Ring circuits

It is now necessary to describe the determination of conductivity in ionic solids. The circuit shown in Figure 8.9 is only a schematic representation. This type of measurement in high-impedance samples of ionic solids causes experimental problems that are avoided with the help of the so-called guard ring circuit, which prevents leakage currents affecting the measurement [16]. Besides, these circuits use an amplifier to deal with the high resistance of the sample. 

A small, single-point over-sink-type water heater may be considered as a permanently connected appliance and so may be connected to a ring circuit through a fused connection unit. A water heater of the immersion type is usually rated at a maximum of 3 kW, and could be considered as a permanently connected appliance, fed from a fused connection unit. However, many immersion heating systems are connected into storage vessels of about 150 litres in domestic installations, and the On Site Guide states that immersion heaters fitted to vessels in excess of 15 litres should be supplied by their own circuit (OSG Appendix H5). 

The object of the test is to ensure that all ring circuit cables are continuous around the ring that is, that there are no breaks and no interconnections in the ring, and that all connections are electrically and mechanically sound. This test also verifies the polarity of each socket outlet. 

Ring circuits are very similar to radial circuits in that each socket outlet is fed from the previous one, but in ring circuits the last socket is wired back to the source of supply. Each ring final circuit conductor must he looped into every socket outlet or joint box which forms the ring and must be electrically continuous throughout its length. The numher of permitted socket outlets is unlimited but each ring circuit must not cover more than 100 m of floor area. 

The total number of non-fused spurs must not exceed the total number of socket outiets and pieces of stationary equipment connected directly in the circuit. The cable used for non-fused spurs must not be less than that of the ring circuit. The requirements concerning spurs are given in Appendix 15 of the lEE Regulations but the various circuit arrangements may be expressed by the block diagrams of Fig. 14.14. 

Non-fused spurs may be connected into the ring circuit at the terminals of socket outlets or at joint boxes or at the origin of the circuit. 

The live and neutral conductors should now be temporarily joined together as shown in Fig. 14.68. An ohmmeter reading should then be taken between live and neutral at every socket outlet on the ring circuit. The readings obtained should be substantially the same, provided that there are no breaks or multiple loops in the ring. Each reading should have a value of approximately... 

The testing of a ring circuit protected by a general-purpose RCD to BS EN 61008 in a split-board consumer unit is carried out as follows ... 

Fig. 17.2 Measuring the continuity of the phase and neutral conductors in a ring circuit.

Standard househoid -13A ring circuit 30A or 32A - max floor area 100 m wired in 2.5 mm cabie... 

Circuit 4. A ring circuit of 13A socket outiets instaiied in accordance with Section H of the On Site Guide and protected by a 32 amp MCB. 

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