The complete guide to R1+R2 continuity testing for UK electricians. What R1+R2 is, why it matters for Zs calculation, the long lead method, ring circuit continuity testing, acceptable values, and common mistakes to avoid. BS 7671 compliant.
Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians
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.
1,000+
UK electricians
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer — DP Electrical
R1+R2 is the combined resistance of two conductors measured in series: R1 is the resistance of the line (phase) conductor from the distribution board to the furthest point of the circuit, and R2 is the resistance of the circuit protective conductor (CPC — the earth wire) over the same path. When you measure R1+R2, you are measuring the total resistance of the outgoing line conductor and the return earth conductor connected together at the far end.
This measurement is one of the most important values recorded during electrical testing because it directly feeds into the calculation of earth fault loop impedance. The formula Zs = Ze + (R1+R2) means that your R1+R2 measurement, combined with the external earth fault loop impedance (Ze), gives you the expected total earth fault loop impedance (Zs). This is the value that determines whether the protective device (MCB, RCBO, or fuse) will disconnect the supply quickly enough to prevent electric shock in the event of an earth fault.
Continuity testing is test number one in the GN3 testing sequence — it is the very first electrical test carried out on an installation, performed with the circuit de-energised. This is because the integrity of the earth path must be confirmed before any other test can be relied upon. The governing regulation is BS 7671 Reg 643.2.1, which requires verification of the continuity of conductors and connections to exposed-conductive-parts and extraneous-conductive-parts before energisation.
Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.
The R1+R2 value is not just a number on a certificate — it represents the actual resistance of the fault current path. When an earth fault occurs (for example, a live conductor touches a metallic enclosure), fault current flows from the supply, through the line conductor (R1) to the fault point, through the fault, and back through the CPC (R2) to the distribution board. The total impedance of this path determines how much fault current flows, which in turn determines how quickly the protective device operates.
If R1+R2 is too high, the fault current will be too low to trip the protective device within the required disconnection time. BS 7671 requires disconnection within 0.4 seconds for final circuits supplying socket outlets and within 5 seconds for distribution circuits. The maximum permitted Zs values in BS 7671 Tables 41.2, 41.3, and 41.4 are calculated to ensure these disconnection times are achieved.
Measuring R1+R2 also serves as a verification tool. After measuring Zs during live testing, you can check that Zs is approximately equal to Ze + (R1+R2). If the measured Zs is significantly higher than this calculated value, there may be a high-resistance connection in the earth path that was not detected during the continuity test — for example, a loose main earthing terminal connection or a corroded earth clamp.
Elec-Mate's R1+R2 calculator lets you look up expected values by cable size and length. Compare your measured R1+R2 against the calculated value to…
Try it free for 7 days
There are two main methods for measuring R1+R2: the long lead method and the temporary link method. Both achieve the same result — measuring the series resistance of R1 and R2 — but the long lead method is more commonly used in practice because it allows you to test multiple circuits from a single setup at the distribution board.
Before testing, you must have completed safe isolation. The circuit must be de-energised, locked off, and proved dead. All loads should be disconnected from the circuit to prevent parallel paths affecting your readings.
Select the continuity function on your multifunction tester (MFT). This applies a low test voltage of between 4 and 24 V DC and measures resistance in ohms. The instrument should comply with BS EN 61557-4 for low-resistance measurement.
The long lead method is the standard technique for R1+R2 measurement on radial circuits. The procedure is as follows:
The temporary link method is an alternative where you create a temporary link between line and earth at the furthest point of the circuit (for example, by connecting a short lead between the L and E terminals at the last socket), then measure at the distribution board. This is less commonly used because it requires someone at the far end of the circuit to make the connection.
GN3 Reg 2.12 describes two widely used methods for protective conductor continuity testing. Both are accepted under BS 7671 Reg 643.2.1; the choice is based on practicality and circuit accessibility.
A temporary lead links the line busbar to the earth bar at the DB, creating a series path through R1 and R2. The instrument reads R1+R2 at the furthest point. Best for: socket circuits, radial final circuits, any installation where the DB is accessible and the circuit has accessible end-points.
A supplementary test cable is connected to the CPC at the DB or MET; the other probe is moved along the circuit to each luminaire or accessory. This measures the CPC (R2) resistance alone without shorting the full circuit. Best for: luminaire circuits (where Method 1 would require shorting at each fitting), three-phase installations, bonding conductors, and situations where shorting the circuit is impractical. GN3 Reg 7.41 confirms Method 2 is the preferred approach for bonding and protective conductors in medical and specialist locations.
