Electrical and
Electronic Technology
for CSEC®
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John Blaus
Vernon Daniel
Wilbert Nunes
Bachan Ramdhan
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Contents
Introduction ������������������������������������������ 1
4. Electrical installations .................... 62
1. Electrical principles and
measurements���������������������������������� 2
4�1 Introduction �������������������������� 62
1�1 What is electricity? ����������������� 2
4�2 Circuits �������������������������������� 64
1�2 Separating charges ����������������� 5
4�3 Wiring methods and
terminations ������������������������� 70
1�3 Electric current ����������������������� 6
4�4 Lighting circuits �������������������� 74
1�4 Basic laws of
electromagnetism ������������������� 9
4�5 Lamps ��������������������������������� 78
1�5 Ohm’s law ���������������������������� 14
4�6 Conduits ������������������������������ 82
1�6 Resistivity ���������������������������� 21
4�7 Testing and
commissioning ��������������������� 90
1�7 Circuit protective devices������� 24
4�8 Fault diagnosis ��������������������� 94
1�8 Principles of a�c� and
d�c� current flow �������������������� 29
4�9 Fuses and circuit
breakers ���������������������������� 102
1�9 Resistance, inductance,
and capacitance ������������������� 35
5. Electronics ................................... 104
1�10 Cells and batteries ������������� 44
2. Electrical and electronic
drafting .......................................... 45
2�1 Drawings and circuit
diagrams ������������������������������ 45
2�2 Electrical symbols ����������������� 47
2�3 Scale drawings ��������������������� 51
3. Electrical power, power and
machines ....................................... 52
3�1 How is electricity produced? �� 52
5�1 Resistors ��������������������������� 104
5�2 Capacitors ������������������������� 108
5�3 Semi-conductor devices������� 111
5�4 Rectification ����������������������� 114
5�5 Transistors ������������������������� 116
5�6 Integrated circuits ��������������� 119
5�7 Thyristors ��������������������������� 119
Glossary ������������������������������������������� 124
Index ����������������������������������������������� 125
3�2 Transformers ������������������������ 56
Access your support website for additional content and activities here:
www.oxfordsecondary.com/9780198395478
ii
Introduction
Electrical and Electronic Technology for CSEC® A CXC Study Guide has
been developed by experienced teachers and examiners, working closely with
the Caribbean Examinations Council (CXC®) and focuses on the development
of competencies. It concentrates on the areas of the syllabus that are most
challenging to learn and are considered essential to the development of skills
required by the programme and entry into the world of work.
The content has been constructed around workshop investigations to support the
process of relating theory to practice and practice to theory. These are designed
to align with the school-based assessment and allow students to review progress
effectively. This gives the student a positive role in managing their own learning.
Additionally, there are opportunities for students to use reflective techniques to
identify what went well, what might have been done more effectively and how
similar activities might be approached in the future (skill transfer).
The study guide and associated activities support a range of pedagogy to make
learning engaging, interactive and efficient, leading to a deeper understanding.
The range of pedagogy includes:
1. Assessment of and for learning
2. Cooperative learning
3. Differentiation
4. Embedding language, literacy and numeracy
5. Experiential learning
6. Learning conversations
7. Relating theory and practice
8. Using e-learning and technology.
Remember, where applicable, candidates who successfully complete the CSEC
examinations in the Technical Syllabuses will receive two awards: the CSEC
Technical Proficiency Certificate and a CVQ* (Caribbean Vocational Qualification)
Statement of Competence.
We are confident that this book will provide students with the skills to succeed in
their course of study and beyond.
* CVQ is the Registered Trademark of the Caribbean Association of National Training Authorities (CANTA).
1
1. Electrical principles
and measurements
1.1 What is electricity?
Can you imagine life without lighting, heating, computers, televisions, fridges, and air
conditioning? The development of electricity as a useful supply of energy in the home
is relatively recent. It is credited principally to Thomas Edison, the inventor of the first
efficient light bulb in the 1870s.
Energy can be converted from one form to another, but cannot be created or destroyed.
Electrical energy (electricity) coming through a cable to a television makes it work – the
energy is converted into light, sound (and heat).
