The descriptions in this part of the Electricity course refer to the wiring systems used in the UK. Wiring practice in Canada and N America is different—ring mains do not exist, and power outlets are connected to a form of loop wiring which may also serve lighting circuits. Cable colour coding and types also differ from those in the UK.
In some areas of N America and Canada, you may not be allowed to do your own electrical work: check your local electrical code to find out what regulations apply in your area.
Whatever country you live in, electricity must be treated with respect. Never work on a circuit until your certain that it is not live.
Today, the average home is equipped with a far greater number of electrical appliances than was the case a few years ago. And if many of these appliances are in use at the same time, there may not be enough socket outlets to go round.
In the UK, the sockets which have three rectangular holes to accept the pins of 13 amp plugs are used on ring mains circuits and on some partially modernized radial circuits. Socket outlets with round holes for 2 amp, 5 amp and 15 amp plugs are used only on radial circuits and usually indicate that the circuit is over 30 years old. The rubber insulation on the fixed wiring in these older circuits eventually begins to break up, so a house with such a circuit may need rewiring altogether.
Modern socket outlets can be flush-or surface-mounted. The flush type is screwed onto a metal box— sometimes called a pattress—which houses the cables and is recessed into the wall. For surface mounting, the same sort of socket is used, but this time screwed to a plastic box which is fixed to the wall surface. Surface-mounting is usually easier, but because the sockets protrude they may be more easily damaged.
The boxes for flush-mounted sockets are usually 25-35mm deep, though some sockets can be mounted semi-flush on a 16mm deep box. These have the advantage that, when cutting the housing in a masonry wall, you do not have to cut away much, if any, of the wall material behind the plaster.
Socket cover plates and, in the case of surface-mounted sockets, the box as well, are usually made of white plastic. But for installations in places m like garages where outlets have to be more durable, sockets with metal cover plates and boxes are available.
In the UK, regulations require the live and neutral holes of 13 amp sockets to be fitted with protective shutters to prevent people from poking metal objects inside with possibly fatal results. Most sockets also have a built-in switch to minimize the danger of touching live parts when you insert or remove a plug. Some switched sockets include a neon indicator light to show when the socket is on. Switches are usually single-pole, cutting off the supply in the live wire only. This is usually sufficient: to totally isolate an appliance, remove the plug.
In a bathroom, where the presence of water increases the risk of electric shocks, the only sockets that can be installed are those specially designed for electric razors.
In rooms where there are too few sockets to go round, or where appliances are too far from free sockets, plug adaptors—sometimes called socket adaptors—are often used to plug two or three appliances into the same socket.
A two-way adaptor with a fuse of the correct size is satisfactory for temporarily connecting low wattage appliances, such as table lamps or the hi-fi. But it is not advisable to use an adaptor for long periods or to plug in a high wattage appliance, such as a bar heater. By far the best approach is to add extra sockets.
Adding socket outlets
If a room has an inadequate number of socket outlets and you have a ring-main circuit, you can add extra ones without too much difficulty.
Even if you have enough sockets in your home, they may not be positioned where they are most needed. An extra socket or two can help.
A ring circuit can have an unlimited number of socket outlets and ideally, each room should have about four. The circuit should serve a floor area of not more than 100m2, which is more than the area of an average two-storey house. So, if you decide to install extra sockets, there is no limit to the number you can add as long as they do not extend the circuit beyond the 100m2 maximum.
Sockets are almost always mounted on a wall and should be positioned at a minimum height of 150mm above the floor level, or the working surface in a kitchen.
Though new sockets can be installed directly from the ring, it is often easier to wire it on a spur—an extension taken from the back of an existing socket.
Each existing socket outlet, or double outlet, can supply no more than two additional outlets in this way.
A spur from the ring main
Running a spur from an existing socket means that you can place the new socket almost anywhere you like. Another advantage is that you do not have to run cables from the ring main under floorboards. You can simply feed it around a channel—or chase—cut into the wall.
