Electricity

Electricity

In England and Wales, the Central Electricity Generating Board is re­sponsible for the power stations and the national grid. Twelve Area Elec­tricity Boards are responsible for the distribution networks and the supply and sale of electricity. In Scotland, there are two Boards each responsible for generation, transmission and dist­ribution within their areas; Northern Ireland also has a separate service.

The standard voltage for the dom­estic supply of electricity in Britain is 240V. (In Northern Ireland the supply is 230/240V.) Under normal conditions the voltage is constant – the permitted variation is plus or minus 6 per cent. Although excep­tionally heavy demand in cold weather may cause a regional drop, a drop is usually only of brief duration seen as dimming lights or a failure of the TV picture.

The supply

Electricity is supplied to most homes through an underground cable. In country districts, electricity is often distributed by overhead line on wood­en poles the live, neutral and earth conductors entering the house through a porcelain tube under the eaves.

The main supply cable goes to a sealed unit that holds the service fuse. The fuse is usually rated 60 or 100 amps, depending on the total house­hold load and is designed to blow if a serious fault occurs in a particular house, thus preventing the neigh­bourhood being affected. If this fuse fails, you have to call the emergency service of the Electricity Board (it is in the telephone directory, but it is a good idea to write the telephone number on the wall close to the meter – keep a torch there too). Check all the other fuses in the house first as it is rare for this fuse to ‘blow’ and check with neighbours that it is not a temporary power failure.

There is usually another terminal on the outside of the sealed unit to which the household earth connection is made. Adequate earthing is vital for the safety of any electrical installation and inside the consumer unit there is an earth connector block to which all the circuits in the house are connec­ted. This block can usually be connec­ted to the household earth terminal using 60mm² conductor wire pro­tected by a green-and-yellow striped sheath. The main earth connection is provided either by the outer metal sheath of the Electricity Board’s incoming cable or, where the system known as protective multiple earthing (PME) is used, by the neutral conduc­tor in a special mineral-insulated PME-type cable. Where the PME system is used, it is essential that all metal water pipes, gas pipes, central heating pipes and radiators and large metal objects, such as baths, are bonded to the main earth terminals.

Electricity is supplied to most homes through underground service cables and goes through the Boards service fuse and meter before It gets to the consumer unit (or fuse box) where the house circuits begin.

 

 

The meter

Two cables are taken from the service fuse to the meter which records the quantity of electricity used. Both the sealed fuse unit and the meter are the property of the Electricity Board and must never be tampered with. From the meter onwards, the electrical installation is your property and your responsibility.

For homes using off-peak elec­tricity, the meter works on two rates with the energy taken at a cheaper rate during the specified night hours.

The house fuses

The meter is connected to the house­hold installation by meter tails – hefty lengths of single-core cable. These belong to the householder but can be connected to, or disconnected from, the meter only by the Board. The meter tails run to one or more switches the mains switches.

In houses thirty or more years old it is still common to find two or more fuse boxes each with its own switch – sometimes in a separate box along­side. In modern homes the mains switch and the fuses are contained in a box called a consumer unit. This has rewirable fuses, cartridge fuses or miniature circuit breakers.

Rewirable fuses consist of plastic fuse holders with a length of fuse wire held in position by brass screws. New consumer units rarely have rewirable fuses and they are not allowed as the protection for some types of circuits —see page 51.

Cartridge fuses are also in a plastic holder but can be replaced like the car­tridge fuse in the 13 amp plug – you must keep spares handy. Cartridge fuses are better than rewirable ones. One of the advantages is that the fuse is made more precisely than a length of fuse wire and therefore blows at currents which can be pre­dicted more accurately. Also cartridge fuses of different ratings are different sizes so it isn’t possible to mend a fuse with one of the wrong rating.

Miniature circuit breakers look like ordinary switches or push buttons but they automatically flick themselves off if any circuit is overloaded, or if a fault in a piece of electrical equipment fails to blow the fuse in the plug. The circuit is brought back into use by resetting the switch. MCB’s give even more precise protection than car­tridge fuses. The ease of resetting is also a considerable advantage and means that an MCB can be used as a switch to turn off an individual cir­cuit. In some consumer units, MCB’s are interchangeable with cartridge fuse holders and you can opt for one or other or a mixture of both.

Modern fuse carriers are colour coded: 5A is white, 15A is blue, 20A is yellow, 30A is red, 45A is green. The job of fuse finding is easier if each fuse in the box is labelled with the cir­cuit, rooms or equipment it serves.

The circuits

From the fuseways in the consumer unit, cables form separate circuits supplying socket outlets, lights and fixed appliances. Flat twin and earth cable is used for most circuits. It contains two supply conductors, live and neutral, and an earth conductor inside a pvc outer sheath. The live is sleeved with red insulation and the neutral with black. The earth con­ductor is not insulated separately but should be protected by a green/yellow striped pvc sleeving wherever the cable sheath is stripped to make a con­nection – at a socket outlet for in­stance. Cables sometimes run in steel, aluminium or plastic tubes (called conduits) but modern practice is simply to bed cable in the walls with a protective plastic capping.

The last link to electric equipment is usually made in pvc-sheathed flex­ible cable. Flex must be sized to suit the current it will carry.

THE REGULATIONS

There are strict rules for elec­trical wiring. These – the Reg­ulations for Electrical Instal­lations are laid down by the Institution of Electrical Engi­neers. Known popularly as the Wiring Regs, they are not legally binding (except in Scotland where they are mandatory under the Building Regula­tions) but are followed by all re­sponsible electricians

Reference libraries generally keep a copy of the Wiring Regs. They are not easy reading for the layperson. The latest edition – the Fifteenth – was brought out in 1982; for a while it runs concurrently with the Four­teenth Edition which is gradu­ally being phased out.