Introduction to electric wiring

The purpose of electric wiring is to make available electrical energy whenever it is required: (1) With maximum safety. (2) With the capability of supplying the current for the usage required and for possible future extended usage. (3) With maximum reliability. (4) With maximum flexibility to provide for change in usage and extension. (5) Economically.

Terminology

Terms used throughout this post include the following: Wiring

The fixed installation of insulated electric cables between the intake point in a particular installation and the appliances that use the current, including the fuses, switches, socket-outlets, lampholders and all other parts permanently fixed in the building.

Appliances

All current-consuming apparatus, whether fixed, or portable.

Socket-outlet

A properly designed and permanently fixed device installed so that a portable appliance can be safely plugged in.

Area electricity board

In Britain, all electricity supplies are given by one or other of the twelve area electricity boards and equivalent authorities in Scotland. For the purposes of this post, the term ‘area board’ may be taken as referring to the electricity supply authority, whether it is a board, a power company, or any other body.

Series connection

A form of connection in which all the current passes through the circuits or appliances one after the other.

Parallel connection

A form of connection in which all current-consuming parts of the circuit are individually connected across the two wires providing the supply.

Power supply

The source of power

Electricity is generated in power stations where coal or oil is burnt to produce steam which turns a turbine coupled to an electrical generator. In Britain a number of nuclear power stations also providing power, the heat from the atomic reaction serving the same purpose as burning fuel in the furnace. In Britain there is a small proportion of power generated by falling water, driving a water wheel, or water turbine, but in some countries the majority of the power supply comes from this source.

The power is almost universally generated as alternating current, where the direction of flow of current changes fifty times a second. In America and in some other parts of the world, the frequency is 60 Hz. Alternating current is used in preference to direct current mainly for ease of transforming from high to low voltage, and vice versa.

Obviously, the more current that is needed the larger the conductor necessary at a given pressure. This can be understood by reference to a water pipe system. To fill a given tank in a given time, either a high-pressure hose, of small diameter, can be employed, or else a low pressure and a large diameter hose. It is the same with electricity, where the pressure is represented by the voltage, and the flow by the current. To carry power from a power station to the point of consumption, pefhaps fifty or one hundred miles away, overhead grid systems are used. It is impossible to increase the size of electrical conductor carried on pylons beyond a certain practical limit. The only way, then, to carry more power, is to increase the pressure, or voltage.

By the use of the transformer, which will be mentioned later in this post, alternating current can be transformed up or down in voltage, as required, and the voltage used for bulk transmission on the grid system is as high as 750,000 V.

This voltage is transformed down until it reaches the transformer at the end of one’s own street, or somewhere on an industrial or housing estate site, at a voltage of 11,000 V; and the final transformation, to feed the power into the cables connected to the consumer’s premises, is to a voltage of 415/240 V, 3-phase.

AH alternating current power is generated on the 3-phase system. This means that the part of the generator where the rotating magnetic field sets up the current we ultimately use is divided into three equal sectors. The three separate windings, in which the power is generated, are brought out by means of six wires, one at the end of each winding, and one end of each winding has the wires connected to a common point, known as the neutral. The other ends – the free ends – are the supply mains, usually known as phases, and for ease of identification are called the red, yellow and blue phases.

The 3-phase system continues all the way to the transformer near to the consumer’s premises, which we will call the local transformer, and on the low voltage side of this transformer we thus have four wires, the red, yellow and blue phases and the neutral. These wires are identified with red, yellow and blue markings, with black for neutral.

In the 3-phase system, two voltages exist. Between each phase and the neutral wire, on the low voltage side of the local transformer, the voltage is 240 V, the standard voltage in Britain. But between the red and yellow phases there is a voltage of 415 V, and 415 V also exists between the yellow and blue, and between the blue and red. This is the reason why the output voltage of the local transformer is given as 415/240 V.

The cables running out from the local transformer to the premises of the consumer are tapped off, for each house, by taking a connection, say for the first house in the street from the red phase wire in the 3-phase cable and from the neutral, thus giving a 240 V supply to that house; for the second house, from the yellow phase wire and the neutral; and for the third house from the blue phase wire and the neutral.