Electricity is produced by converting the energy in one of three ways:
●
From chemical energy, e.g. batteries
●
From magnetic and kinetic energy, e.g. wind turbines, power stations which use
various forms of fuel (wood, coal, gas, oil, nuclear, tidal)
●
From the Sun (solar), e.g. converting energy
from the Sun to charge a battery, or to feed into
the main power grid.
Gas
Every known substance is composed of
particles – atoms or molecules, which are held
together by chemical bonds. Atoms and molecules
Solid
are always in a state of rapid motion. In a solid
they are densely packed together and movement is
restricted. In a liquid the particles are less tightly
Liquid
bound, and there is more free movement. In a gas,
the particle movement is almost unrestricted, the
Figure 1.1 The three states of matter
substance can expand and contract in any direction.
Atoms consist of even smaller particles, called sub-atomic particles. At the centre of each
atom is the nucleus, which is made up of protons and neutrons. Protons possess a
positive charge, and neutrons have no electrical charge. Electrons circle the nucleus and
possess a negative charge. All atoms possess equal numbers of protons and electrons,
so the atom is electrically neutral.
–
–
Orbiting
electrons
–
–
+
+
+
+
+ +
+ +
+ +
–
–
Neutrons
–
Protons –
Nucleus
–
–
Figure 1.2 Sub-atomic particles (this shows an atom of neon)
2
The simplest atom is that of hydrogen which has one
proton and one electron, whereas aluminium has 13
protons and 13 electrons. Electrons orbit the nucleus in
shells at varying distances. Those nearest the nucleus
are held in place more strongly than those furthest away.
These distant electrons are easily moved from their orbits
and so are free to join those of another atom, whose
own distant electrons may in turn leave to join another
atom, and so on. Movement of these ‘free’ electrons is
electricity. Atoms or molecules that have gained or lost
electrons are called ions.
Proton
Electron
A material that allows the movement of free electrons is a
conductor, and one that doesn’t is an insulator.
Metals are made up from a lattice of positive ions
surrounded by a sea of free electrons which can move
about. This means that it is easy for charges to move
freely through metals, so they are good conductors of
electricity, e.g. copper, aluminium, tungsten.
Al
Most other solid materials do not have free electrons or
other charge carriers and so it is difficult for a current to
flow. These materials are called insulators, e.g. rubber,
plastic, wood.
Figure 1.3 A hydrogen atom
(top) and an aluminium atom
Some materials lie between the two extremes of
conductivity and can conduct current under certain
conditions. These are known as semi-conductors. Silicon
is a widely used semi-conductor which will only conduct
current when another substance such as germanium is
added to the silicon.
Iron
Copper
Conductors
Germanium
Silicon
Semi-conductors
Glass
Diamond
Fused silica
Insulators
Conductivity
Figure 1.4 Conductivity
Conductivity is the ability of a material to carry electrical charge. Put
simply, if a material can carry electrical charge easily, then this material is
considered to have good conductivity, for example, copper and aluminium.
However, materials with a poor conductivity do not carry electrical charge
easily, for example, wood, glass and plastic products.
3
Why don’t you?
Test a range of materials to see
whether they are conductors or
insulators. You can use a simple
lamp or an ammeter.
Place the material in the circuit
to find out if it can conduct electricity
Figure 1.5 A simple conduction test
Electricity – movement of free electrons.
Atom – the smallest part of an element that can take part in a chemical change.
Molecule – a group of atoms bonded together.
Key terms
Nucleus – the positively charged central core of an atom made up of protons
and neutrons.
Proton – a stable sub-atomic particle occurring in all atoms with a positive
charge.
Neutron – a stable sub-atomic particle with no charge.
Electron – a stable sub-atomic particle with a negative charge.
Ions – atoms or molecules that have gained or lost electrons.
Conductor – a material that conducts electricity.
Insulator – a material that doesn’t readily conduct electricity.
Semi-conductor – a material that can conduct electricity under certain
conditions.
Questions
For each of the following questions choose the correct answer.
