6.1 Continuity of Protective Conductors (R1+R2)

Purpose of Protective Conductor Continuity

The continuity of protective conductors is the first test in the dead-test sequence and one of the most heavily examined topics on the 2391-52, sitting inside LO4, which carries 56% of the multiple-choice paper. Its purpose is to verify that the circuit protective conductor (CPC) — the earth path back to the main earthing terminal — is continuous and of low enough resistance that, under a fault, the protective device operates within the required disconnection time. This is the physical basis of automatic disconnection of supply (ADS), the fundamental protective measure in BS 7671.

If the CPC is broken, high-resistance, or poorly terminated, fault current cannot flow, the protective device will not trip, and exposed-conductive-parts will remain live long enough to cause a dangerous shock. Continuity testing catches open circuits, loose connections, and incorrect conductor sizes before the installation is energised.

The R1+R2 Method (Method 1)

The standard method for verifying CPC continuity on a radial circuit is the R1+R2 test:

• R1 = resistance of the phase (line) conductor from the distribution board to the accessory under test.
• R2 = resistance of the CPC from the distribution board to the same accessory.

The procedure:

1. Isolate and prove dead — this is a dead test; the installation must not be energised.
2. Link the phase and CPC at the distribution board — connect a temporary link between the line conductor and the CPC terminal at the outgoing way for the circuit under test.
3. Measure at each accessory — using a low-resistance ohmmeter, test between the phase and earth terminals at every socket, switch, light point, or other accessory on the circuit.
4. Record the highest reading — the highest value measured is the R1+R2 for that circuit, entered on the schedule of test results.

The highest reading is recorded because resistance increases with conductor length; the furthest point from the distribution board has the longest conductor run and therefore the highest resistance. If the path is continuous to the furthest point, it is continuous at every nearer point. A reading that drops suddenly partway along a circuit suggests a loose connection or a spur branching off the radial.

Nulling the Test Leads

Before taking any continuity reading, null the test leads — connect the two leads together and either press the null/zero button on the instrument or record the lead resistance and subtract it from each subsequent measurement. A typical pair of leads has 0.05 to 0.20 Ω of resistance, which is significant when the value you are measuring may itself be only 0.30 Ω. Failing to null is one of the most common practical-test failures and a frequent exam question.

Instrument Requirements

GN3 10th Edition (2026) and BS 7671:2018+A4:2026 require the instrument to be a low-resistance ohmmeter delivering a test current of at least 0.2 A and a resolution of 0.01 Ω or better. The 0.2 A minimum ensures the reading is meaningful for the low resistance values involved — a standard multimeter's ohms range, which uses a tiny test current, can be fooled by a high-resistance joint that passes a small current but would not carry fault current. Always verify the instrument is on the correct range before testing.

Using R1+R2 to Verify Zs

The R1+R2 value is not recorded for its own sake — it feeds directly into the earth-fault loop impedance check. The total earth-fault loop impedance is:

Zs = Ze + (R1 + R2)

where Ze is the external loop impedance measured at the origin (the supply-side contribution). Adding the measured R1+R2 gives the expected Zs for the circuit, which you compare against the tabulated maximum Zs for the protective device (BS 7671 Table 41.3 for circuit-breakers, or Table 41.2 for fuses, applying the 0.8 rule for measured values).

Worked Example

A radial socket circuit is wired in 2.5 mm² phase / 1.5 mm² CPC. You measure R1+R2 = 0.80 Ω at the furthest socket. The measured Ze at the origin is 0.35 Ω.

• Expected Zs = 0.35 + 0.80 = 1.15 Ω
• The circuit is protected by a 32 A Type B MCB. The tabulated maximum Zs in Table 41.3 is 1.37 Ω (0.95 × 230 ÷ 160 A), so the 0.8 on-site value is 1.37 × 0.8 = 1.10 Ω.
• The expected Zs of 1.15 Ω exceeds the 1.10 Ω on-site limit but is still below the 1.37 Ω tabulated maximum, so it falls in the indeterminate band: the simple 0.8 rule cannot confirm compliance and the full Appendix 3 temperature adjustment must be applied (see section 8.2). Investigate the terminations and the circuit length before accepting it.
• Beware of older revision material quoting 1.44 Ω for a 32 A Type B device. That is the pre-18th-Edition figure (230 ÷ 160) calculated without the Cmin = 0.95 voltage factor that BS 7671:2018 applies.

This is why R1+R2 is recorded on the schedule of test results: it lets you predict Zs before energising and gives a benchmark for the live test that follows.

The Long-Lead (Wander-Lead) Method

Where you cannot link both ends of a conductor — typically when verifying main protective bonding — use the long-lead method: one instrument lead stays on the bonding conductor at the main earthing terminal (MET), and a long ("wander") lead is run to the far end of the extraneous-conductive-part. Null the long lead first, because its resistance is much higher than a standard lead. This method is covered in detail in section 6.3.

Note: the (R1+R2)/4 value that appears in the syllabus is the ring-final-circuit crossover result (section 6.2), not a separate protective-conductor method. For radial protective conductors, the recorded value is the highest R1+R2 measured at the accessories.

Key Points to Remember