Socket outlet circuits

A 20 A radial socket outlet circuit is wired in 2.5 mm PVC cable incorporating a 1.5mm CPC. The cable length is 30m installed in an ambient temperature of 20°C and the consumer s protection is by 20 A MCB Type B to BS EN 60898. The earth fault loop impedance of the supply is 0.5 fi. Calculate the total earth fault loop impedance Zs, and establish that the value is less than the maximum value permissible for this type of circuit. 

PVC cables enclosed in heavy-duty PVC conduit are suitable for installations in most agricultural buildings. All exposed and extraneous metalwork must be provided with supplementary protective bonding in areas where livestock is kept (lET Regulation 705.415.2.1). In many situations, waterproof socket outlets to BS 196 must be installed. All socket outlet circuits must be protected by an RCD complying with the appropriate British Standard and the operating current must not exceed 30 mA. 

Consumer units can now be supplied which incorporate an RCD, split boards, where half of the final circuits are RCD protected so that any equipment supplied by the consumer unit to socket outlet circuits or out of doors circuits which are outside the zone created by the protective equi-potential bonding, such as a garage or greenhouse, can have the special protection required by lEE Regulations 411.3.3 and 415.1.1. 

Additional protection by 30 mA RCD is required in addition to overcurrent protection for all socket outlet circuits to be used by ordinary persons and intended for general use. 

Where the distribution circuit feeds a stationary item of equipment, not socket outlets and not portable equipment, the disconnection time may be taken as 5.0 seconds. This apphes to motors. 

The nominal time of 0.4 seconds is intended for circuits supplying socket outlets, regularly moved portable equipment and Class 1 hand-held equipment. For voltages (Vp ) different from 240 Vac, the disconnection time (fdis) of 0.4 seconds becomes approximately related as. 

It has become standard practice in recent years to use sensitive earth leakage current detectors in circuit breakers to further protect against electric shock. The current sensitivity can be chosen from a range of standard current values, e.g. 30, 100, 500 and 1000 mA. The 30 mA sensitivity is used at individual consumer sub-circuits, e.g. feeders to domestic and small power socket outlets, feeders to luminairs. The higher sensitivities are used in the upstream circuit breakers so that protection discrimination is achieved. 

An isolator is a mechanical device which is operated manually and used to open or close a circuit off load. An isolator switch must be provided close to the supply point so that all equipment can be made safe for maintenance. Isolators for motor circuits must isolate the motor and the control equipment, and isolators for discharge lighting luminaires must be an integral part of the luminaire so that it is isolated when the cover is removed or be provided with effective local isolation (lET Regulation 537.2.1.6). Devices which are suitable for isolation are isolation switches, fuse links, circuit-breakers, plugs and socket outlets. They must isolate all live supply conductors and provision must be made to secure the isolation (lET Regulation 537.2.2.4). 

The maximum permitted value given in Table 41.3 of the lET Regulations for a 20 A MCB protecting a socket outlet is 2.19 as shown in Table 3.2. The circuit... 

A 230V ring main circuit of socket outlets is wired in 2.5 mm single PVC copper cables in a plastic conduit with a separate 1.5 mm CPC. An earth fault loop impedance test identifies Zg as 1.15Verify that the 1.5 mm CPC meets the requirements of lET Regulation 543.1.3 when the protective device is a 30A semi-enclosed fuse. 

Touring caravans must be supplied from a 16A industrial-type socket outlet adjacent to the caravan park pitch, similar to that shown in Fig. 3.75. Each socket outlet must be provided with individual over-current protection and an individual residual current circuit-breaker with a rated tripping current of 30 mA (lET Regulations 708.553.1.12 and 708.553.1.13). The distance between the caravan connector and the site socket outlet must not be more than 20 m (lET Regulation 708.530.3). These requirements are shown in Fig. 3.77. 

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. 

The object of this test is to verify that all fuses, circuit-breakers and switches are connected in the line or live conductor only, that all socket outlets are correctly wired and that Edison screw-type lamp holders have the centre contact connected to the live conductor. It is important to make a polarity test on the installation since a visual inspection will only indicate conductor identification. 

Mechanical maintenance switching is intended for use by skilled but nonelectrical persons. Switches for mechanical maintenance must be manually operated, not have exposed live parts when the appliance is opened, must be connected in the main electrical circuit and have a reliable on/off indication or visible contact gap (Regulations 537.3.2.2). Devices which are suitable for switching off for mechanical maintenance are switches, circuit breakers, plug and socket outlets. 

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. 

Where two or more circuits are installed in the same premises, the socket outlets and permanently connected equipment should be reasonably shared out among the circuits, so that the total load is balanced. 

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. 

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 Regulations allow us to install an unlimited number of socket outlets, the restriction being that each circuit must not exceed a given floor area as shown in Figs. 14.12 and 14.13. 

Ring final circuit supplying twin socket outlets (total protective conductor current exceeding 10mA). 

The multi-core cable used for wiring extension socket outlets should be of a type intended for use with telephone circuits, which will normally be between 0.4 and 0.68 mm in cross-section. Telephone cable conductors are identified in Table 14.10 and the individual terminals in Table 14.11. The conductors should be connected as shown in Fig. 14.62. 

---