Electricity in the home is something which we all take for granted—and would be lost without. Yet electricity is also highly dangerous if it is not treated with the respect it deserves. For the do-it-yourself enthusiast, this means having a sound knowledge of the way in which domestic installations work before tackling any electrical job with confidence.
Electricity and the law
In the UK, regulations covering wiring are compiled by the Institute of Electrical Engineers. Anyone may do his own wiring, but the IEE regulations must be complied with. These require that all electrical installations be tested on completion by the relevant electricity supply board.
In the US and Canada, the ordinances laying down what you are permitted to do vary from area to area. In some communities you may do complex installations; in some, only partial installations; in others, you may not touch the permanent wiring at all. So check with your local building inspector before you do any work.
An electric current consists of a flow of minute particles called electrons. This flow can be likened to the flow of water from a tap connected by a pipe to a tank.
For water to flow when the tap is opened, the tank must be at a higher level than the tap. And the greater the height of the tank, the higher the pressure of the water that comes out of the tap. So water at high pressure has a greater rate of flow, or current, than water at low pressure.
The voltage in an electrical circuit corresponds with the pressure of the water in the pipe. The rate of flow of an electric current is measured in amperes and is equivalent to the flow of water along the pipe—that is, how much comes out at any given time.
Electrical power is measured in watts. This term applies to the electrical equipment itself and is a measurement of the rate at which it
The comparison between the flow of water in a pipe and an electric current in a wire is not exact: electricity requires a closed loop—a circuit—in order to work.
Electricity comes into the home from a local transformer through an armoured service cable or via overhead wires. The service cable is connected to a fuse unit—called the company fuse— which is sealed by the electricity board or company. From here, power flows along the live supply wire and through the meter to the consumer unit—a combined fuse box and main switch. The live supply wire is usually encased in two separate sheaths. The electricity then flows through your lights and appliances before returning to the local transformer along the neutral wire. A third wire, the earth, connects the appliance casing to the main fuse box casing and then to the ground. Unless a fault develops, it-carries no current.
In the US and Canada—and most other parts of the world—power and lighting outlets are served by a system called loop wiring. In this system the live, neutral and earth wires run outwards from the main switch to the first light. From there they are ‘looped off’ to the second light; from the second light to the third; and so on. The number of lights and/or power outlets on each circuit is limited by local ordinances, so that every house needs a number of circuits. In some such systems, an individual circuit will supply both lighting and power outlets. In others, the lighting circuits and those that supply wall sockets are separate installations.
In the UK, lighting circuits are still of the loop type. And up until 1947, power circuits were also of a similar type called radial wiring, in which each socket was served by a live, neutral and earth cable direct from the main fuse board. These older British systems are fitted with three varieties of round pin sockets rated at 15 amps, 5 amps and 2 amps. The configuration of the pins is the same in all three cases but reduces in size with current rating.
In all loop and radial systems, because the plugs that fit the sockets have no fuse of their own to help protect appliances, the circuit fuses in the fuse box must be kept to as low an amp rating as possible in order to give both the cable and the appliance a reasonable degree of protection from overload.
In Britain, houses wired since 1947 use, for power socket circuits only, a different wiring system known as the ring-main circuit. In this system, the live, neutral and earth wires run in a complete circle from the main switch to each socket in turn, and then back to the consumer unit. There is generally one ring for each floor of a house, with ‘spurs’ reaching out from it to supply isolated sockets and appliances which take a lot of power, such as the stove.
Plugs and socket outlets in ring-main circuits are of the 13-amp rectangular pin type. These are much safer than the old round-pin types because the sockets have shutters inside which automatically close when a plug is withdrawn. Furthermore, ring-main plugs, unlike other types, carry their own cartridge fuses. This means that should an individual appliance become faulty, only the fuse in its own plug—and not the main fuse for the whole circuit— will ‘blow’.
Should a live wire come into contact with the metal casing of an appliance, anyone who touches the appliance is liable to receive a severe electric shock. For this reason, domestic appliances—apart from ones that are double insulated—have an earth wire connected to their outer casings and led indirectly to ground.
This is so that, if a live wire makes contact with the casing, the electricity will follow the path of least resistance to the ground. That is, it will flow through the earth wire instead of the person’s body. At the same time, a live wire coming into contact with earthed metalwork will result in a large current flow that will blow the fuse.
The electricity flows from the live wire in this way because it is trying to reach the neutral—which is connected to earth back at the electricity board transformer. This system has been found to be the safest way of disposing of unwanted current.
A fuse is a deliberately weak link in the wiring, thinner than the wires on either side. If an overload occurs, the fuse wire melts and cuts off the current before the heat from the overload can damage equipment or cause a fire.
Fuses should always be of the nearest available size above the amperage of the appliance or circuit that they protect. Most electrical appliances have their wattage marked on a small plate fixed to the back or base of the unit. So, for an appliance connected to a ring-main, you can use the formula above to find the amp rating and hence the correct fuse to go in the appliance’s plug.
For example, say an electric fire has a rating of 3 kilowatts and the voltage of the mains is 240 volts. The current taken by the fire is found by dividing the watts—3,000—by the volts—240— which gives a result of 12.5 amps. Therefore, the fire should be protected with a 13-amp fuse, the nearest higher size available.
In Britain, it is recommended practice to use 3-amp cartridge fuses, colour coded red, for all appliances rated up to 720 watts, and 13-amp fuses, colour coded brown, for everything else up to a maximum rating of 3 kilowatts.
Most plugs come fitted with 13-amp fuses, the largest available size. But some appliances, portable TV sets for example, can be damaged by a current of less than 13 amps. So it pays to use the formula and check that the plugs on appliances are fitted with the correct size of fuse.