introduction to electrical principles unit 202: electrical principles and processes for building...

Introduction to Electrical Principles

Unit 202: Electrical principles and processes for building services engineering

Principles of electricity

• At its simplest, an electric current is a flow of electrons.

• In order for an electric current to flow in a simple circuit two requirements are

necessary.

• A source of chemical energy

• A continuous loop of a conducting metal which will allow transfer of that

energy.

2


• The source of chemical energy most

commonly used is the electric cell (a group of

cells forms a battery).

• Batteries are relatively safe because they

produce small amounts of electricity and

causes a one way electron flow.

• They are said to produce a direct current as

opposed to the alternating current.

3

Electricity

4


• The chemical reaction which takes place in the battery produces an excess

of electrons at one pole of the battery, and since electrons have a negative

electrical charge, this is the negative terminal or cathode.

• Similarly, there’s a net positive charge at the other pole of the battery, and

this is the positive terminal or anode.

5


6


• If the two terminals of the battery are connected to a continuous loop (or

circuit) of materials that will allow the transfer of those electrons (electrical

conductors), the difference in electrical potential between the two battery

terminals will cause the electrons to “flow” through the material, as a current

of electricity.

• This will flow from the negative (cathode) to the positive (anode) poles of the

battery.

7


8


• There are three important variables relating to electrical circuits:

– Current

– Voltage

– Resistance

9

Current

• The electric current in a circuit “the flow of electrons” can be measured as

the quantity of charge passing through any particular circuit in a given time.

• The unit of electrical charge is known as the coulomb, and when one

coulomb of charge flows in one second, the current is said to be one

ampere (or 1 amp or 1 A)

10

Current

• This can be shown by the formulae:

• Charge in coulombs (Q)= current in amperes (l) x time (t): Q=lt

• And current in amps (l) = charge flowing in coulombs (Q) per second (t):

l=Q/t

• What effects the flow of current in a simple electric circuit?

• For the current of electrons to flow at all there must be a difference in

electrical potential between different parts of the circuit.

11

Current

• The potential is measured in Volts, and a potential difference of one Volt (V)

between two points will allow one joule of work to be done per coulomb of

electric charge passing between the points.

• The voltage in a battery is therefore a measure of how much energy it can

provide.

• A 12 volt battery will therefore transfer twice as much energy (12 joules per

coulomb) as a 6 volt battery.

12

Current

• If an electric current can be represented by the rate of flow of water in a pipe,

the voltage would correspond to the water pressure.

• This, in a closed circuit such as a central heating system, would be governed

by the size and power of the water pump ( or in an electrical circuit by the

battery)

13

Current

• The second factor affecting the rate of flow of electrons in a circuit is the

resistance of materials in the circuit.

• Some materials, even though they allow the passage of electrons (i.e. they

are conductors of electricity) nevertheless they can slow down electron

transfer.

• Such materials are known as resistors, and their resistant properties is

measured in ohms.

• A resistance of one ohm will need a voltage of one volt to drive a current of

one amp through it.

14

Current

• The resistance of different materials can be compared to pipes of a different

diameter through which water must pass in a closed system, the smaller the

diameter of the pipe the greater the resistance to the flow.

• These relationships between current, volts, resistance is summarised in the

principle known as ohms law.

• This states that the current flowing in a circuit is proportional to the potential

difference (the voltage), providing the temperature of the conductor remains

constant.

15

Ohms law

• The formulae in ohms law can be written three different ways in order to

isolate each of the variables in turn.

Current (l) = voltage (V)

resistance (R)

Voltage (V) = current (l) x resistance (R)

= V=lR

Resistance (R) = voltage (V)

Current (l)

16

Ohms law

17

Fuses

• Fusing is a safety measure which aims to prevent high electrical current passing

through wires that are not designed to carry such high charges.

• This is important because if a current is too high for the wire it passes through,

overheating of the wire presents a serious risk of fire.

