. On modern multifunction instruments this is done automatically. On analogue instruments, adjust the zero setting. Failing to null lead resistance causes every reading to be higher than the true conductor resistance by the lead resistance value.
Use a wander lead for distant tests: A wander lead (a long single-conductor test lead, typically 10m to 25m) allows one instrument terminal to remain at the consumer unit while the other is taken to the furthest point of the circuit. Factor the wander lead resistance into the null reading.
Instrument calibration: The continuity test instrument must be calibrated to a traceable standard and within its calibration period. Record the instrument make, model, serial number, and calibration expiry date on the certificate or test schedule.

08 · Testing Guide

Recording Results on the Schedule of Test Results

Continuity test results are recorded on the Schedule of Test Results, which forms part of the Electrical Installation Certificate or EICR. The following information must be recorded for each circuit:

R1+Rn (Ω): The measured line-to-neutral resistance from the cross-connected test, in ohms.
R1+R2 (Ω): The measured line-to-earth resistance. Record the highest value found on the circuit — on a radial that is normally the furthest point, and on a ring it is usually a socket wired as a spur. This is the value used to calculate Zs without live testing.
Ring final continuity: For ring final circuits, record the end-to-end values (r1, rn, r2) from Test 1, and the R1+Rn and R1+R2 values from the cross-connected test.
Bonding conductor continuity: Record the measured resistance of each bonding conductor tested. Note where the bonding conductor was disconnected for the test.

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Continuity Testing (R1+R2): Acceptable Results (BS 7671)

How to test CPC continuity (R1+R2) and what results are acceptable. Step-by-step method, expected values and pass or fail criteria to BS 7671.

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09 · Testing Guide

For Electricians: Continuity Testing Efficiently

Continuity testing, done methodically, takes a few minutes per circuit. The steps are straightforward but must be followed in order — particularly the nulling of lead resistance and the disconnection of bonding conductors before testing.

Complete Test Schedules On Site

Use the Elec-Mate schedule of tests to enter continuity, IR, loop impedance, and RCD results on your phone as you test. The schedule auto-populates and the PDF is ready before you leave.

Derive Zs Without Live Testing

Recording accurate R1+R2 values during the continuity test means you can calculate Zs for every circuit without applying a live loop impedance test. Particularly useful where live testing is not practical or where RCDs prevent accurate live loop impedance measurement.

Frequently Asked Questions About Continuity Testing

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