To wire the spur, you need a sufficient length of 2.5mm2 cable ‘twin with earth’ to stretch from the existing socket to the site of the new one. You also need a similar length of oval PVC conduit with securing clips and about 1m of green and yellow PVC sleeving.
Choose as your source for the spur a socket as close to the proposed site as possible. The work will also be easier if the source is a twin socket— the larger box will have more room for an extra cable.
Before starting work, isolate the supply. Check on the suitability of your source socket by unscrewing the cover plate, easing it from the wall and comparing the wiring. If a spur has already been taken from it or the socket itself is on a spur, it is not suitable. When you have located a suitable socket, unscrew the terminal screws and re-remove and untwist the wires. Prise the box away from its recess and knock out one of the punched holes on the relevant side.
Hold the new box in position on the wall and draw around it to mark out its recess. Using a straight edge as a guide, draw two parallel lines about 25mm apart, between the new position and the existing recess, for the cable chase. Next cut the new recess and the chase with a hammer and bolster. The chase should be about 6mm deeper than the thickness of the conduit.
When the recess and chase are completed, knock out one of the punched holes in the new box and install it in its recess. Refit the box of the other socket as well, then cut the conduit to length so that it will protrude about 10mm into each box when installed in the chase. Fit the conduit, secure it with its clips, then make good the plaster.
Wait until the plaster is dry before you start wiring. Push the cable through the conduit so that about 200mm of cable protrude into each box, then remove the outer sheathing back to the edge of the box at both ends. Strip about 20mm.of insulation from the live and neutral cores and sleeve the earth wires with lengths of green and yellow PVC sleeving, leaving about 20mm of bare wire protruding. Wire up the new socket cover plate and screw the plate into place.
Returning to the other box, connect the three red wires together and the three black wires together. If the original earth wires have not been sleeved, sleeve them at this stage then twist them together. Wire the plate up and screw it back on to the box. Turn the power on again and plug an appliance into the new socket to check that it is wired correctly.
Single into twin socket
An even easier method of adding sockets is by converting single socket outlets into double ones. Where single sockets have been installed, the number of outlets in a room can be doubled, though none will be in new positions.
If you are working in an area of good natural light, turn off the electricity supply at the mains. If not, isolate the supply by removing the relevant circuit fuse and leave the lighting circuits functioning.
You must now find out whether or not your chosen socket is suitable for conversion. If it is a single socket on a spur, or if it is on the main circuit, the socket can be converted without any problems. But if it is one of two sockets on a spur, no more outlets can be added to the spur.
To check the position of the socket in the circuit, unscrew the cover plate, ease it from the wall and examine the wiring. Two sets of wires connected to the plate suggests that the socket is probably not a spur. To make quite sure, examine the wiring in the nearest sockets on either side. If either of these has one set, or three sets of wires, the socket is on a spur and should not be converted. If the selected socket has three sets of wires, it is supplying a spur and can be converted without any problems.
When you have satisfied yourself that the socket outlet is suitable, undo the terminal screws holding the cable cores into the plate and gently remove the cores inside. If the live, neutral and earth cores are twisted together, untwist them and then undo the mounting screws holding the metal box into its recess. Prise the box from the recess with a screwdriver or an old chisel, taking care not to damage the cables.
To house the new, twin socket box, the recess must be extended. Mark the position on the wall and cut out the plaster and brickwork to the depth of the new box with a hammer and bolster. Knock out the punched holes in the box to accept the cables, feed these through, and position and screw the box into place using wall plugs. Twist the wires together again and wire up the twin cover plate. Screw the plate on to the box, switch on the power and test the socket.
The above procedure also applies to fitting a surface-mounted twin socket though in this case, no recess is needed—the box simply screws to the wall. If you replace a flush single socket with a surface-mounted twin, remove the old cover plate but leave the recessed box in place—removing it is only likely to damage the wall, and make the job more difficult.