How to test continuity of a circuit

Helpful video reference. We use GSH Electrical's "Continuity, Polarity and Insulation Resistance of our 2 way and Intermediate Lighting Circuit" as the video reference here. GSH Electrical are UK electrical college lecturers who demonstrate the complete initial verification sequence — continuity, polarity and insulation resistance — on a training rig wired to BS 7671.

1. Safe isolation and dead testing

Switch off the circuit breaker at the consumer unit. Before doing anything else, check your voltage indicator is working by testing it on a known live supply or using a proving unit — this is required procedure, not optional. Then test between line, neutral and earth at the furthest accessible point on the circuit. All three combinations should read zero. Lock the breaker off if you have a lock-off device.

2. Null the test leads

Connect your multifunction tester's test leads to the continuity range and touch the probe tips together. The display will show a small resistance value representing the leads themselves — typically 0.01 to 0.05 Ω. Press the null or zero button to subtract this from all subsequent readings. If you skip this step every reading will be slightly high, which matters when you are checking values in the milliohm range.

3. Link line to CPC at the consumer unit

Connect a temporary short-circuit link between the line conductor and the circuit protective conductor (CPC, the earth conductor) at the distribution board. Use a proper test lead, not a length of loose wire. This creates a closed loop: current travels from your probe at any outlet, through the line conductor to the board, across the link and back through the CPC to the second probe. The resistance you measure is R1 (line conductor resistance) plus R2 (CPC resistance), which is why this is called the R1+R2 test.

Make sure the link bridges only the correct line and CPC terminals — do not link across to another circuit's conductors.

4. Set the multifunction tester

Select the low-resistance or continuity function. Some testers have a dedicated R1+R2 range. The test applies a current of around 200mA: enough to give a stable reading and to detect a high-resistance joint that a lower test current might miss. A tester with a 200mA test current will satisfy BS 7671 requirements for this test.

5. Test each outlet

Working from the distribution board outwards (or in any convenient order), connect your test leads between the line terminal and the earth terminal at each socket outlet, light fitting, junction box or switch. Record the reading on your schedule of test results. The highest reading you see will be at the point furthest from the board — that reading is the one that matters most for earth fault loop impedance calculations.

If the tester shows OL (over-limit), infinity or no reading at any point, the protective conductor is open-circuit at that point. Stop and investigate before continuing.

6. Record and check the readings

For twin and earth cable using a 2.5mm² line conductor and 1.5mm² CPC, the combined R1+R2 per metre of cable run is approximately 19.51 mΩ/m (at 20°C). Multiply by the one-way cable length from the board to the outlet to get the expected value. A reading significantly above this figure suggests a poor joint or damaged section of cable worth locating before the circuit is energised. A small deviation is acceptable — cable temperature and joint resistance account for a few percent of variation.

7. Remove the link and proceed

Once all readings are recorded, remove the temporary link from the board terminals. Return any covers removed to access the testing points. Continuity testing is complete. The next steps in the initial verification sequence are polarity testing (confirming the correct conductor is at each terminal) followed by insulation resistance testing (checking for faults between conductors). Carry out both of these before energising the circuit.

When to call us

Initial verification testing is required after any notifiable electrical work under Part P. Richard carries out the full test sequence — continuity, polarity, insulation resistance, earth fault loop impedance and RCD testing — and issues the Electrical Installation Certificate the same day for work he completes in Sandwich and east Kent.