1
2
3
4
At the centre of each atom is the nucleus. This is made up from:
a) Electrons
b) Protons
c) Molecules
d) Free electrons
Which of the following is not a good conductor?
a) Tungsten
b) Aluminium
c) Brass
d) Magnesium oxide
Which of the following is not a good insulator?
a) Mercury
b) Impregnated paper
c) Mica
d) Rigid plastics
1.2 Separating charges
ro
d
Electrons
gained
by cloth
Pe
rsp
ex
Po
ly
th
en
er
od
Most materials are
Atoms
become
uncharged because they are
positive as
Atoms
a result of
composed of neutral atoms.
become
losing electrons
positive as
When some materials
Electrons gained
a result of
by rod
are rubbed together, the
losing electrons
––
–
frictional forces can cause
–– + +
+
+ 1+
–
++
electrons to be transferred
+
– –
++
+
+
–
from one material to another.
+
+
– –
Because the protons are
–
Dry cloth
tightly bound in the nucleus,
Dry cloth
they cannot move from place
to place. This transfer of
Figure 1.6 Charging a plastic rod
electrons results in both
of the materials becoming
electrically charged.
●
When extra electrons enter an object, the object becomes negatively charged.
●
When electrons leave an object, the object becomes positively charged.
Rubbing a polythene rod with a dry cloth will cause electrons to move from the cloth to
the polythene rod. The rod will become negatively charged because it has extra electrons,
and the cloth will now become positively charged as it has fewer electrons than protons.
Did you
know?
If a Perspex rod is rubbed, electrons will move from the rod to the cloth, leaving the rod
positively charged and the cloth negatively charged.
Charged particles produce a force on each other in a similar way to magnets:
●
Opposite charges attract each other.
●
Similar charges repel each other.
Static electricity
Key
term
Why don’t
you?
Have you ever experienced a small electric shock when touching the handrail of a moving
escalator? Have you ever rubbed a balloon against you and then stuck it on a wall? Have
you noticed how particles of dust are attracted to computer monitors and TV screens?
Static electricity causes this. It is a type of induced charge.
Investigate the effect of induced charge by cutting a sheet of paper into small
pieces and placing on the desk. Charge up a plastic rod using a cloth and place
the cloth near the pieces.
●
What happens to the pieces of paper?
●
Explain why the charged rod affects the uncharged pieces of paper.
Static electricity – a build-up of charge on the surface of an object. The charge
remains until it is discharged, for example, by touching the object. This can
cause a mild electric shock.
5
1.3 Electric current
Table 1.1 Terminologies relating to electrical measurement and quantities
Measured
in units
Unit
symbol
I
This is the flow of electrons, or
the flow of charge. The rate of
this flow of charge is measured in
amperes
ampere
A
Charge
The quantity of electricity. One
coulomb of charge is conveyed in
one second by a current of one
amp
Q
coulomb
C
Potential
difference
One volt is the energy transferred
by one coulomb as it passes
between two points in a circuit
V
volt
V
Electromotive
force (e.m.f.)
The voltage produced by an
electrical power source, e.g. an
e.m.f. of 9 V for a battery or an
e.m.f. of 115 V for a mains supply
E
volt
V
Power
The rate at which energy is
transferred by electrically
powered items, e.g. a light bulb
may be 60 W, a fridge may be
200 W
P
watt
W
Energy
This is the ability to perform work
or to move or change things
W or E
joule
J
Resistance
The level of opposition to the flow R
of current in a circuit
ohm
Ω
Capacitance
The ability to store electric charge C
microfarad
μF
nanofarad
nF
picofarad
pF
Name
Description
Current
Symbol
Inductance
The opposition created by a
changing current in a magnetic
field which induces a reverse
voltage
L
henry
H
Frequency
The rate at which alternating
current (a.c.) completes a cycle
f
hertz
Hz
6
Electric current
The flow of electric charge is a current. This flow of charge is caused by a potential
difference between two points of a circuit. The current is driven through the circuit by the
electromotive force (e.m.f.).
The size of a current is the rate of flow of charge, and this is measured in a unit called
the ampere (A). The relationship is normally written as:
Charge transferred = current × time
Did you
know?
Worked example
Q=I×t
What is the current in a wire if a charge of 300 C is transferred in 30 seconds?
Q=I×t
Q
I=
t
300
I=
30
I = 10 A
The symbol for current is I, which related to the intensity of electron flow and is
measured in amperes (A).
