How to carry out initial verification on a new electrical installation

Helpful video reference. "Electrical Test Procedure For A New Fuseboard" is from the Electricians Life UK channel, a UK electrician who documents real domestic electrical work. The video follows the test sequence on a newly installed consumer unit, covering the dead tests and then the energised tests in the correct order, with explanations of what each reading means and where to record it on the EIC.

1. Read the design documents and carry out the visual inspection

Start with the paperwork, not the tester. Confirm that the cable sizes, MCB ratings, RCD types and circuit descriptions on the circuit schedule match what has actually been installed. An error on paper that contradicts what is wired in is worth catching now, not during a fault six months later.

Then work through the visual inspection. Look at every termination in the consumer unit: conductors should be fully inserted with no insulation under the terminal, earth conductors should be sleeved in green and yellow, and there should be no stray strand bridging adjacent terminals. Check every socket, switch and light fitting in the building: faceplate screws tight, no cracked plates, no signs of heat damage. Work systematically around each floor rather than randomly, so you do not miss a circuit.

2. Test continuity of protective conductors (R1+R2)

With the installation isolated and the supply disconnected, null your test leads against each other to remove the lead resistance from the readings. Then measure from the main earth bar in the consumer unit to the earth terminal at every point on each circuit: sockets, switches, light fittings, fixed appliances. Record the R1+R2 value for each circuit on the Schedule of Test Results.

On a 2.5 mm twin and earth ring circuit, the combined R1+R2 reading at the furthest socket should be well under 0.5 ohms. Higher readings indicate a poor joint, a missing earth connection, or undersized cable. Do not move on until every circuit passes.

3. Test continuity of ring final circuit conductors

For ring final socket circuits, the test is slightly more involved. First, measure the end-to-end resistance of the line conductor, neutral conductor and CPC by connecting your tester across both ends of each conductor in turn. Record those values as r1, rn and r2.

Then cross-connect line to neutral and CPC to nothing, and measure R1+Rn at each socket along the ring. The readings should be approximately equal at every socket and roughly equal to (r1+rn)/4. Do the same with line and CPC cross-connected to get R1+R2. If any socket reads significantly higher than its neighbours, there is a break or a poor joint at that point in the ring.

4. Test insulation resistance

Before applying 500 V DC to the wiring, disconnect any voltage-sensitive equipment: LED drivers, dimmer modules, surge protection devices, electronic timers, and any hardwired appliances with sensitive electronics. Leaving them connected risks damage to the equipment and false readings on the test.

Apply 500 V DC between line and earth for each circuit, then between neutral and earth, with the MCB closed and all switches in the on position. The minimum reading under BS 7671 Table 61 is 1 MOhm, but a new installation in good order should read tens or hundreds of MOhms. Record the actual reading for each circuit. A value close to 1 MOhm on a new circuit means something is wrong: check for nicked insulation, a rogue conductor touching earth, or a device that should have been disconnected.

5. Verify polarity

Before energising, confirm polarity throughout the installation. At the consumer unit, trace each line conductor from its MCB terminal to the MCB line terminal input (not the load side), and confirm neutrals go to the neutral bar not the earth bar. At sockets, verify that the line (brown) goes to the live terminal (bottom right when viewed from the front) and neutral (blue) goes to the neutral terminal (bottom left). At lighting points, confirm the switched line goes to the live terminal of the lampholder.

Polarity errors -- particularly line and earth reversed at a socket -- are difficult to detect in normal use but can cause a fatal shock under certain fault conditions. Spending ten minutes on this check is far better than finding it on a live installation tester with a surprised homeowner watching.

6. Energise and test RCD and RCBO trip times

With dead tests complete, energise the installation. Using a calibrated installation tester, test each RCD and RCBO at 1x In (the rated residual operating current): for a 30 mA device that means a 30 mA test signal. Under BS 7671 Table 41.1, the device must trip in 300 ms or less. Then test at 5x In (150 mA for a 30 mA device): the device must trip in 40 ms or less.

Record the actual trip time in milliseconds, not just a pass or fail. Also test the physical test button on each device to confirm it trips the mechanism. A device that passes the timed tests but whose test button does not work has a defective button mechanism and must be replaced.

7. Measure earth fault loop impedance (Zs)

Using the loop impedance test function on the installation tester, measure Zs at the furthest point on each circuit from the consumer unit. Compare each reading against the maximum Zs values in BS 7671 Appendix 3 for the MCB or RCBO type and rating protecting that circuit.

For example, the maximum Zs for a 32A Type B MCB under a 0.4 s disconnection time is 1.44 ohms. If your measured Zs is, say, 0.35 ohms you have plenty of margin. If it is 1.40 ohms you are right at the limit, and the reading at operating temperature (which will be higher) may push you over. Investigate any circuit that is close to the limit rather than just recording it as a pass.

8. Measure prospective fault current (PFC) at the origin

At the main isolator or first protective device of the installation, measure the prospective fault current using the PFC test function. This is the highest possible fault current that could flow if a bolted short-circuit occurred at the origin. The value must not exceed the rated breaking capacity of the protective devices installed in the consumer unit.

Most modern UK domestic MCBs and RCBOs have a breaking capacity of 6 kA. Standard BS EN 61009 domestic consumer units are rated to 6 kA. In most UK homes, the measured PFC falls between 800 A and 3 kA, well within these limits. Record the actual measured value on the Electrical Installation Certificate.

9. Complete and sign the Electrical Installation Certificate

With all tests passed and readings recorded on the Schedule of Test Results and Schedule of Inspections, complete the Electrical Installation Certificate. The certificate names the designer, constructor and inspector (one person on most domestic jobs), states the earthing arrangement, describes the work, and includes the next recommended inspection date.

Sign the certificate, provide the original to the building owner, retain a copy for your records, and notify the installation through your Part P registered competent person scheme. For a consumer unit replacement or new circuit, the scheme will issue a Building Regulations compliance certificate to the local authority. The homeowner should store this with their property documents: they will need it if they sell the property or make an insurance claim.

When to call us

Richard carries out initial verification testing on new consumer unit installations, extension circuits and rewires across east Kent. All work comes with a fully completed Electrical Installation Certificate, Schedule of Test Results, and Part P Building Control notification where required.