18

Fuses

• The various types of fuse that exist all

contain fuse wire, the fuse wire will melt or

blow if electric current above the specified

amount is passed through the wiring.

• Fuses come in various sizes to protect

against different levels of current.

19

Miniature circuit breaker

• Another form of circuit protection is the miniature circuit

breaker or (mcb).

• This device will trip a switch to break the electrical

current if an excessively high current is detected.

• These are far more accurate and more expensive than

fuses but they can be reset and are found in all new

domestic properties.

20

Traditional fuse type and mcb type consumer unit

21

Residual current device

• The Residual current device (rcd) this is a highly sensitive

device providing a high degree of protection to high risk

parts of electrical systems such as plug socket outlets and

electric showers.

• The rcd measures the difference between the different

electrical conductors in the system e.g. live and neutral

and measure changes in the electrical current, if a small

change occurs then the system is isolated.

22

Fuse rating

• Ensuring the appropriate size fuse

is used or fuse rating can be

worked out using this formulae:

• watts÷volts=amps

• Often fuses in houses are

overrated i.e. a lamp with a 100

watt bulb should be rated thus.

• 100÷240(volts) =0.426amps

• Therefore a 1 amp fuse would

be sufficient for use, though

usually a 3 amp fuse would be

fitted.

• If a 13 amp fuse is used it does

not give sufficient protection and

is unsafe.

23

Fuse rating

• There's a potential for confusion here because 1amp

of current is more than sufficient to kill at 240 volts.

• Remember the purpose of the fuse is to protect the

wiring.

• The earth is to protect you the user.

24

Circuits

• There are two very basic types of electrical circuits:

• Series circuits

• Parallel circuits

25

Series circuits

• This describes a system where the current flow is made to pass

through each component (e.g. a bulb) in a circuit.

• The current should be the same in any part of the circuit, but the

voltage will vary depending upon the resistance of each component.

• The total voltage of all components must not exceed the total

available voltage; otherwise the bulbs will not glow sufficiently.

26

Series circuit

27

Parallel circuit

• In a simple parallel circuit however there are alternative routes open to the

flow of electrons, and the current will flow along both.

• This results in a very different effect to a series circuit when two electric

lamps are connected in parallel.

• With this system, when a bulb blows the other stays alight, where as a

series system, when one bulb blows, it breaks the circuit all the bulbs go

out. E.g. lighting in a house is wired in parallel and a Christmas tree lights

are wired in series.

28

Parallel circuit

29

Direct and alternating current

• Direct current (d.c.), in a d.c. electrical circuit,

the electron flow is in the same direction all the

time.

• One example would be from cathode to anode

of a battery around a simple circuit.

30

Alternating current (a.c.)

• Alternating current is found in the majority of domestic properties, the usual rate

at socket level usually being 240V a.c.

• Within the alternating current, electrons travel continually back and forth. The

reason for this is a result of the way the electricity is produced.

• Alternating current is produced as a result of electromagnetism.

31


• All electrical current produces magnetic force: this is a basic fact that

underpins the creation of almost all the electricity used in today's world.

• The application of this fact was first demonstrated by Michael Faraday in the

1830’s who discovered that electricity could be generated by moving a

magnet in and out or around a coil of wire, which is wound around a soft iron

core.

32


• Electric generators at power stations still produce a.c. electricity on this

principle today.

• When a.c. electricity is used it’s essential that appliance be “earthed” as this

completes the formation of a circuit necessary for current flow.

• The way this works is that the current flows to an appliance from the phase

(live) wire and then from the neutral wire (which is in effect connected to

earth) the current flows continuously back and forth in the UK at a rate of 50

times a second (50 Hertz)

33

Recap

• Q. what is the unit for the measurement for electrical current?

• Q. the positive terminal on a battery is called what?

• Q. what three issues is ohms law concerned with?

• Q. What type of circuit protection is used on a new build circuit?

• Q. a.c. current is produced as a result of what?

