In this post we are going to cover the often-misunderstood requirements for testing radial single-phase standard socket-outlet circuits here in the UK.
A radial socket circuit starts at the consumer unit / distribution board ‘CU/DB’ and ends at the final socket on the circuit. Unlike a ring socket circuit, the wiring does not return to the CU/DB from this final point.
Guidance on cables and accessories used on both ring and radial socket circuits can be found in the 18th Edition Wiring Regulations BS7671:2018+A2:2022 Appendix 15.
For example
Radial socket circuits wired in 2.5mm Live, 1.5mm CPC. PVC/PVC cable should be protected by a 20A overcurrent device ‘circuit breaker’.
Radial socket circuits wired in 4.0mm Live, 1.5mm CPC. PVC/PVC cable should be protected by a 30A or 32A overcurrent device ‘circuit breaker’.
Additional protection using a 30mA RCD should be used especially in domestic installations through either an RCBO or RCCB.
Testing
Before you start, please ensure that all safe isolation procedures are in place and that you have isolated the CU/DB. This is done by switching off and placing a lock and label on the main switch for the CU/DB. Using an approved voltage indicator ‘see GS38’ and a proving unit, prove that your voltage testers are working correctly. Then on the load side of the main switch test between N – L, N – E and E – L, they should all show 0v.
Continuity of Protective Conductors
At the CU/DB connect the Line and the CPC of the radial circuit together. Then at each socket-outlet using a continuity tester set to ohms measure across Line (R1) and Earth (R2). Ensure that you have nulled your test leads if needed and use a plug-in lead or adapter so there is no need to remove the socket-outlet fronts as you go.
You should notice that the resistance reading increases as you work your way from the CU/DB to the final point. Record the highest reading and this will be the R1+R2 reading for this circuit. This reading should be recorded as it is used on the schedule of test results on any certificates used.
To confirm that your reading is accurate you can compare your resistance reading to the values given in Guidance Note 3 table B1 which shows the values of resistance per metre of each available conductor size.
For example, a 2.5/1.5 cable has a resistance of 19.51mΩ per metre. If we know that our circuit is 9.5 metres long, we can calculate what the value should be by…
(19.51 x 9.5)/1000 = 0.19 Ω. You now have a benchmark to compare your measured value against and a higher reading could indicate a possible loose connection along the circuit.
Polarity
This is to ensure that the Line, Neutral and CPC conductors have been connected correctly at the rear of each socket-outlet. Similar to the previous test but this time at the CU/DB connect the Line and Neutral conductors together and measure across Line and Neutral at each socket-outlet.
This reading again should rise as you work your way from the CU/DB. There is no need to record this reading as we are only checking that the connections at the are the correct way round.
NOTE: If when completing this test and the previous Line – Earth test you fail to get a reading when checking both L – E and L – N at any socket-outlet this will indicate a cross connection at the rear of the socket and should be investigated.
Conclusion
By correctly performing both tests we have confirmed the continuity of the protective conductor and the correct polarity at each socket-outlet on the radial circuit. It’s also worth noting that the R1+R2 reading forms part of the overall earth fault loop impedance value ‘Zs’ for this circuit as Zs = Ze+(R1+R2).
This Zs value when measured as part of your live testing should be compared to 80% of the values given in BS7671 table 41.3 to ensure that the overcurrent protective device will meet its maximum disconnection time under fault conditions.
Also, if an RCD is present that should be tested as part of the live testing to ensure that it meets its maximum disconnection time of 300 milliseconds at 1 x its rating.
Hope that you found this helpful for more information and online training see…
City & Guilds 18th Edition online course with at home exam…
https://the-regs.co.uk/18thEdition/onlinecourse.php
Inspection and testing City & Guilds 2391-52…