How to test a radial circuit (UK)

Helpful video reference. We use Learn Electrics' UK tutorial "Testing Radial Circuits: Lighting, Cookers, Showers and Radial Socket circuits" as the video reference here. It covers the full test sequence for the common radial circuit types found in UK domestic properties, using the methods required by BS 7671 and the On-Site Guide.

1. Safe isolation

Switch off the circuit at the consumer unit. Lock it out with a lockout hasp if the board allows, or pull the fuse and pocket the carrier. Use an approved voltage indicator (not a neon screwdriver) to confirm the circuit is dead at every socket and outlet on the circuit. Test the tester on a known live source before and after to prove it is working correctly.

2. Continuity of protective conductors (R1+R2 method)

Connect a temporary wander lead between the earth terminal at the consumer unit and the line or earth conductor at the furthest point. This links the earth conductor back to the meter point so your tester can measure the combined resistance of the line conductor and the circuit protective conductor together (R1+R2). Note the reading at each socket or accessory along the radial. The highest reading (at the furthest point) is used to calculate the maximum earth fault loop impedance later.

For a 2.5mm² twin and earth radial, a typical R1+R2 at 20 m from the board might be around 0.3-0.4 Ω. If it is significantly higher, look for a loose or corroded connection.

3. Insulation resistance test

With all socket faceplates in place and anything plugged in removed, apply 500 V DC between the line conductors and the earth. Leave the neutral and line linked at the consumer unit so both conductors are tested together. Hold the test for one minute. The result must be 1 MΩ or above. A reading well below that, particularly one that falls as you hold the test, usually means damaged cable insulation or moisture in a back box.

On a circuit with LED drivers, smart sockets or any electronic accessories, you may need to disconnect those items first as their internal components can give a misleadingly low reading or be damaged by the test voltage.

4. Polarity check

With the wander lead still connected at the consumer unit earth bar, use the low-resistance continuity function to confirm that the line is connected to the L terminal and neutral to the N terminal at every socket and accessory. A reversed polarity (line in the neutral terminal) will show a low reading between the L terminal and the earth, rather than the N terminal. This is a notifiable defect and must be corrected before the circuit goes live.

5. Earth fault loop impedance (Zs)

Restore power to the circuit. At the furthest socket on the radial, measure the earth fault loop impedance (Zs) using the loop test function on your multifunction tester. Compare this result against the maximum permitted Zs for the protective device protecting the circuit. For a B-type 20 A MCB, BS 7671 Table 41.3 gives a maximum Zs of 2.30 Ω. If your measured value is below the maximum, the device will disconnect fast enough to protect against electric shock.

If the measured Zs is higher than the permitted maximum, check for a poor neutral connection at the consumer unit or a high-resistance earth path. Do not put the circuit into service until you have resolved the issue.

6. Record results

Enter all readings on a schedule of test results. Record the date, the instrument used (and its calibration date), the circuit details, and each measured value alongside its acceptable limit. Sign and date it. This document forms part of the electrical installation certificate or EICR report and is a legal record of the work carried out.

When to call us

Electrical testing is notifiable work under Part P of the Building Regulations and must be signed off by a registered electrician. If you need a circuit tested, an EICR for a rental property, or a certificate following new installation work in east Kent, call Richard.