34

Electricity supply and control

• At the end of this section you should be able to:

• State the main principles behind the generation and supply of electricity

• Explain the main features of domestic circuits.

35

Generation of electricity

• The principles of electricity generation were

discovered by Michael Faraday in 1831. He

found that moving a bar magnet through a

wire coil generated electricity. Modern

generators are more complex, but the

difference is mainly one of scale.

36

Generating electricity

• Power stations range in size from single wind driven devices to

major industrial sites, employing many hundreds of staff, but what

they are all doing is converting one kind of energy into another.

Different stations use a variety of energy sources but they all

generate electricity in the same way.

37


• Simplified to its essentials, a power station consists of just two major items.

First, there is a machine that generates electricity when its shaft is turned -

the generator. Secondly, there is some kind of engine to turn the shaft. The

generated voltage can be up to 25,000 volts, which is transformed to a higher

voltage for transmission on the grid.

• Generators need to turn fast and continuously, and the most efficient type of

engine for this is the turbine. In the United Kingdom, most power stations use

steam-driven turbines.

38


• In a power station generator, the equivalent of Faraday’s bar magnet is a

powerful electromagnet - a coil energised by direct current to produce a

magnetic field. This is mounted on the central rotating shaft, and is called the

rotor. Around the rotor is a series of coils called the stator, in which the

electrical voltage is generated by the rotating magnetic field. Both rotor and

stator may weigh several hundred tonnes.

39


40


• The rotor turns at 3000 revolutions per minute - 50 per second - to produce

alternating current with a frequency of 50 hertz (cycles per second). Modern

generators typically produce 500 megawatts of power, the largest generating

up to 700 megawatts - enough to light seven million 100 watt bulbs!

41

Distribution

• Electricity arrives in your area from the national supply network (the National

grid) at 275,000 or 400,000 volts. It is reduced to 132,000 volts at a substation

for distribution within each area of the country, travelling to further substations

known as grid supply points. From these it is distributed on overhead lines or

underground cables at 33,000 volts - the primary distribution networks - to the

intermediate substations.

42

Distribution

• At the intermediate substations, electricity at 33,000 volts is reduced to

11,000 volts for secondary distribution. The secondary distribution networks

then carry it at 11,000 volts to individual towns, industrial areas and groups

of villages.

• Particularly heavy users such as manufacturing industries are supplied at

33,000 volts. Electrified railways have their own substations which draw

electricity direct from the grid supply point - the latest overhead-line systems

run at 25,000 volts.

43

Distribution

• At the final substations, transformers reduce the 11,000 volt supply to 230 volts

for small scale customers such as homes and shops. A typical substation

serves 200 to 300 houses. Larger users such as farms take electricity at 415

volts.

44

Distribution

45

Distribution

• Upon entering the customers home you will

find the following.

• A sealed over current device that protects

the supply companies cable.

• An energy metering system to determine

the customers usage.

• This is then fed into the consumer unit.

46

Basic domestic circuits• The next section will show the final stage from production to consumer unit

to domestic output devices such as sockets and electric lighting.

• Lighting circuit: this is a radial circuit which feeds each overhead light or wall

in turn.

• To stop the light being on continuously the live or phase wire is passed

through a wall mounted switch, used by the property owner to turn lights on

and off at will.

• Two way switches are used usually on stairways and these require special

switch controls.

47

Basic domestic circuits• The lighting circuit is usually fed by

a 1.5mm2 twin and earth pvc

insulated cable and is protected by

a 6amp fuse or mcb at the

consumer unit.

• Commonly lighting is split into an

upstairs and downstairs circuit.

48

Ring main circuit – 13amp socket outlets

• The sockets you will see in domestic properties feeding televisions and

stereos will normally be 13 amp socket outlets fed from a continuous ring

circuit.

• As with the lighting circuit, cables circulate from the consumer unit round

each socket and then return to the consumer unit, hence the term ring

main.

• The ring main permits the cables to be kept to an optimum size as

electricity is permitted to flow in two directions to reach the socket.

