Testing a ring circuit
Final testing of a ring circuit
By Andy Wade. From the FAQ
A fairly rigorous method for testing the continuity of ring final circuits is given in the IEE On-Site Guide. It's really aimed at situations where the testing is done by someone other than the installer, and will catch out attempts at 'cheating' -- e.g. a small ring with an excessive number of spurs, or a patched-up broken ring. Arguably, this is OTT for DIY, when you've put the cables in yourself, but anyway, here goes:
The ring should be complete, except for the connection of all six wire ends at the consumer unit or distribution board. You need a low resistance ohm-meter with good resolution, say a range of 0 - 2 ohms with divisions of 0.05 ohm.
- At the board, start by measuring the loop resistance of each conductor (L1-L2, N1-N2, E1-E2, where the 1's and 2's represent the two ends of the ring) and record the values. The L-L and N-N readings should be the same and, with 2.5mm^2 twin & earth cable the E-E value should be a factor of 1.67 times higher.
- Now for a cunning stunt, cross-connect the L and N ends -- L1 to N2 and L2 to N1. This makes a 2-turn loop right round the ring. Now, with meter connected to the L and N pins of a 13A plug, work round every socket on the circuit. Lo and behold, every socket on the ring will (should) give the same resistance reading, equal to half of the L1-L2 reading. Any sockets on spurs will read higher by the combined L N resistance of the spur cable, and so are easily identified (except for very short spurs).
- Finally repeat the previous step with L and E cross-connected (L1 to E2, L2 to E1) and with the meter between the L and E pins of the plug. The readings now will not be constant (unless the CPC is the same size as the live conductors) but will increase toward the middle of the ring, and on spurs. This step effectively checks that the earth is connected at each socket. The highest reading obtained represents the contribution of the circuit to the overall earth fault loop impedance (used for ensuring compliance with 0.4s disconnection time in the event of an earth fault). The highest non-spur reading obtained should equal one-quarter of the sum of the initial L1-L2 and E1-E2 readings. If all is well, remove cross-connections and connect the ends to the supply.