Radial circuits

Therefore, immersion heaters must be wired on a separate radial circuit when they are connected to water vessels which hold more than 15 litres. Figure 3.55 shows the wiring arrangements for an immersion heater. Every switch must be a double-pole (DP) switch and out of reach of anyone using a fixed bath or shower when the immersion heater is fitted to a vessel in a bathroom. 

To avoid this, a clean supply is required for the computer network. Tfiis can be provided by taking the ring or radial circuits for the computer supplies from a point as close as possible to the intake position of the electrical supply to the building. A clean earth can also be taken from this point, which is usually one core of the cable and not the armour of an SWA cable, and distributed around the final wiring circuit. Alternatively, the computer supply can be cleaned by means of a filter such as that shown in Fig. 3.70. 

It Is best to dedicate a ring or radial circuit to the UPS and either to connect the computer equipment permanently or use non-standard outlets to discourage the unauthorized use and overloading of these special supplies by, for example, kettles. 

Socket outlets may be wired on a ring or radial circuit and in order that every appliance can be fed from an adjacent and convenient socket outlet, the number of sockets is unlimited provided that the floor area covered by the circuit does not exceed that given in Appendix 15 of the IEE Regulations. 

In a radial circuit each socket outlet is fed from the previous one. Live is connected to live, neutral to neutral and earth to earth at each socket outlet. The fuse and cable sizes are given in Appendix 15 of the IEE Regulations but circuits may also be expressed with a block diagram, as shown in Fig. 14.12. The number of permitted socket outlets is unlimited but each radial circuit must not exceed the floor area stated and the known or estimated load. 

When designing ring or radial circuits special consideration should be given to the loading in kitchens which may require separate circuits. This... 

Ring and radial circuits may be used for domestic or other premises where the maximum demand of the current-using equipment is estimated not to exceed the rating of the protective devices for the chosen circuit. 

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. 

Radial circuits are tested by connecting the phase and protective conductors together at the distribution board and then applying the test between the phase and earth terminals at each outlet point. A higher than expected reading should be investigated and will probably be due to a defective connection. Where the disconnection times are found to exceed those of Table 41A of BS 7671, it will be necessary to measure the protective conductor resistance on its own. 

The allowance for diversity for 13A socket outlets installed according to the standard arrangements of ring and radial circuits given in Table H2.1 of the On Site Guide when installed in individual households is ... 

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