With probes in one hand and the instrument in the other, use Elec-Mate's voice entry: 'Ring 1…
Try it free for 7 daysThe R1+R2 reading is recorded in ohms (not megohms — that is insulation resistance). For most domestic circuits, R1+R2 values are typically between 0.1 and 2.0 ohms. The exact value depends on the cable length, the cross-sectional area of the conductors, and whether the CPC is the same size as the line conductor.
The value is recorded in the R1+R2 column of the schedule of test results on the EICR or EIC. It is used alongside the Ze measurement to calculate the expected Zs: Zs = Ze + (R1+R2). This calculated value is compared against the measured Zs obtained during live testing — the two should be approximately equal. Any significant discrepancy indicates a problem that requires investigation.
When recording results, ensure you note which method was used (long lead or temporary link), confirm the leads were nulled, and record the value to two decimal places. The instrument should display readings to at least 0.01 ohm resolution.
AI board scanner, voice test entry, and automatic BS 7671 validation — finish the certificate before you leave the property. From £6.99/mo.
Try the certificate tools freeBS 7671 does not specify a single maximum R1+R2 value. Instead, the acceptability of R1+R2 depends entirely on the resulting Zs value for the circuit. The key question is: when you add R1+R2 to Ze, does the resulting Zs fall within the maximum permitted Zs for the protective device?
The Zs limits in BS 7671 Table 41.3(a) are tabulated at conductor operating temperature. On site, measurements are taken at ambient temperature, so you must apply the GN3 0.80 correction factor before comparing your reading against the tabulated limit:
Source: BS 7671:2018+A4:2026 Table 41.3(a); GN3 0.80 site-test correction factor.
R1+R2 is measured with the conductors at ambient temperature, but conductors in service operate at up to 70 °C (PVC insulation) or 90 °C (thermosetting). Resistance increases with temperature, so the effective Zs at operating temperature is higher than the cold-measured value.
Per GN3 Reg 1.08, the correction factor is: (1 + 0.004 × (operating temp − 20 °C)). For 70 °C PVC conductors: 1 + 0.004 × (70 − 20) = 1.20. This means the measured R1+R2 will be approximately 20 % higher at operating temperature. GN3 Reg 5.78 provides worked examples of adjusting (R1+R2) measurements from 20 °C to 70 °C for Zs verification.
Practical rule: where the GN3 0.80 factor is applied to the tabulated Zs limit, this implicitly accounts for both the ambient-temperature measurement and the operating-temperature conductor resistance, making the two approaches equivalent for site compliance checks.
If your measured R1+R2 is significantly higher than the calculated expected value, this indicates a problem — typically a high-resistance joint, a damaged conductor, or a longer cable run than expected. If R1+R2 is lower than expected, check that the leads were properly nulled and that there are no parallel earth paths providing a lower-resistance route.
Ring final circuit continuity testing follows a fundamentally different procedure from radial circuit testing. You cannot simply use the long lead method on a ring because both ends of the ring connect to the distribution board. The ring circuit test is a three-stage process that verifies the ring is complete, identifies faults, and provides the R1+R2 value at the furthest point.
Disconnect both ends of the ring at the DB. Measure the resistance of each conductor separately: r1 (line-to-line), rn (neutral-to-neutral), r2 (CPC-to-CPC). For a healthy ring with no breaks, r1 and rn should be approximately equal (since L and N are the same conductor size). r2 may differ if the CPC is a different size (for example, 1.5 mm² CPC in 2.5 mm² twin-and-earth).
Cross-connect the line and neutral conductors at one end of the ring — connect L1 to N2 and N1 to L2, where subscripts indicate the two ends. Measure between line and neutral at each socket outlet on the ring. Readings should rise to a maximum at the midpoint of approximately (r1+rn)/4, then fall back symmetrically. This confirms the ring is intact with no breaks or cross-connections.
Cross-connect the line and CPC conductors and repeat the measurements at each socket outlet. The maximum reading at the midpoint gives the R1+R2 value at the furthest point of the ring, approximately (r1+r2)/4. This is the value recorded on the schedule of test results and used for Zs verification.
Anomalies in the readings indicate specific faults: readings that do not rise and fall symmetrically suggest cross-connections between two rings; a very high reading at one socket suggests a high-resistance joint; readings that plateau rather than peaking suggest an interconnection; and readings that jump abruptly suggest a break in one leg of the ring operating as two radials.
