Understanding ELECTRICS

In theory, you could do electrical jobs knowing nothing about electricity, given accurate step-by-step instructions.

But you can’t deal with any part of an electrical installation in isolation — everything is linked. And unless you understand how each part of the system works you have no way of knowing if you are making a mistake. With electricity, ignorance is dangerous.

We’re all familiar with lights and power sockets, but how does the electricity reach them so we can use it? In fact, electricity enters your home along one thick cable (the service cable), passes through a large ‘service fuse’ and into a meter which records the amount you use. Everything up to and includeing that meter belongs to the electricity board, and is their responsibility. Everything beyond is the householder’s property, which is perhaps why installations vary so much.

In a modern installation — one wired in the last 30 years — there are two wires carrying electric current that lead from the meter to what is called the consumer unit. These wires are known as the meter tails — one is termed live, the other neutral.

On the inlet side of the consumer unit there’s a switch with which you can turn off the power altogether, but the unit’s principal job is to divide up the power and send it round your home through a network of cables.

These cables are organized into circuits. There are circuits for lights, power sockets and so on, each with its own fuse in the consumer unit. The cables themselves run under the floor, above the ceiling and may even be visible on wall surfaces, although more often they are buried within them.

In older installations, instead of a consumer unit there may be individual fuse boxes protecting separate circuits. And each of these fuse boxes will have an isolating switch to cut off power to the circuit it controls. These fuse boxes are connected direct to the meter by

WARNING: live and neutral meter tails. Alternatively the fuse boxes may be supplied from a distribution board which in turn is connected to the meter. Sometimes, even with a consumer unit you may find separate fuse boxes. This is normally the result of the system having been extended.

What are circuits?

If you take a battery, and connect a wire to the positive (+) terminal, and another to the negative (-), then bring the free ends of the wires together, electricity will flow from positive to negative along them. That’s a circuit. You can build a torch bulb and holder into it to prove it works. Break the circuit by cutting one wire, and the light goes out (the flow of current has stopped), and it will stay out until the cut ends are rejoined. That’s a simple switch.

Of course, the circuits in your home are a good deal more complex than that, and their design varies according to whether they supply lights, power sockets or whatever. Even the electricity is different. Instead of flowing in one direction, it goes back and forth 50 times a second — hence its name or AC for short.

But the principle is the same., Think of ‘live’ as positive, ‘neutral’ as negative, and you will see that for any appliance such as an electric fire to work it must have wires connecting it to the live and neutral terminals in the consumer unit. Those wires may be contained in a single cable, but the link must always be there, with switches to make or break it, and for safety reasons, switches are on the live wire.

What are fuses?

The main service cable has its fuse; the various circuits have theirs in the consumer unit or fuse box and if you remove the back of a flat-pin plug you’ll find a fuse in there.

Think of an electric light bulb. It gives out light because electricity passing through the filament (the fine wire just visible inside the bulb) makes it very hot. If you pass enough electricity through any wire, it will also heat up. If that wire happens to be a circuit cable, an appliance flex, or the service cable to the meter, then the consequences would be serious. So, to protect them, a weak link called a fuse is built into the circuit.

Most fuses are just thin pieces of wire. They can be fitted to rewirable fuse carriers, in which case you can replace them, or they may be in ceramic cartridges, in which case you throw them away and fit another. In any event, the fuse’s thickness is described in terms of how much electricity — expressed in amps— is theoretically needed to melt it.

The word ‘theoretically’ is important be-cause, in fact, fuses aren’t particularly accurate or reliable. For this reason, a more sensitive device called a miniature circuit breaker (MCB) may be used instead. It’s just a switch that turns off automatically when danger threatens. Once the fault responsible for the overload is put right, you switch on again.

Why cables?

It would be far too complicated to wire a house like a battery and bulb circuit using individual wires. Instead, the copper wires carrying the electricity are encased in PVC insulation to stop them touching and making their circuit in the wrong place — what’s called a short circuit — and then bound together in PVC sheathing to form a cable. In this way, the live, neutral and earth wires can be run as one, even though each one is still connected up separately.


The purpose of the earth wire within the cable is to make up the earth continuity conductor (ECC). This is an essential safety feature of any electrical installation. Its role is to act as a ‘safety valve’ in the event of a fault, causing a fuse to blow or an MCB to trip to isolate a faulty circuit or faulty appliance from the mains supply. In doing so it could prevent the risk of fire or someone being electrocuted.

Earth wires are connected to the metal parts of switches, socket outlets, fittings and appliances (and even plumbing) in a really up-to-date system. Electricity will flow along the line of least resistance, so that if by some mishap any of these parts became live (by coming into contact with a live conductor) the earth wire would offer a line of ‘less’ resistance. In effect the faulty current would travel along the earth wire rather than through a person touching the live metal part. And the extra current passing through one circuit would be sufficient to blow the fuse or activate the MCB. Unfortunately this doesn’t always happen.

– so, for added safety, a special device called an earth leakage circuit breaker (ELCB) can be fitted to detect the slightest leakage of current to earth. It shuts off the power within milliseconds — quickly enough to save a life.

– at the first sign of a fault.

ELCBs can be added to an existing system, or included within the consumer unit in a new installation. They usually protect all the circuits in the house and also act as a mains on/off switch.

Circuit cables

Distribution board

Consumer unit for off-peak circuits Consumer unit ‘White’ meter


Ampere (amp, or A for short) is the measure of the rate at which current flows through a circuit. The more powerful the appliance the more current it uses. Volt (V) indicates the force driving the current between live and neutral. The mains voltage is usually 240 volts. Watt (W) is the measure of electric power and is worked out by multiplying amps by volts. Also, 1,000W = 1 kW (kiloWatt).


Special regulations apply for safety reasons:.

