Home Electrics

Were all familiar with lights and power sockets, but how dose 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 including the metre belongs to the electricity board, and is their responsibility. Everything beyond is the householders 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 metre 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 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 floor, above the celling and may even be visible on wall surfaces, although more often they are buried within them.

In order 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 live and neutral metre tails. Alternatively the fuse boxes may be supplied from 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 a negative (-), then bring the fee ends of 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 alternating current, or AC for short.

But the principle is the same. Think of "live" as positive, "neutral" as negative, and you will seek 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 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 meter, then consequences would be serious. So, to protect them, a week link called a fuse is built into the circuit.

Most fuses are just thin pieces of wire. They can be fitted to rewirable fuse carries, 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 the terms of how much electricity - expressed in amps - is theoretically needed to melt it.

The word "theoretically" is important because, 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 own circuit in the wrong place - what's called a short circuit - and the bound together in PVC sheathing to form a cable. In this way, the live, neutral and earth wires can run as one even though each on is still connected up separately.

Different kinds of cables are used for different jobs.

Earthing

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 MBC 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 really up-to-date systems. 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 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 MBC.

Unfortunately this doesn't always happen - so, for added safety, a special device called a residual current circuit breaker (RCCB) 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 sigh of a fault. RCCBs 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 mains on/off switch.

Ring circuits

For getting electricity to the power points, the most common system of wiring is what called a 'ring' circuit. Wired in 2.5mm 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 sope 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 supply one single, double or triple socket, or one fixed appliance via a fused connection unit. Until a recent change in the IEE Wiring Regulations, a spur could feed two single sockets, and you may find such spurs on your existing circuits.

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 off power all switched on together. It's doubtful that 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 MBC will switch off.

Radial circuits

Unlike ring circuits consist of a single cable that leaves the fuse box and runs to one or more sockets. In order 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 form 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 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 2.5mm 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 are not exceeding 20 sq metres (about 215 sq ft).

The other type of circuit is the 30A radial which is wired in 4mm  cable and can feed a floor area of up to 50sq m ( 540 sq ft). It can be protected by a 30A cartridge fuse or MCB, but not by a rewirable 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 special circuits which supply only one outlet or appliance. Cookers, immersion heaters, instantaneous showers and the like are wired 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-sized cooker might need a 45A fuse, and 6mm or even 10mm cable.

Because electric night stroge heaters all come on together they could overload a ring circuit; consequently each one is supplied by a seperate radial circuit protected by a 20A fuse. The fuses are housed in a seperate consumer unit which is linked to a sealed time clock and uses off-peak electricity.