In a ring final circuit the end to end continuity readings of the live (r1) and neutral (rn) should be very similar to one another (within 0.05 Ohms) whilst the CPC is likely to be 66% higher when wired in Twin & CPC due to the CPC reduced CSA.
The readings matching these rules could just be a coincidence. When the length of conductors is known the expected value of these end to end readings can be calculated using Table 11 on Pg 196 of the OSG. This table gives 12.1mΩ and 7.41mΩ for 1.5 and 2.5mm squared copper conductors.
Even if the end to end resistance readings of the 3 conductors, (live, neutral and cpc) match the expected values calculated there may be other possible issues with ring circuits such as multiple loops or or spurs off spurs). To help detect these non standard arrangements other tests can be taken to flag up through readings rather than visual inspection (a spur feeding a spur has two cables and looks like and socket on the main loop).
By cross linking the live of one leg of the ring to the neutral of the other leg and linking the remaining live and neutral together at the board, and then taking a resistance reading at each socket outlet from the front (with an appropriate adaptor and the outlets switch turned on) a very similar reading should be obtained at all sockets on the main loop of the ring with spurs having increased readings. The expected reading of this crossover or figure of eight test should be equal to the end to end readings of the live and neutral added togather and divided by 4.
By cross linking the live of one leg of the ring to the CPC of the other leg and linking the remaining live and CPC together at the board, and then taking a resistance reading at each socket outlet from the front (with an appropriate adaptor and the outlets switch turned on) a very similar reading should be obtained at all sockets on the main loop of the ring with spurs having increased readings. The expected reading of this crossover or figure of eight test should be equal to the end to end readings of the live and cpc added togather and divided by 4.
If the sockets pass both of these tests it confirms how they are positioned in the ring and that they are wired correctly (have correct polarity) If it has a low reading on one and is open circuit on another further investigation is needed of that socket as passing one test but not the other indicates crossed wires. Eg, passing the first test but failing the second indicates L and N have been switched in the socket as that would still produce a low reading on the first test but the second test taken between L & CPC via the front of the socket is actually making contact between N & CPC so unable to obtain a reading via the currently interinked L & CPC at the board.
