Electrical DIY – Source of supply

When you turn on a switch you naturally expect the light to come on or a particular appliance to work. Circuits feed electricity all round the home, so you can use the power just where and when you want it.

Electricity is generated at a power station and conveyed across country by either underground cables or overhead cables strung between tall pylons. Its voltage is reduced by an enclosed transformer or sub-station in your locality and from there electricity is fed to your home at 240 volts.

Main supply

Cables which conduct electricity from the transformer to the consumer consist of insulated live (red) and neutral (black) conductors protected by an earthed metal sheath. The underground cable terminates in the house at ground level; in districts where there is overhead distribution, the supply is fed into the top of the house through a porcelain tube under the eaves.

In both cases the main supply cable terminates in a sealed unit which holds the service fuse. This fuse is designed to blow if there is a serious fault in the house which has failed to blow the fuse in the consumer unit (or fuse box), thus preventing the supply to neighbouring homes being affected. A 60 or 100amp fuse is connected to the live feed and the neutral conductor is connected to a brass terminal. There is another terminal on the outside of the sealed unit to which the household earth connection is made. The sealed unit containing the fuse belongs to the electricity company and must never be opened by the consumer; if the fuse does blow, you should call in the electricity company.

Two cables run from the service fuse to the meter, which measures the amount of electricity used. The meter is also the property of the electricity company and should never be tampered with by the consumer. The installation from the meter onwards is the consumer’s responsibility and includes the two leads from the meter to the consumer unit.

Consumer unit

This contains a main switch and circuit fuses for the whole installation. Most houses have a lighting circuit for each floor so the whole house is not plunged into darkness in the event of a fuse blowing. A high wattage fixed appliance such as a cooker or immersion heater has its own sub-circuit. The cooker is wired directly to its own control switch — into which can be incorporated a power plug for use with other electrical kitchen gadgets. The control switch must be within two metres (or six feet) of the cooker, but separate from it. Make sure you have a long enough lead to the box so you can move the cooker out when cleaning.

Ring circuit

Socket outlets and small fixed appliances such as wall heaters and extractor fans are connected to a ring circuit, formed by a cable comprising a live (red) wire, a neutral (black) wire and a bare earth wire (which is encased in green PVC sleeving where it leaves the sheath). The cable used in domestic ring mains is 2.5sq mm twin core and earth, sheathed in PVC. This cable runs from a 30amp fuse in the consumer unit, serves each outlet and returns to the fuse, forming a ring.

The total load for a ring circuit is 30amps (7200 watts). So although it is possible to have, for example, twenty 13amp socket outlets on a ring, it is unlikely the number of domestic appliances being used at one time will exceed 30amps. If the circuit were overloaded, the 30amp fuse would blow and therefore maintain the safety requirements. However it is advisable to have a ring circuit for each floor in the house. If the kitchen has a large number of electrical appliances and/or a freezer, you should have a separate ring circuit in the kitchen to prevent overloading and ensure the freezer is not affected by a fault elsewhere.

Plugs used on a ring circuit have square pins and are fitted with 3 or 13amp fuses. The 3amp fuse covers all appliances up to a loading of 720 watts, which includes small appliances such as lamps. A 13amp fuse takes up to 3000 watts.

Additional socket outlets may be connected to a ring circuit, but the area served by the circuit should not exceed 100sq m (or 120sq yd).

Non-fused spur extensions

You can connect non-fused spur extensions to the circuit provided each spur supplies no more than two 13amp socket outlets or one fixed appliance and that the total number of spurs does not exceed the number of socket outlets on the main ring. No more than two spurs may be connected from each outlet on the ring; for this purpose a junction box wired into the ring can be classified as an outlet.

Fused connection units

These outlets, also used on a ring circuit, have the same fuse rating as plugs and are connected to a fixed appliance by a cable or flex. The unit may be switched or unswitched and fitted with an indicator light to show when the supply is connected to the outgoing flex or cable. Like socket outlets, the units can be flush or surface-mounted.

Flex outlets are simply a means of converting from a cable to a flex. A fused, switched connection unit, for example, outside a bathroom may supply a cable leading into the bathroom to feed a wall-mounted heater. Adjacent to the heater would be an outlet to feed the flex into the heater.

The clock connector is a similar type of unit, with a fuse fitted in a special plug into which the flex runs. The plug is retained in the socket by a knurled thumbscrew. Though called a clock connector, this is also suitable for small appliances such as window and wall extractor units.

Lighting circuits

Although lighting circuits are separate from the ring main or power circuit, there is nothing to prevent an individual light being taken from the ring. However, every home should have at least two lighting circuits protected by 5amp fuses. A 5amp fuse will carry up to twelve 100 watt lamps and it is usual, for example in a two-storey house, to plan one circuit for downstairs and another for

upstairs. This will provide enough lighting points

for decorative effects as well as general illumination.

Shaver units may also be connected to lighting circuits. For use in a bathroom, or room containing a shower, you must buy a unit which incorporates an isolating transformer. Here the socket is isolated from earth to remove the risk of an electric shock.

Lighting cable

Unlike the ring circuit, lighting cable does not return to the consumer unit. The cable now used is a l sq mm PVC twin core and earth rated for up to 12amps. It is made up of a red insulated core for live, black insulated core for neutral and a bare earth conductor between them. The three conductors are laid side by side, surrounded by a PVC sheath.

The lighting cable travels from the consumer unit to a series of lighting points for ceiling roses or wall light fittings. It also connects to the switch or switches controlling the lamps and must do this in such a way that the individual switch, unless planned otherwise, does not affect other lights.

Two systems

There are two methods of wiring lights — by junction box or the loop-in ceiling rose. With the junction box system, cable is taken to a series of up to ten junction boxes. These are generally sited between ceiling joists or under floorboards close to where the cable is chanelled under the wall plaster to the switch. The loop-in system is more widely used and the ceiling rose incorporates the function of the junction box.

Extra lighting points may be added to an existing system by connecting cable from the junction box or ceiling rose to the new lighting point and switch.

Lamps may be controlled by switches in more than one place in a house — switches on the ground floor and first floor can both control a half-way or landing light. Or two lamps at different places—possibly in a large room — could be controlled by a double switch.

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