– power sockets are not allowed.

– lights should be operated by a pull-cord.

– wall heaters should not be placed over a bath and must be operated by a pull-cord, or a switch outside the bathroom


These have a special socket and plug connection and are safe to use in a bathroom because they isolate the razor from the earthed mains supply.


– flexes under carpets.

– too many adaptors

It’s easier in the long run to have the system changed to meet your needs.


If this involves installing a new consumer unit or fuse box, you must call in the electricity board to make the final connection between it and the meter.


– turn off the supply.

– pull person clear immediately but his/her body – grab the clothes or use a wooden stick, length of rubber or leather.

– if breathing stops, give mouth-to-mouth resuscitation Don’t wrap the person in blankets or try to give him brandy. If severely shocked call the doctor.

Ring circuits

For getting electricity to the power points, the most common system of wiring is what’s called a ‘ring’ circuit. Wired in 2.5mm2 two-core and earth cable, most homes have one such circuit for each floor of the house.

The two-cores and the earth wire are connected to their terminals in the consumer unit (or fuse box) and then pass through each power socket in turn before returning to their respective terminals in the consumer unit (fuse box). The circuit is protected by a 30A fuse. The advantage of this system is it allows the cable to cope with more sockets than if it made a one-way trip. In fact, you are allowed as many sockets as you like on the ring, so long as the floor area served by the circuit doesn’t exceed 100 sq metres (1,080 sq ft). What’s more, you can increase the number of sockets by adding ‘branch lines’ off the ring. These are called ‘spurs’ and break into the ring via a junction box, a spur connection unit, or an existing socket. You are allowed as many spurs as there are sockets on the ring, and each spur can carry two single sockets, one double socket, or one fixed appliance via a fused connection unit. From 1983, when new wiring regulations come into force, spurs will be able to supply only single socket,one double socket or one fused connection unit.

Of course, with all those sockets, there is a risk of overloading the circuit, but in the average family home it’s unlikely that you’ll have enough sockets in use at any one time. The circuit may carry up to 30 amps of current which is equivalent to having appliances and portable lamps using 7,200 watts of power all switched on together. It’s doubtful that you would want all this on at the same time, but it’s wise not to go above this level of power use. If the circuit does overload, the fuse will blow. Or the MCB will switch off.

Radial circuits

Unlike ring circuits, radial circuits consist of a single cable that leaves the fuse box and runs to one or more sockets. In older homes in the UK, before ring circuits were introduced, all power circuits were wired as radials. Since homes had (and needed) only a few sockets. Individual circuits were usually run to each one from the fuse box. The sockets themselves were rated at 2A, 5A or 15A, and had round holes to accept round-pin plugs. Such circuits will probably have been wired in rubber- or lead-sheathed cables, which deteriorate with age , and are not able to satisfy the far greater electrical demands of a modern household. It’s wise to have such circuits examined by a qualified electrician, and best of all to have them replaced.

Radial circuits are, however, also used in modern wiring systems where a ring circuit could be inappropriate for some reason. There are two types, with different current-carrying capacity.

A 20A radial circuit uses 2.5mm2 cable and is protected by a 20A fuse (rewirable or cartridge) or an MCB in the consumer unit (or fuse box). It can supply an unlimited number of 13A socket outlets and fixed appliances using 3kW of power or less, providing they are within a floor area not exceeding 20 sq metres (about 215 sq ft).

The other type of circuit is the 30A radial which is basically the same as the 20A circuit, but it can carry more power to a larger floor area (50 sq metres/540 sq ft). The circuit is protected by a 30A fuse.

These restrictions on floor area mean that several radial circuits would have to be installed to cover the same area as a ring circuit. This is one of the reasons why the ‘ring’ is now the most common method of wiring in the UK, but radial circuits can supplement an overworked ring circuit.

Special purpose circuits

In addition to rings and radials, your home may have speciai circuits which supply only one outlet or appliance. Cookers, immersion heaters, instantaneous showers and the like are wired in this way and each has its own individual fuse. In effect, these circuits are just radials that have had both the cable and fuse sizes ‘beefed up’ to cope with the often heavy demands of the appliances they supply — for example, a large family-size cooker might need a 45A fuse, and 6mm2 or even 10mm2 cable.

Because electric night storage heaters all come on together they could overload a ring circuit; consequently each one is supplied by installation to contain a little bit of each. One is called the loop-in system; the other the junc-tion (or joint) box system.

With the loop-in system, a cable (normally 1.0mm2 but sometimes 1.5mm2) leaves a 5A fuse in the consumer unit (or fuse box) and is connected to the first in a series of special loop-in ceiling roses. From this rose, one cable

LIGHTING CIRCUITS a separate radial circuit protected by a 20A fuse. The fuses are housed in a separate consumer unit which is linked to a sealed time clock and uses off-peak electricity.

Lighting circuits

Two systems of wiring lighting circuits are in common use, and it is not unusual for an goes onto the next in the series, and another takes the power down to the switch controlling the light and back up through the light itself.

The junction box system uses the same dea but, instead of going from rose to rose, the cable from the consumer unit (or fuse box) passes through a series of junction boxes. From each box, one cable goes to the ceiling rose or light, and another to the switch that controls it. This system is particularly useful, for example, when fitting wall lights as there is little space at the back of a wall light fitting for looping-in.

Lighting circuits are rated at 5 amps, which means they can take a load of up to 1,200 watts. In effect, they could supply 12 lamp-holders containing bulbs of 100w each or smaller. But as you may want to fit bulbs with higher wattages, it is usual for a lighting circuit to supply up to eight outlet points, so separate circuits are required for each floor.

Strictly speaking it’s better to arrange the circuits so that there is more than one on each floor — this means that you won’t be in total darkness if a fuse in the consumer unit blows.

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