How The Electrical System Works

Electricity is brought into your home from the main supply by a service cable, which ends at a terminal box called the service head or sealing chamber. This box contains a fuse that protects the whole installation and allows engineers to switch off the supply when they need to carry out maintenance work. Close to this box is the meter. From the main supply to your meter, the installation is the property and responsibility of your electricity supplier and at no time should you interfere with it. Beyond the meter, the electrical installation is the responsibility of the consumer.

The heart of the consumer's installation is the consumer unit, or fuse box. This box houses the main switch that governs the entire electrical supply to your home and the fuseways, which protect the individual circuits in the house. It also incorporates the main isolating switch with which you can turn off the supply of power to the whole house.

The Earthing System

All of the individual earth conductors of the various circuits in your house are connected to one heavy cable in the consumer unit. This cable is sheathed in green, or in green and yellow, and it runs from your consumer unit to the consumer's earth terminal, which looks like a hexagonal bolt situated below the unit. In most houses in towns and cities, the earth cable continues from the earth terminal to a clamp on a metal sheath of the main service cable, just below the sealing chamber. Until recently, most electrical installations were

earthed to the cold water supply. This meant that the earth leakage current passed along the metal water pipes into the ground in which they were buried. However, with more and more water systems being replaced by non-metallic, non-conductive pipes, this type of earthing is no longer reliable.

You will find that your pipework is connected to the earth terminal just in case one of the live conductors in the house touches a pipe at some point. The same earth cable is usually clamped to a nearby gas pipe on the householder's side of the meter before running to the consumer's earth terminal. This means that both the water and gas pipes are cross-bonded so that the earth leakage current passing through either of them will run without being hindered on to the clamp on the cable service sheath and so to the earth. These clamps should never, under any circumstances, be interfered with or removed.

The PME (Protective Multiple Earth) System

In country areas, electricity suppliers often use a different method of earthing the system known as PME (Protective Multiple Earth). In this method, the earth leakage current is fed back to the electricity board's sub-station along a neutral return wire, and then to earth.

Regulations regarding the PME method of earthing are very stringent and cross bondings to gas or water services usually need to be larger. If you are in doubt about the type of earthing method used in your area, always check with your local electricity supplier.

The RCCB (Residual Current Circuit Breaker)

Although the electricity companies go to great lengths to provide earthing, it is in fact your responsibility as a consumer. This is often done by installing an RCCB (Residual Current Circuit Breaker) into the house circuitry. Under normal conditions, the current that flows out through the neutral conductor is exactly the same as the one flowing in through the live conductor. If there is an imbalance between the two caused by an earth leakage, the RCCB will detect it and isolate the system. An RCCB can be installed as a separate unit or it can be incorporated into the consumer unit with the main isolating switch.

The MCB (Miniature Circuit Breaker)

In some consumer units you will find an MCB (Miniature Circuit Breaker), sometimes called a 'trip switch', instead of fuse carriers. There are two differences between MCBs and fuse carriers: current ratings for MCBs tend to be slightly different from fuse ratings, and the MCBs automatically switch to the 'off' position so a faulty circuit is clearly visible when you inspect the consumer unit.

If your MCB 'trips' to off, turn off the consumer unit's main supply switch and close the switch on the MCB to reset it - there is no fuse to replace. If the MCB switch or button won't stay in the 'on' position when you turn the main power supply back on, then there is still a fault on the circuit, which must be rectified.

Ring Circuits

The most common form of 'power' circuit used to supply the socket outlets into which portable appiances are plugged is known as the ring circuit or 'ring main'. This method of wiring consists of a loop of cable 2.5 sq. mm in cross section, that starts at the consumer unit and goes around your house, connecting socket to socket and returning back to the same terminals. This allows power to travel in both directions to the socket outlets or fused connection outlets and reduces the load on the cable.

The ring circuit runs from a 30 amp fuseway or a 32 amp MCB. It can supply appliances up to a rating of 3 kilowatts: table lamps, TVs, washing machines, portable room heaters, personal computers. Anything above that rating must have its own circuit.

The advantage of the ring circuit is that only one size and type of plug is used no matter what the appliance. There is, however, a cartridge fuse inside the plug and this should be changed according to the amount of power the appliance takes.

In theory, there is no limit to the number of socket outlets or fused 13 AMP connection units that can be fitted to one ring circuit provided that it does not serve a floor area of more than 100m sq. (120 sq. yd). This limit is based on the number of heaters, which would be required to heat the space. In practice, two-storey homes usually have one ring circuit for the upper floor and one for the lower floor. You can increase the number of sockets on the ring circuit by adding extensions known as 'spurs'.

Radial Circuits

The radial power circuit feeds a number of sockets of FCUs (Fused Connection Units) but, unlike the ring circuit, the cable on the radial circuit terminates at the last outlet. FCUs are sockets to which applances are permanently wired and which have a fuse housed inside the unit. FCUs are useful for appliances that you don't want to keep unplugging - waste disposal units, freezers and fridges, or dishwashers - and have a switch and a neon light indicator to show when the current is flowing.

The size of the cable and the fuse rating depend on the size of the floor area to be supplied by the circuit: in an area of up to 20m sq. (24 sq. yd), the cable must be 2.5mm sq, protected by a 20 amp MCB or 20 amp fuse (which can be of any type). For areas up to 50m sq. m (60 sq. yd) you must use 4mm sq. cable with a 30-amp cartridge fuse or a 32-amp MCB (a re-wireable fuse is not permitted in this instance).

Any number of sockets or FCUs can be supplied from a radial circuit and spurs can be added if required. The circuits are called multi-outlet radial circuits. Powerful appliances such as electric cookers or showers must have their own radial circuit.

Lighting Circuits

Domestic lighting circuits are also radial circuits, but there are two types currently in use: the 'loop-in' system and the 'junction box' system. In many instances, lighting systems in the home are in fact a combination of the two. The loop-in system has a single cable that runs from ceiling rose to ceiling rose and ends at the last rose on the circuit. From the ceiling roses, single cables also run to the various light switches.

In the older junction box system, each light has its own junction box situated on the single supply cable. A cable runs from each junction box to the ceiling rose while another runs from the box to the light switch. One single circuit of 1mm sq. cable can serve the equivalent of twelve 100-watt light fittings - you can check the load by adding together all of the light bulbs on the circuit.

If you find that your load exceeds 1200 watts, then your circuit should be split to have two or more lighting circuits running from the consumer unit. If you have a very large house, needing long runs of cable, it is better to use 1.5mm sq. two-core-and-earth cable instead of 1mm sq. Lighting circuits also need to be protected by 5 amp fuses or 6 amp MCBs.

IEE Wiring Regulations

In the UK the rules regarding electrical installation are governed by the Institution of Electrical Engineers (IEE) and laid down in a document known as 'IEE Wiring Regulations'. If you follow its recommendations, you can be confident that your work will satisfy your local electricity board. If your work doesn't comply with the regulations, the board will refuse to connect you to the main supply. You can buy a copy of the IEE Wiring Regulations from the IEE itself or borrow one from your library.

The methods on this web site comply with IEE Wiring Regulations, but if you are planning on undertaking a job that is outside of the scope of this site, you will need to consult an original copy of the regulations. If you are unsure at any time about your competence, always ask for advice and help from a professional electrician. Always check that he or she is registered with the NICEIC (National Inspection Council for Electrical Installation Contracting). To be a member of this association, an electrician must be fully cognizant and must comply with the Regulations for Electrical Installations, the code of practice published by the Institute of Electrical Engineers.

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