Current in circuits
Electrical charges move round circuits made up of metal wires and components. Copper
or aluminium are usually used for these as they are both good electrical conductors. This
current is not lost or used up as it travels around the circuit, but is transformed to power
electrical devices. (However, a small amount of energy is lost as heat.)
Some examples of electrical transformations include:
Electrical motors transform electrical energy into kinetic energy, allowing objects to move.
Electricity can be converted into light using filament light bulbs, LEDs or fluorescent tubes.
Conventional current is described in terms of a flow
of positive charge. In circuits the energy is actually
carried by a flow of negative charge. This means
that the particles carrying charge are moving in the
opposite direction to the conventional current.
The current in a metal wire is caused by a flow
of electrons from a negative terminal to a positive
terminal. Conventional current is a flow of positive
charge from a positive terminal to a negative
terminal.
Direction of conventional
current
Battery +
Did you know?
Television sets convert electrical energy to sound and images.
Direction of
electron
flow
Figure 1.7 Flow of charge
7
The causes and effects of electric current
To use electrical power we need a current to be pushed by the electromotive force
around a conductive circuit.
Table 1.2 The main sources of an electromotive force
Chemical
An example of this is a battery. A battery is made up from two different
metal plates and an electrolyte. Electrons from one metal plate will flow
through the electrolyte to the other plate. This causes an imbalance in
the two plates: one will be positive, one will be negative. This reaction
causes the electrons to flow and produce a current. Batteries can be
used to run small hand-held devices, e.g. a torch.
Thermal
(heating or
lighting)
This occurs when electrons flow between two different metals at different
temperatures. This arrangement is known as a thermocouple and is used
in appliances which heat up to extreme temperatures, e.g. in an oven or
a furnace.
Magnetic
A magnetic field can be used to generate a flow of electrons. We call this
situation electromagnetic induction. If a conductor is moved through a
magnetic field, then an e.m.f. will be induced in it. Provided that a closed
circuit exists, this e.m.f. will then cause an electric current to flow. An
example of this would be an alternator or generator.
Potential difference is the cause of the movement of charge. A charged particle will move
to a position where it will have less potential energy and so will move to a lower point
in an electric field. The reference point that potential is measured against is the Earth
which is taken as zero potential or 0 V.
A voltage of 115 V means a potential of 115 V above 0 or Earth.
Table 1.3 The effects of electricity
Chemical effect
Did you
know?
Cells
Batteries
Electroplating
8
Thermal (heating/
lighting) effect
Light bulb
Fan heater
Cooker
Iron
Kettle
Magnetic effect
Motors
Transformers
Circuit breakers
Bell
Electromotive force (e.m.f.) is the total force measured in volts causing a
potential difference between two points, thus causing a flow of electrons.
A current cannot exist without a potential difference.
The bigger the difference, the bigger the flow. So a 115 V mains supply has a
greater flow than a 9 V cell battery.
1.4 Basic laws of electromagnetism
Magnetism is hard to define – you may
know about the attraction or repulsion
of one material by another material, but
why does this happen? Why do you only
see it in some materials, particularly
iron? Materials that are attracted by
a magnet such as iron, steel, nickel
and cobalt have the ability to become
magnetised. These are called magnetic
materials.
A magnetic field is a region of space
in which a magnetic material will
experience a force. The shape of a
magnetic field is not as simple as that
of an electric field, as there are always
two poles involved.
Lines of force
N
S
Figure 1.8 The poles of a magnet
Why don’t you?
Place a bar magnet under a sheet of paper and sprinkle iron filings on the
paper. These filings will align with the field lines and the shape of the field will
be clearly seen. What happens if you place a second magnet under the paper?
Figure 1.9 Field lines shown with iron filings
There are two types of magnets – the permanent magnet and the electromagnet. A
permanent magnet is a material that displays a magnetic field of its own. Examples of
permanent magnets are the bar magnet and the horseshoe magnet.
9
Why don’t you?
An electromagnet is a temporary magnet as the magnetic field will only be present whilst
there is a current flowing through a coil of wire wound around it. If current is moving
along a wire, a field of magnetic flux will be created. The direction of the field will depend
upon the direction of the current. Examples of electromagnets include doorbells and
relays. An example of a relay is used in cars, where a small current from the ignition
circuit is used to turn on a larger current in the starter motor.
