TECHNICAL TOPIC POWER CIRCUITS

Standard circuit arrangements for socket-outlet circuits

Bill Allan Introduction cable and can have an unlimited number of socket- Fig 1 requires that joints and the number of socket-outlets which can be Last month, we looked at the accessories for socket- outlets as long as the floor area served is no greater must be accessible supplied by a fused spur is also unlimited. continues outlet circuits. This month, we’ll consider the standard than 100 m2 . However, consideration must be given for inspection, circuit arrangements for socket-outlet circuits. In this to the value of the connected load to ensure that it testing and his look at Fused Non-fused spur Radial circuits article, we hope to cover the main questions which does not exceed the rating of the circuit’s overcurrent spur maintenance (see circuits are asked about socket-outlet circuits. protective device. Regulations 131-12- A radial circuit differs from a ring circuit in that no Regulation 553-01-07 of BS 7671 requires that an 01, 513-01-01 and cable is brought from the furthest socket-outlet back Standard circuit arrangements adequate number of socket-outlets be installed, 526-04-01). A few to the (see Fig. 2). Reference to Table 1 taking account of portable equipment likely to be Fused exceptions to this indicates that two types of radial circuits are permitted connection There are two standard circuit arrangements for final used, in order to discourage the use of extension unit (FCU) requirement are using BS 1363 socket-outlets: circuits using BS 1363 socket-outlets and fused leads and long flexible cords on equipment. given in Regulations • those wired in 2.5 mm2 cable and protected by a connection units (FCU): Due to the number of items of electrical equipment 526-04-01 and 543- 20 A overcurrent device (these can supply an • Ring circuits (called ‘ring final circuits’ in BS 7671) commonly located in kitchens, these may require a 03-03. unlimited number of socket-outlets up to a • Radial circuits separate socket-outlet circuit. A non-fused spur maximum floor area of 50 m2) Table 1 below shows the maximum floor area which Socket-outlets in kitchens must be located so as to can supply only one • those wired in 4 mm2 cable and protected by a can be served by these circuits. provide reasonable sharing of the load in each leg of single or twin or 30A or 32A overcurrent device (these can supply multiple socket- an unlimited number of socket-outlets up to a TABLE 1 outlet or one item of maximum floor area of 75 m2 ) Circuit Minimum conductor Copper conductors, Rating of Maximum floor stationary Radial circuits are often considered to be more size (pvc insulated, MI cable overcurrent device area served equipment. economical to install than ring circuits, especially in copper conductors) large premises where bringing a cable from the Fused spurs furthest socket-outlet back to the consumer unit or Ring 2.5 mm2 1.5 mm2 30A or 32 A 100 M2 A spur which is could effectively double the length Radial 4 mm2 2.5 mm2 30A or 32 A 75 M2 connected to the of cable used. Neutral conductors Protective conductors Radial 2.5 mm2 1.5 mm2 20A 50 M2 ring circuit by means Phase conductors of a FCU is called a Conclusion fused spur (Fig. 1). Table 1 permits, for each type of circuit shown, an the ring circuit. Alternatively, a 4mm2 radial circuit Consumer unit The number of fused Further information on socket-outlet circuits can be Neutral Bar unlimited number of socket-outlets to be installed up could be installed. Regulation 433-02-04 gives the Earth bar spurs which can be obtained in IEE Guidance Note 1 Selection and to the maximum floor area indicated. requirements for ring circuits. Protective device supplied by a ring Erection (Appendix C in particular) and the IEE On- The Table is applicable for circuits protected by: Larger items of current-using equipment, such as circuit is unlimited Site Guide (especially Appendix 8). • Fuses to BS 3036, BS 1362 and BS 88 immersion heaters, cannot normally be plugged into a • Circuit breakers types B and C to BS EN 60898 ring circuit but require a separate circuit. • RCBOs to BS EN 61009-1 • Circuit breakers to BS 60947-2 Spurs Fig 2 Radial circuit • MCBs to BS 3871, types 1, 2 and 3 A spur is a socket-outlet or FCU supplied by a single NOTE: The cross-sectional area of a conductor may cable which is connected to the ring circuit. When be reduced for fused spurs, which are discussed adding a socket-outlet to a ring circuit, it is often below. easier to do so by using a spur.

Non-fused spurs Ring circuits Neutral A spur which is connected directly into the ring circuit Neutral Protective conductors Bar conductors Ring circuits, as the name implies, are connected in a is called a non-fused spur. (Fig. 1). The number of Phase conductors ‘ring’, which is formed by the cable running from the non-fused spurs connected to a ring circuit must not consumer unit to the first socket-outlet, then to the exceed the total number of socket-outlets and items second socket-outlet and so on, with the cable of stationary equipment connected to the ring. Consumer unit

returning back to the consumer unit from the last A non-fused spur can be connected to a socket- Earth bar See overleaf for socket-outlet in the circuit (see Fig. 1). Ring circuits outlet on the ring circuit or it can be connected by Student Activities are protected by either 30 amp or by 32 amp means of a suitably rated . If a junction Protective device protective devices, are normally wired in 2.5 mm2 box is used, it must be borne in mind that BS 7671

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