However, it is usual, where the demand in a particular consumer’s premises exceeds 15 kW, for a 3-phase supply to be given, because the load would be too great to be balanced out properly if connected only to a single phase. Therefore more and more consumers, including private houses, are being fed with a 3-phase supply, as the loads grow.

At the local transformer, the neutral connection of the low voltage side, that feeds the consumers, is connected to earth. But this does not mean that the neutral wire can be considered as safe. There are circumstances under which it could become alive, and although in general the neutral connection appears to be at the same voltage as the general mass of earth, it must at all times be treated as a live wire. The neutral wire is not the earth connection, to which reference will be made later, except in special circumstances, which will be mentioned later under protective multiple earthing.

The circuit

It is fundamental to electric current that there must be a circuit. This means that the current must be able to flow from the point where it originates, at the local transformer, on, say, the red phase terminal, through the supply cable to the consumer’s premises, through his wiring to the appliance, through the appliance, and back to the neutral connection and so back to the other end of the red phase wiring at the local transformer.

This is the fundamental point to be appreciated in all considerations of electrical wiring; there must be a circuit.

Fundamental considerations

Safety

Electricity is a good servant but a dangerous master. The lowest recorded voltage at which death occurred from an electric shock is 38V. In general, 240 V seldom kills a fully dressed person wearing dry footwear : but it is a fatal voltage for anyone wearing damp shoes, or perhaps with bare feet standing on a damp floor or touching earthed metal. The circuit, in this case, is from the live, or phase, conductor through the person’s body and back to the earth point where the neutral connection is earthed in the local transformer. Since the whole mass of earth including all the buried metal and so on usually has very little resistance to the passage of current, the full current that could flow from the live wire through the person is limited only by the resistance offered by the person’s body. Damp skin is a much better conductor than dry skin, and damp shoes offer very little insulation.

Anyone attempting to carry out electric wiring must at all times remember that he is dealing with a potentially lethal form of energy.

In Britain, it is still permissible for anyone to extend his or her electric wiring system, or to install new wiring, without special qualifications. In many countries this is not the case. It is a punishable offence, for example, in New Zealand, for anyone other than a registered and qualified electrician to install wiring of any kind.

However, if the work is undertaken with a full sense of responsibility, proper materials are used and proper methods employed, there is no reason why a safe installation should not result. But the person installing wiring should always have in the front of his mind the possibility that he might have to give evidence at a coroner’s court.

The safety of electrical appliances and wiring is ensured, basically, in three ways. First, by insulation; secondly, by earthing; and thirdly, by proper protection against fire risk.

Insulation

Insulation is the method whereby the live electric wires or other equipment are covered in such a way that it is impossible for anyone to come into contact with live metal. Wiring, for example, is made up of copper conductors covered with insulating material, and if the right type of cable is used, there can be no danger from touching the outside of the insulating covering.

Appliances of all kinds, if of proper design, have the live parts completely encased in porcelain or plastic materials so that it is impossible even for an inquiring child to insert its finger into any part that is live. Otherwise the live equipment is fitted inside a sealed part of the appliance, and access can only be obtained to it by deliberate interference.

Double insulation

Certain appliances are of what is known as the ‘double-insulated’ type. These appliances have first the normal functional insulation, as in all other appliances, and then a separate protective insulation enclosing all metal parts. Such appliances do not need an earth connection, but it must be borne in mind that no appliance can be considered as double insulated unless it complies with the Regulations and has been certified by the British Electrical Approvals Board.

Some designs of shavers, hair dryers, dishwashers, clocks, blankets and similar appliances have been certified as double insulated, and thus need only the phase and the neutral connections to the mains.

Earthing

Earthing is the second line of defence. The whole mass of earth is obviously safe from the electrical point of view. Therefore if. say, a kettle has a wire connecting the body of the kettle, which can be touched, to the earth, then whatever happens to the live wires inside the kettle heating element, or in the connector feeding the appliance with current, the user is safe because any current finding its way to the body of the appliance would be short-circuited straight to earth.