49

Basic domestic circuits

• The ring main circuit is fed using a

2.5mm2 twin and earthed pvc cable

and is protected by a 32 amp mcb or

fuse.

50

Spur outlets

• Spur outlets are usually connected into a ring circuit on an existing system

(you would not usually encounter spurs on new installations) where its

inconvenient to place a socket from the ring main using two conventional

cables.

• The spur is connected to the ring main through a joint box, or is wired

directly from the back of an existing socket. Spurs can be either fused or

non fused.

51

Basic Lighting circuit

52

Basic Ring main circuit

53

Wiring to a ring circuit and spur

54

Recap

• Q. Who discovered the principles with regard to generating

electricity?

• Q. Name three fuel sources of capable of generating electricity?

• Q. What is the frequency electricity is brought into the home at?

• Q. What are the two types of circuit found in a domestic dwelling?

• Q. What size cable is used to feed a socket ring main?

55

What is earthing? • The earth, or ground in America, in electrical terms, carries no current, and it

is this that electricity will make a dash for when it is allowed to escape from its

secure home in an electric cable or flex.

• This is because one side of the electrical supply, the neutral, is intentionally

connected to earth.

• If someone touches a live conductor then a current will flow through the

person, their shoes, the floor, the wall, via earth and back to the supply

transformer via one or more earth connections of the transformer neutral.

56

What is earthing?

• The person has completed the electrical circuit. Should any fault develop in

an electrical system the electricity will always head for earth, taking the

easiest route there.

• The electrical appliances and supplies in the home are of a much higher

potential and if any of these become available to touch and are electrically

charged at a different voltage to earth the possibility of an electric shock

exists, with the current passing through the connection between the

charged parts and earth.

57

What is earthing?

• If, for instance, a person comes into contact with a conductive part

that is at a potential difference to earthed metalwork and that

metalwork; then a very serious shock can result

• In order to eliminate this possibility, all electrical earths of circuits

supplying equipment in the bathroom and all extraneous conductive

parts are bonded together.

58

What is earthing?

• In this way, even if a potential does develop, such as during an

earth fault on one of the electrical circuits, all the conductive parts

that someone could touch will be at substantially the same voltage.

• No dangerous shock current can then flow.

59

What is earthing?

• On its way to earth, leaking current may pass through walls, floors

or anything capable of carrying it.

• This is made much easier when the connecting substance is wet.

Water is an excellent conductor of electricity which is why special

care must be taken in the bathroom.

60

Metal pipework bonding

61

Earthing

• The bonding of all exposed metal components in a dwelling that are not

part of the electrical are known as equipotential bonding.

• The equipotential bonding conductor should be found close to the

consumer unit.

• In certain areas of domestic property supplementary bonding may be

required.

• Supplementary bonding is required to link sections of central heating or

cold water together as well as metallic surfaces such sink unit tops, steel

baths etc.

62

Earthing

• When maintenance processes are being undertaken and it is necessary to

remove a length a of metal pipework, its essential that the earth continuity

be maintained.

• This achieved by “bridging” the gap exposed by the removed section of

pipe with a temporary bonding wire.

• Its vital that the temporary bonding wire is securely in place before the

length of pipe is removed.

63

Temporary continuity bonding

64

Temporary continuity bonding

65

Earthing

• Earth clips should be used when

connecting bonding wire to

pipework.

• These are designed to clearly

inform the importance of the

connection and to show that it

ensures safe electrical connection.

66

Plastic pipework bonding

• Plastic is not a good conductor of

electricity and where the use of

plastic pipework systems occur the

earth continuity to components is

broken therefore provision for this

must be made.

67

Recap

• Q. Why is the earth cable so important?

• Q. What is the correct size cable required for equipotential bonding?

• Q. What is the correct size cable required for supplementary

bonding?

• Q. Why is it important to use temporary continuity bonding?

68

introduction to electrical principles unit 202: electrical principles and processes for building...

Documents