Domestic Electrical Installations, by Bishoy E. Sedhom

5- Radial Socket-Outlet Circuits ❑ Although most domestic installations use ring final circuits to supply socket outlets, radial circuits are quite acceptable. The recommendations for such circuits are given in Table 2.1. These radial circuits may have both fused and non-fused spurs (Figure...

5- Radial Socket-Outlet Circuits ❑ Although most domestic installations use ring final circuits to supply socket outlets, radial circuits are quite acceptable. The recommendations for such circuits are given in Table 2.1. These radial circuits may have both fused and non-fused spurs (Figure 2.20). 6- Storage Heater Circuits ❑ These, as discussed, are arranged to be energized during off- peak periods and it is usual to have a separate radial circuit for each heater, terminating in a fused connection unit. 7- Water heater circuits ❑ Most small, over-or under-sink water heaters (less than 15 litres), storage or instantaneous, are fed from fused spurs from ring circuits. However, heaters over 15 L capacity and showers must be fed from their own individual radial circuits. 8- Cooker Circuits ❑ These radial circuits feed cooker units or control switches from which the cooking appliances are supplied. Cooker units are available with or without socket outlets. ❑ If the circuit rating is between 15 and 50 A, one radial may feed two or more cooking appliances in the same room. Typical of this is the split-level cooker arrangement (i.e. an oven and a remote hob). ❑ The control switch must be within 2 m of any appliance and, in the case of two appliances, within 2 m of either one. 9- Ring Final Circuits The general requirements for these conventional circuits are as follows. ❑ Provided that the ring does not serve an area exceeding 100 m2, an unlimited number of sockets may be fed using either 2.5 m2 PVC-insulated copper conductors or 1.5 m2 mineral- insulated copper conductors (MICCs) and protected by a 30 or 32 A device of any type. ❑ The number of fused spurs is unlimited. ❑ There may be as many non-fused spurs as there are points on the ring. ❑ Each non-fused spur may feed one single-socket outlet, one double-socket outlet or one item of permanently connected equipment. 10- Cables ❑ There are a vast number of different types and sizes of cable and conductors available to cater for all applications. Those used in a domestic situation, however, are limited to just a few. Fixed wiring ❑ supplies all the outlets in the installation, sockets, lighting point, etc. ❑ connects together detectors, sensors, etc., for security systems ❑ supplies to telephone points and call systems. Power and lighting ❑ The cable used for these applications will predominately be of the flat twin with Circuit Protective Conductor (cpc) variety (6242 Y) or flat three-core with cpc (6243 Y) (Figure 2.23). ❑ The use of steel or rigid PVC conduit containing single-core conductors is rare in domestic premises, except flats or apartments with solid floors and ceilings and some older properties. ❑ When underground supplies are required to supply remote garages, sheds, workshops and garden lighting, 6242 Y cable can be used, provided it is protected by running in galvanized conduit or is otherwise protected from mechanical damage. It is, however, better to use a steel wire armored (SWA) cable (Figure 2.24), as this type is specifically designed for harsh applications. ❑ Alternatively, PVC-sheathed MICCs can be used (Figure 2.25), although this is expensive and requires special terminating tools. ❑ The only other cables involved in the fixed wiring would be the single-core line, neutral and earth conductors at the main intake (tails) and bonding conductors. ❑ All are categorized as 6491 X, and the line and neutral are PVC- sheathed (usually grey) as well as insulated. ❑ The earth conductors, collectively called protective conductors, are green and yellow. Security ❑ In this case, the cable used to link sensors and detectors will depend on the system, but is usually multicore PVC-sheathed and insulated with stranded conductors typically 7/0.2 (i.e. seven strands of 0.2 mm diameter wire). Fire alarms ❑ An increasing number of homes now have fire alarm systems installed, very often linked to a security system. The cable used to link the sensors is usually FP 200 or Firetuff (both of which are fire retardant). Telephone and call systems ❑ These employ similar cable to that used for security systems. Flexible cables ❑ Flexible cables are used to make the final connection from the fixed wiring to accessories or equipment either directly or via plugs. ❑ The conductors of these cables are made of many strands of thin wire, giving the flexible quality with which we are familiar. ❑ The number and size of these strands depend on the overall size of the conductor, for example, a 0.5 mm2 conductor comprises 16 strands of 0.2 mm diameter wire, whereas a 2.5 mm2 conductor has 50 strands of 0.25 mm diameter wire. ❑ There are a variety of flexible cables used in a domestic situation, varying from 0.5 to 2.5 mm2 (4.0 mm2 would hardly ever be used). ❑ The choice of cable type and size will depend on the appliance or equipment and the environmental conditions. ❑ So, for example, pendant drops from ceiling roses to lamp- holders would be wired with circular general-purpose PVC cables, whereas heat-resistant PVC or butyl rubber flex would be used for connection to heating appliances.