Metal parts of any kind of appliance in which electricity is used should always be earthed. With portable appliances, this is carried out, as will be seen later, by using a 3-core flexible cable and a proper plug and socket-outlet, the third pin of which, the earth pin is properly connected to the earth. This is done by connecting the earth terminal in the socket-outlet on the wall to a proper earth point, which may be earthed to the lead sheathing of the supply cables at the consumer’s terminal point, or may be a special earth prepared properly and provided for this purpose.

At the appliance end, the connector or the cable termination must be so arranged that every metal part of the appliance is properly connected to a terminal to which the third wire in the flexible – green and yellow – is connected, so that whatever leakage of current might take place, in whatever part of the appliance, the current will flow harmlessly to the earth point, through the green and yellow insulated protective conductor into the plug and to earth via the proper earth point.

Protection against fire risk

Protection against fire risk is secured by using properly dimensioned cables and fittings, and by protecting the circuits by means of the correct sizes and types of fuses or circuit-breakers.

Properly designed wiring

As well as the danger of shock, the supply of electric power from a large power station brings with it another danger.

When current passes through a wire, there is a certain resistance in the wire to be overcome. In overcoming this resistance, heat is generated in the wire. In the ordinary open-type electric radiator, this heating effect is usefully employed to give the heat we require from the radiator.

But heat is also being generated in the wiring itself. If the wiring is properly proportioned, the heating effect is small, but if the wire is too small, two problems arise. First, in extreme cases there is danger of fire from the wiring becoming too hot, and setting fire to adjacent materials with subsequent damage to the cable insulation, and secondly some of the voltage in the supply mains will be lost in the cable itself, and the appliance will not receive its full voltage, and thus, for example, lights may be dimmer than they should be.

This question of the proper dimensioning of the wiring installation in relation to the load to be fed with current applies not only to the wiring itself but to all the appliances used. Even the ordinary switch found on the wall of a living room must be suitable for its duty. Poorly designed switches, or switches that have become worn out, cannot carry the current they were intended to carry, or the increased current that wiring extensions have made possible for them to carry, mainly because the contact parts within the switch were never large enough, or have become twisted or bent or overheated so that they no longer make good contact. In consequence, there is too high a resistance within the switch, further heat is generated, and fire may result. This factor also applies to fuses, to junction boxes and to any part of the installation such as lampholders. There is a special aspect of the fire risk to be borne in mind when lampholders are considered.

People are becoming accustomed to higher and higher levels of lighting than were accepted in the past, and they have tended to add larger and larger lamps to existing lampholders. In addition, mushroom-shaped lamps are now available which give a high wattage, or power consumption, in a small volume, and lighting fittings are being made to accommodate lamps of a greater power than those for which they were designed.

A lamp gives out almost all its power in the form of heat. The overheating which may take place when oversized lamps are used may give rise to serious consequences, because the flexible or other wiring connected to the lampholder may become overheated to the stage where its insulation is damaged, and a short circuit may occur, which at the least may black out a number of lights, or at worst give rise to a fire.

Protection of the installation

The most commonly used type of protection is the fuse. A fuse is like a weak link in a chain, carefully designed to break when the maximum permissible load is exceeded, so that the crane, for example, to which a chain might be connected cannot be overloaded and perhaps overturned.

In the electric circuit, a fuse consists usually of a fine wire which has been carefully selected so that, when the maximum current which the circuit should carry is exceeded, the wire melts and effectively switches off the current, so saving the wiring itself and all the appliances on the circuit from damage.

Fuses are usually contained in fuseholders and installed in a consumer unit, in such a way that the designed overheating of the fuse and its ultimate rupture by melting cannot give rise to any fire risk or other serious consequences.

The circuits in the installation must be fused according to the Regulations mentioned later, and care must be taken to see that a blown fuse is replaced with a fuse of the proper type for the duty required, and not with a larger type which would invalidate the protection it gives to the circuit.