Using your right hand, imagine curling
your fingers around the wire while sticking
your thumb out in the direction of the
conventional current. The curved direction
of your fingers shows you which way the
magnetic field loops.
Field
nt
Curre
Did you know?
Figure 1.10 The right-hand grip rule
●
Opposite poles attract each other: north poles attract south poles.
●
Like poles repel each other: north poles repel north poles, and south poles
repel south poles.
●
The closer the magnets are to each other, the stronger the force (flux)
between the magnets.
●
The lines of flux flow from the north pole of the magnet to its south pole.
●
The lines of flux never cross each other.
●
Magnetic materials placed near either pole of a magnet will be attracted
towards it.
Direction of screw travel
Direction
of turning
The corkscrew rule
The corkscrew rule helps us to understand the
relationship between current flow and magnetic fields.
If we think of a normal right-hand threaded screw, the
rotation of the tip represents the direction of the current
through a straight conductor. The direction of rotation
of the screw represents the direction of rotation of the
magnetic field.
Current
flow
Magnetic flux
Figure 1.11 The corkscrew
If a wire is formed into a coil, and a current is passed
rule
through it, it becomes an electromagnet, otherwise
known as a solenoid. Figure 1.12 is a section through a
coil, with the arrows showing the direction of current flow for
different magnetic fields.
Because the turns of the coil would be wound close together,
all the individual magnetic fields shown in Figure 1.12 would
Figure 1.12 Direction of
combine to form one overall large magnetic field. In order
current for different fields
to make the electromagnet stronger, an iron core may be
10
Why don’t you?
fitted in the centre of the coil. This helps to
concentrate the flux here. This is the principle
used to produce magnetic fields in relays,
contactors and motors, etc.
Build your own electromagnet using
a large iron nail, a low voltage power
supply (a 9 V battery), and a length of
insulated wire. Wrap the wire around
the nail in a tight coil, and connect
each end of the wire to the battery.
Test the power of the electromagnet by
seeing if the nail will attract other iron
or steel objects.
N
S
Figure 1.13 Using an iron core
First finger
pointing in the
direction of the
field (N to S)
Fleming’s left-hand rule
Fleming’s left-hand rule (also known as the
motor rule) enables us to determine the
direction of the force acting on the wire when
we know the direction of the current and the
magnetic field.
To understand this rule, using your left hand:
●
Separate your thumb, first and second
fingers so that they all point at right angles
to each other.
Thumb points
in the direction
in which the
conductor
tends to move
Second finger
pointing in the
direction of the
current in the
conductor
Figure 1.14 Fleming’s left-hand rule
●
Point your First finger in the direction of the magnetic Field (i.e. north pole to south
pole).
●
Point your seCond finger in the direction of the Current (from the positive terminal to
the negative terminal). You may have to twist your hand to do this but make sure you
keep your fingers at right angles to each other.
●
Your thuMb will show the direction of the force acting on the wire (Movement).
Why don’t you?
Have a go at making an electric motor using the following items:
●
an AA cell battery
●
a strong cylinder magnet
●
a small nail
●
a short piece of wire with insulation removed at each end.
You should be able to create a motor which will spin rapidly. Explain how this
happens.
11
Electromagnetic induction
When a conductor is moved through a magnetic field so that it cuts through the flux, the
electrons in the conductor experience a force which attempts to make them move. This
is the electromotive force (e.m.f.). If the conductor is connected to a circuit, then the
e.m.f. will produce an induced current in the circuit.
To understand this induced current flow, you need to
imagine the magnetic field lines around the magnet
being cut by the conductor as it moves through them.
It is the cutting of these lines of magnetic flux which
places a force on the electrons. Electromagnetic
induction occurs when there is a relative movement
between a magnet and a conductor, so that the
magnetic flux lines are being cut.
You can see the effect of induction by moving a
wire rapidly through a magnetic field. The current
induced in the circuit will be very small, but can
be measured using a sensitive ammeter. Note the
current only exists when the wire is moving through
the magnetic field. There is no current when the wire
is stationary.
Movement
of wire
Ammeter
Magnets
Figure 1.15 Demonstrating
electromagnetic induction
Did you know?
These factors affect induced current:
●
The stronger the magnetic flux, the larger the induced current.
