8.4.2 - Ring final circuit continuity

The ring final circuit, feeding 13 A sockets, is extremely widely used, both in domestic and in commercial or industrial situations. It is very important that each of the three rings associated with each circuit (phase, neutral and protective conductors) should be continuous and not broken. If this happens, current will not be properly shared by the circuit conductors. {Fig 8.4} shows how this will happen. {Fig 8.4(a)} shows a ring circuit feeding ten socket outlets, each of which is assumed to supply a load taking a current of 3 A. In simple terms, current is then shared between the conductors, so that each could have a minimum current carrying capacity of 15 A. {Fig 8.4(b)} shows the same ring circuit with the same loads, but broken between the ninth and tenth sockets. It can be seen that now one cable will carry only 3 A whilst the other (perhaps with a current rating of 20 A) will carry 27 A. The effect will occur in any broken ring, whether simply one live conductor or both are broken.

Fig 8.4 - illustrating the danger of a break in a ring final circuit
a) unbroken ring with correct current sharing
b) broken ring with incorrect current sharing

It is similarly important that there should be no 'bridge' connection across the circuit. This would happen if, for example, two spurs from different points of the ring were connected together as shown in {Fig 8.5}, and again could result in incorrect load sharing between the ring conductors.

The tests of the ring final circuit will establish that neither a broken nor a bridged ring has occurred. The following suggested test is based on the Guidance Note on Inspection and Testing issued by the IEE.

Fig 8.5 - A 'bridged' ring final circuit

Test 1
This test confirms that complete rings exist and that there are no breaks. To complete the test, the two ends of the ring cable are disconnected at the distribution board. The phase conductor of one side of the ring and the neutral from the other (P1 and N2J are connected together, and a low resistance ohmmeter used to measure the resistance between the remaining phase and the neutral (P2 and Ni). {Figure 8.6} shows that this confirms the continuity of the live conductors. To check the continuity of the circuit protective conductor, connect the phase and CPC of different sides together (P1 and E2) and measure the resistance between phase and CPC of the other side (P2 and El). The result of this test will be a measurement of the resistance of live and protective conductors round the ring, and if divided by four gives (Ri + R2) which will conform with the values calculated from

Fig 8.6 - Test to confirm the continuity of a ring final circuit

Test 2
This test will confirm the absence of bridges in the ring circuit, see {Fig 8.7}. First, the phase conductor of one side of the ring is connected to the neutral of the other (P1 and N2) and the remaining phase and neutral are also connected together (P2 -and Ni). The resistance is then measured between phase and neutral contacts of each socket on the ring. If the results of these measurements are all substantially the same (within 0.05 Ohms), the absence of a bridge is confirmed. If the readings are different, this will indicate the presence of a bridge or may be due to incorrect connection of the ends of the ring. If they are connected P1 to NI and P2 to N2 then readings will increase or reduce as successive measurements round the ring are taken, as is the case where a bridge exists. Whilst this misconnection is easily avoided when using sheathed cables, a mistake can be made very easily if the system consists of single-core cables in conduit. It may be of interest to note that the resistance reading between phase and neutral outlets at each socket should be one quarter of the phase/neutral reading of Test 1.

Measurements are also taken at each socket on the ring between the phase and the protective conductor with the temporary connection made at the origin of the ring between P1 to E2 and between P2 to El. Substantially similar results will indicate the absence of bridges.

Fig 8.7 - Test to confirm the absence of bridges in a ring final circuit