It is extremely unwise to repair a fuse that has blown without finding out why it blew. It might well happen that the danger still exists, for example if a flexible cord is frayed through it may have shorted its two conductors together sufficiently to have blown the fuse, and then left the bare copper exposed, so that a shock could result if the fuse is replaced. The faulty part of the installation should either be repaired or temporarily isolated, before replacing the fuse that has blown.

The miniature circuit breaker, or MCB as it is often referred to, may be used to protect a circuit. The MCB is a switch which operates automatically in the event of an overload or short circuit. It has the advantage that once installed, with the correct current rating for the circuit, its rating cannot be changed.

Reliability

Since most of us rely entirely on our electrical systems, anyone installing any kind of wiring must be careful to ensure that the wiring is absolutely reliable. This means that not only must it be properly designed but that it must be laid out in the building in such a way that it is not likely to be subjected to casual damage, and that any failure of wiring, or of appliances connected to any particular circuit, must not be allowed to give rise to wholesale blackouts and failures of supply in other parts of the building.

Flexibility

Wiring requirements are constantly changing. With the addition of more and more domestic appliances, and more and more electrical services in offices, while in small factories and workshops additional electrical appliances are constantly being added, thought should be given to planning the installation so that it does not become overloaded. This may be effected by providing for growth of the installation by allocation of additional fuses etc.

Cost

There are many people who naturally feel that wiring must always be carried out as cheaply as possible. It is safe to say that good wiring pays for itself in peace of mind and in ease and convenience of usage. Money skimped on a wiring installation is money unwisely skimped. The minimum cost consideration should never be in the forefront of the mind of anyone planning or executing a wiring installation.

The electrician’s responsibility

In every installation, the area electricity board or the electricity supply company or authority provides a service terminal point, on which all parts are sealed. This usually consists of a main fuse, or cut-out, and a meter. Some consumers take advantage of off-peak current, with the supply to the water-heating or space-heating circuits being restricted to the off-peak hours that occur during the night and sometimes during the afternoon period. If this type of usage is contemplated the area electricity board will provide, in addition, a time switch to adjust the hours of usage of the off-peak current, and a separate meter.

All these appliances are the property of the area board, and must not be interfered with in any way by the electrician. To break the seal on the main fuses or any part of the metering circuit is to invite prosecution.

After the meter, the responsibility for the installation lies entirely with the consumer, and if it is a new installation, the electrician is therefore responsible for handing over to the consumer a proper wiring installation, which the electricity board will test, and if found in good order will connect up to the meters.

The electrician’s bible

The main guidelines for electric wiring are the Regulations of the Institution of Electrical Engineers, known as the Regulations for Electrical Installations, and available from the Institution. These Regulations are constantly being amended, so that the user must make certain of providing himself with the latest copy.

These Regulations, although they are not part of the law of the land, are very nearly in the same category. For example, an insurance company, if it is asked to insure a building, will nearly always specify that the electric wiring must be in conformity with the practice laid down in the Regulations of the Institution. In cases where accidents occur, such as fires or electrical shocks, the best possible defence, on the part of the person who installed the wiring, is that it is in conformity with the Regulations. One would have a very poor defence indeed if the wiring did not conform to the Regulations.

Tests by electricity board

In addition, the area electricity board has the right to refuse to connect up an installation which it considers to be unsafe, and such an installation obviously does not comply with the Regulations. The tests the board’s engineers will apply will be directed to ensuring that the Regulations have been properly carried out. The electricity board also has the right to be notified when any alteration or addition is made to an installation, and may inspect the altered or added parts before they are connected to the main system.

Some definitions

Voltage

The pressure that forces the current round an electric circuit is measured in volts. The normal domestic supply is at 240 V: a flashlamp bulb works at 1.5 V: most of the National Grid System works at 275,000 V, parts at 400,000 V. The voltage is equivalent to the head of water causing a flow along a pipe. The following voltage levels are defined:

Extra-low voltage: below 50V between conductors or to earth.