●
The longer the conductor, or the more loops of wire, the greater the induced
current.
●
The faster the wire is moved in relation to the magnet, the greater the
current.
●
Reversing the direction of the magnetic field (i.e. changing north and south
poles) will also reverse the direction of current.
Questions
1
In your own words, describe a magnetic field and state the first law of magnetism.
2
List four factors which affect the resistance of a conductor.
3
Describe what is meant by the resistivity of a material.
12
Measuring instruments
There are two types of meters measuring voltage, current and resistance, presenting the
results in either analogue or digital form.
Analogue
Analogue meters use a pointer moving
over a scale and within limits. They are
capable of a continuous indication of
Moving-coil
voltage or current. The most important on aluminium
analogue instrument is the moving-coil former
meter.
Soft-iron core
N
S
Pole piece
The meter consists of a coil of fine
+
–
copper wire, mounted on an aluminium
forma and pivoted at either end so it is Figure 1.16 The moving-coil meter
free to rotate in a strong magnetic field.
The magnetic field is provided by a small but powerful, permanent magnet with shaped
pole pieces. The coil itself is provided with a soft iron core to produce an intense radial
magnetic field. When a direct current is passed through the coil, it takes up a position
dependent on the size of the current.
Digital
Digital meters are devices which show the readings as a numerical display using a liquid
crystal display (LCD) or light-emitting diode (LED) technology, rather than pointing to a
measure on a scale.
Why don’t
you?
Why don’t
you?
Key terms
Ammeter – an instrument used for measuring current.
Voltmeter – an instrument used for measuring voltage. It measures the
potential difference between two points.
Multimeter – combines the features and characteristics of both an ammeter
and a voltmeter, and measures current, voltage and resistance. Digital
multimeters are the most commonly used measuring devices.
Using a voltmeter, measure the voltage at socket outlets in your workshop,
classroom and other college/school socket outlets. Record these voltages in
a table and compare your readings with those from other students. If there
is a variation in readings at socket outlets, discuss reasons why this may be
the case.
Describe using diagrams how you would take a current reading from a power
circuit when a load has been switched on.
13
1.5 Ohm’s law
So far we have established that current is the amount of electrons flowing in a conductor
and that a force known as the e.m.f. (or voltage) is pushing them. We also know that the
conductor will try to oppose the current by offering a resistance to the flow of electrons.
Ohm’s law is the result of research by a 19th century German physicist G S Ohm into the
relationship between current, voltage and resistance in an electrical circuit. His findings
are summarised as Ohm’s law:
The current flowing in a circuit is directly proportional to the voltage applied to the circuit
and inversely proportional to the resistance of the circuit, provided that the temperature
affecting the circuit remains constant.
Put simply, Ohm’s law means that the amount of electrons passing by every second will
depend on how hard we push them, and what obstacles are put in their way.
Did you know?
Ohm’s law:
V
voltage (V )
Current (I ) =
resistance (R)
or
V
I=
R
I
V
R
V =I×R
I
V
R
I = V
R
I
R
R = V
I
Figure 1.17 The equation triangle
(the vertical line is multiply, the
horizontal is divide)
Worked
example 1
Using Ohm’s law
If a voltage of 100 V is applied to a 5 Ω resistor, calculate the current flow.
Using Ohm’s law, V = I × R or
Then
100 = I × 5
100
I=
= 20 A
5
Worked
example 2
If the resistance in this circuit is increased to 10 Ω, it can be seen that the current flow
reduces:
100
Then I =
= 10 A
10
If you double the resistance, the current is halved.
If the voltage is increased to 200 V and produces a current of 20 A, what is the
resistance?
V
Using the formula R =
I
200
Then
R=
= 10
20
For a constant current, voltage is proportional to resistance.
14
Question
1
Using Ohm’s law copy and complete the following table.
V
I
1
110
5
2
240
3
4
5
115
R
4
10
12
15
6
3
We now understand the relationship between resistance, voltage and current, but
electrical circuits may contain many resistors and they may be connected in different
ways. For example, series circuits, parallel circuits or a combination of both series and
parallel circuits.
R1 = 16 Ω
Series circuits
A series circuit is when a number of resistors (or other
components) are connected end to end, and then
connected to a power supply or battery.
