Installation Circuits and Systems Block diagrams - These show, using squares, rectangles etc., the sequence of a system without too much technical detail. e.g. Layout diagrams - These are very similar to block diagrams, but they indicate more technical detail and tend to show items in their correct geographical location. Circuit/schematic - These show how a circuit functions and takes no account of exact locations of terminals or equipment. Wiring diagram - These indicate how a circuit or system is physically wired. Lighting Circuits One-way Switching Circuit Diagram Wiring Diagram
Two-way Switching Two-way and Intermediate Switching
Lighting layouts There are two main methods of wiring a lighting installation. 1. Each ceiling rose or junction box is fed from the previous one in the form of a chain. 2. The main feed is brought into a central junction box and each point is fed from it (like the spokes of a wheel). Power Circuits
Radial circuits Radial circuits are arranged in the way that each socket outlet is supplied via the previous one. IEE Regulations 1. The number of socket outlets, complying with BS 1363, is unlimited for a floor area of up to 50 m2 if the circuit protection is a 30 A or 32 A cartridge fuse or circuit breaker, the cable being 4 mm2 PVC copper or 2.5 mm2 mineral insulated (m.i.). 2. The number of socket outlets is unlimited for a floor area up to 20 m2 with any type of circuit protection of 20 A using 2.5 mm2 PVC copper cable or 1.5 mm2 m.i. 3. The total number of fused spurs is unlimited. Ring Final Circuits These circuits are the same as radial circuits except that the final socket outlet is wired back to the supply position. In effect any outlet is supplied from two directions. IEE Regulations 1. Every twin-socket outlet counts as two single-socket outlets. 2. The number of non-fused spurs must not exceed the total number of points on the ring.
3. The fuse rating of a fused spur box must not exceed 13 A and the current rating of all points supplied by the fused spur must not exceed 13 A. 4. Non-fused spurs must supply no more than one single or one double socket or one stationary appliance. 5. For domestic premises of area less than 100 m2 , a ring circuit may serve an unlimited number of points. 6. Cable sizes for ring circuits using copper conductors are: 2.5 mm2 PVC insulated or 1.5 mm2 m.i. Water Heater Circuits Water heaters of the immersion type (i.e. having an uninsulated element completely immersed in water) are available in several different forms, two of the more popular being the large 140 liter storage type, and the smaller open-outlet type (10–12 liters) used for small quantities of instant hot water. Both types are stationary appliances and could therefore be connected into a ring circuit. However, it is recommended that heaters over 15 liters have their own circuit. IEE Regulations 1. The heater shall be effectively connected to earth, and the inlet and outlet pipes must be made of metal. An earthing terminal must be provided.
2. The heater must be supplied from a double-pole linked switch (not a plug and socket). Cooker Circuits A cooker exceeding 3 kW should be supplied on its own circuit. As it is rare to have every heating element working at once, diversity is applied to calculate the assumed current demand as follows. The first 10 A of the total rated current of the connected cooking appliance, plus 30% of the remainder of the total rated current of the connected appliance, plus 5 A if there is a socket outlet in the control unit. Example The full-load rating of a 230 V cooker is 11.5 kW. Calculate, using diversity, the assumed current demand. The cooker control unit has a socket outlet. Actual full load demand: P = I x V = = 11500 230 = 50 Assumed demand using diversity: = 10 + 30 (50 − 10) 100 + 5 = 27 This means that a 27 A cable could be used safely rather than a 50 A cable. IEE Regulations 1. If the assumed current demand of a cooker circuit exceeds 15 A but does not exceed 50 A, two or more cooking appliances may be fed, if they are in the same room. 2. Every cooker must have a control switch within 2 m. If two cookers are installed, one switch may be used, provided that neither cooker is more than 2 m from the switch.