Point your phone at the distribution board and Elec-Mate's AI reads the MCB/RCBO ratings, circuit references, and board layout.
Try it free for 7 daysContinuity testing appears simple but several common mistakes can produce incorrect results and lead to errors on the certificate.
Failing to null (zero) the leads before testing adds the lead resistance to every reading. For short circuits with low R1+R2 values, this can be a significant error — a 0.2 ohm lead resistance added to a 0.3 ohm circuit gives a 0.5 ohm reading, a 67% error. Always null before testing and re-null if leads are changed.
The R1+R2 value must be measured at the furthest point of the circuit, not the nearest. Testing at the first socket on a radial gives a low R1+R2 that does not represent the worst case. The schedule of test results requires the value at the furthest point because this gives the highest R1+R2 and therefore the highest Zs.
Leaving the temporary long lead connected between line and earth bars at the DB after testing creates a direct short circuit path. When the circuit is re-energised, the long lead will carry fault current and the MCB will trip immediately — or worse, the lead may overheat. Always remove the long lead after testing.
If metallic services (gas pipes, water pipes) are bonded to the earthing system, they provide parallel paths that can reduce R1+R2 readings and mask genuine faults. For accurate ring circuit testing, supplementary bonding connections should ideally be disconnected during testing, though this is not always practical.
Elec-Mate provides several tools specifically for continuity testing and R1+R2 recording. The schedule of tests auto-validates every R1+R2 value by comparing the resulting Zs (Ze + R1+R2) against the maximum permitted Zs for the protective device on that circuit. If the calculated Zs exceeds the BS 7671 limit, the app flags it immediately.
Enter your measured R1+R2 and Ze values, and Elec-Mate calculates the expected Zs automatically. Compare against your measured Zs to verify consistency.
Try it free for 7 daysVoice-to-test-results lets you speak R1+R2 values while on site — just say the circuit number and reading. The insulation resistance readings, Zs values, and RCD trip times are all recorded in the same digital schedule of tests with automatic BS 7671 validation across every value.
Step-by-step R1+R2 continuity testing procedure using the long lead method, per BS 7671 and IET Guidance Note 3.
Follow the safe isolation procedure per HSE GS 38. Identify the circuit at the distribution board, switch off and lock off the MCB (or remove the fuse), and verify the circuit is dead at the point of work using a proved voltage indicator. Safe isolation is mandatory before continuity testing. BS 7671 Reg 643.2.1 requires verification of the continuity of protective conductors and connections to exposed-conductive-parts before the installation is energised.
Connect a temporary test lead between the line busbar (or the line terminal of the circuit MCB) and the earth bar at the distribution board. This creates the series path through R1 and R2. The long lead must be clearly identifiable and of adequate length to reach comfortably.
With the long lead connected and the multifunction tester in continuity mode, touch the probes together and press the null/zero button. This subtracts the resistance of the test leads and the long lead from all subsequent readings. Verify the instrument displays 0.00 ohms after nulling.
At each circuit endpoint (the furthest socket, light fitting, or accessory), measure between line and earth terminals. The reading is the R1+R2 value for that circuit. Record it on the schedule of test results. For radial circuits, measure at the last accessory on the circuit.
Calculate the expected R1+R2 from the cable data: multiply the resistance per metre (from BS 7671 tables) by the cable length for both R1 and R2 conductors. The measured value should be close to the calculated value. A significantly higher reading indicates a problem. Use the R1+R2 to verify Zs: Zs should approximately equal Ze + (R1+R2).
After testing all circuits, remove the temporary long lead from the distribution board. Record all R1+R2 values on the schedule of test results. Elec-Mate auto-validates each value and uses it in the Zs calculation to verify your live loop impedance measurements.
Verified reviews from the UK App Store.
Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.
I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.
I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.
Real feedback from real sparks
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer
Sole Trader · DP Electrical
“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”
Nathan Perry
Electrician · NP Electrical Services
“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”
Jake Pizey
3rd Year Apprentice · Apprentice
Join 1,000+ UK electricians using Elec-Mate for on-site testing and certification. R1+R2 calculator, Zs lookup, voice test entry, board scanner. 7-day free trial, cancel anytime.
“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”
Daniel Palmer, DP Electrical
From £6.99/mo after trial — less than a coffee a week
We use cookies to improve the app and measure what works. Cookie Policy