Low voltage: exceeding extra-low voltage but not exceeding 1000V between conductors, or 600V between any conductor and earth.

The electrician, dealing with domestic and small industrial installations, will therefore deal mainly with low-voltage supplies.

Current

The flow, or current, in a conductor is measured in amperes. A lkW fire needs a flow of about 4.16 A: a 100 W lamp needs about 0.4 A.

Resistance

When water flows through a thin pipe, it encounters a resistance to its flow. The larger the pipe, the less resistance. Also pipes differ, in resistance to flow, even at the same diameter. A smooth-bore pipe will offer less resistance than a rusted bore.

Similarly, with conductors of electricity, the thicker the wire, in general the less the resistance. Certain wires, like copper, silver and aluminium, have less resistance than similar-sized wires of steel or nickel alloy.

The effect of resistance

The effect of resistance in a conductor is to generate heat as the current passes, as mentioned earlier. In radiator elements, this is the desired effect, and the conductors used in these elements are designed to give a suitable resistance to generate the required amount of heat.

But in the conductors used for wiring a house, the aim should always be to reduce this heating effect to the minimum. Heat is not required in the wiring system: it will reduce the life of the insulation, and if excessive may even cause a fire.

Therefore care must always be taken to reduce the heating effect to the minimum by ensuring that all conductors used – the cables themselves, and all the fittings of every kind – are large enough to present a very low resistance to the current for which the circuit is to-be used.

Resistance is measured in ohms. As an example, a 100 W lamp has a resistance of about 600 Q.

The resistance of the insulation used on electrical appliances is of course very high indeed – otherwise it would not be regarded as insulation. To give a typical example, the resistance of the insulation used in a good, new electric iron, measured between the live conductors in the element and the outer metal case of the iron, ought to be approximately 2,000,000 Q known as 2 megohms.

The resistance of all parts of an electrical installation in a domestic dwelling – that is, the resistance of all the live conductors in the cables, the switches, the socket-outlets, and the consumer unit – measured against earth, must not be less than 1,000,000 Q.

Later we shall see how this insulation resistance is tested.

Two simple formulae

There are two very simple and fundamental formulae that must be understood in relation to all electric circuits.

The ability of an appliance of any kind to consume electricity is measured in watts. A thousand watts equals one kilowatt.

An appliance that is so designed that its resistance allows 1A of current to flow when the appliance is connected to a voltage of 1V is said to have a power rating of 1W.

Therefore power = current x voltage

As an example, a one-bar radiator with an element of 1000 W rating would take current of 4.16 A if connected to a 240 V circuit: power = current x voltage 1000 = 4.16 x

Often it is desired to know what current will be needed to feed a These simple but fundamental formulae apply to direct current and ordinary alternating current circuits in which the appliances used – radiators, immersion heaters, filament lamps, and the like – are mainly users of electricity by means of resistance wires that grow hot when current flows. When appliances that use motors and such devices as the electromagnetic choke coils in fluorescent lamps are considered, the formulae have to be modified to take into account the electromagnetic effects of the current: but the modifications needed for domestic and similar installations are usually so small that they can be ignored. They only become significant where larger industrial installations are concerned.

The unit of electricity

An appliance – say, a one-bar electric fire – is designed so that when it is connected to the usual 240 V mains, it will use 1000 W, that is 1 kW.

If such a 1 kW fire is switched on for exactly 1 hour, it will draw 1 kilowatt-hour of energy from the mains.

One kilowatt-hour is the standard unit of electricity consumption.

You can use one unit of electricity by: burning a 1 kW fire for 1 hour, burning a VikW fire for 2 hours, burning a 2kW fire for Yi hour.

To take another example, a 25 W lamp used for 1 hour: 25 W x 1 hour = 25 Wh = Vtoth of 1000 Wh = Vfoth of a unit or, 25 W for 40 hours = 25 x 40 Wh = 1000 Wh = 1 unit.

One further example: a cooker hotplate has a capacity, or loading, of 2kW. 2kW for 1 hour = 2kWh = 2 units.

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