The total resistance is calculated by adding up the
resistance of each individual resistor, i.e.
Rt = R1 + R2 + R3
Did you know?
Rt = (16 + 24 + 48) Ω = 88 Ω
+
–
Battery
R2 = 24 Ω
R3 = 48 Ω
Figure 1.18 A series circuit
●
The total circuit current (I) is the voltage (V ) divided by the total resistance (Rt).
●
The current will be the same value at every point in the series circuit.
●
Resistors in the same circuit may have different values. The amount of
electrons flowing through each resistor will vary. We use voltage to push
these electrons through a resistor.
●
The potential difference across each resistor is proportional to its resistance.
●
The bigger the resistor the more voltage is used.
So applying Ohm’s law to the series circuit, you would then have three different voltage
readings (known as voltage drop). The supply voltage (V ) will be equal to the sum of
all the potential differences across each resistor. In other words if we add up the p.d.
across each resistor (the amount of volts dropped across each resistor), it should come
to the value of the supply voltage. We can show this as:
Vt = V1 + V2 + V3
Power
The total power in the series circuit is equal to the sum of the individual powers used by
each resistor.
To calculate the power in (watts) in a direct current (DC) circuit we can use the formula:
P=V×I
15
Calculations with a series circuit
6.2 Ω
Two resistors of 6.2 Ω and 3.8 Ω are
connected in series with a 12 V battery.
3.8 Ω
Calculate:
a) the total resistance
b) the total current flowing
c) the potential difference across
each resistor
12 V
Figure 1.19
Worked example 3
d) the total power used by each resistor.
a) To find the total resistance, we add up the value of the individual resistors.
Therefore:
Rt = R1 + R2
b) I =
V
R
I=
12
10
Rt = (6.2 + 3.8) Ω
Rt = 10 Ω
I = 1.2 A
c) Across R1:
V = I × R, therefore V1 = I × R1 = 1.2 × 6.2 = 7.44 V
Across R2:
V = I × R, therefore V2 = I × R2 = 1.2 × 3.8 = 4.56 V
d) P = V × I, therefore the total power will be P = 12 × 1.2 = 14.4 W
Power used by R1:
P1 = V1 × I, therefore P1 = 7.44 × 1.2 = 8.93 W
Power used by R2:
P2 = V2 × I, therefore P2 = 4.56 × 1.2 = 5.47 W
Parallel circuits
If a number of resistors are connected together as
shown they are said to be connected in parallel.
In this type of connection, the total current divides
among the different branches of the circuit. However,
it should be noted that the ‘pressure’ pushing the
electrons along (the voltage), will be the same through
each of the branches. Therefore any branch of a
parallel circuit can be disconnected without affecting
the other remaining branches.
16
R1
R2
Figure 1.20 A parallel circuit
The following rules apply to a parallel circuit:
Did you know?
The total circuit current (I), is found by adding together the current through each
of the branches:
It = I1 + I2
The same potential difference will occur across each branch of the circuit:
Vt = V1 = V2
Where resistors are connected in parallel and, for the purpose of calculation, it
is easier if the group of resistors is replaced by one large resistor (Rt):
1
1
1
=
+
Rt R1 R2
Calculations with a parallel circuit
16 Ω
Three resistors of 16 Ω, 24 Ω and 48 Ω are
connected as shown across a 230 V supply.
Calculate the total circuit current.
24 Ω
48 Ω
There are two ways of doing this:
Method 1
Worked example 4
Find the equivalent resistance (Rt) of all the
V
branches and then use I =
R
1
1
1
1
1
1
1
Remember:
=
+
+
=
+
+
Rt R1 R2 R3 16 24 48
From I =
V
R
230 V supply
Figure 1.21
1 3+2+1
=
Rt
48
R
48
6
=
and Rt = 8 Ω
then t =
1
6
48
I=
230
and therefore I = 28.75 A
8
Method 2
Find the current through each resistor and then add them together.
V
230
I1 =
gives I1 =
and therefore I1 = 14.38 A
R1
16
V
230
I2 =
gives I2 =
and therefore I2 = 9.58 A
R2
24
V
230
I3 =
gives I3 =
and therefore I3 = 4.79 A
R3
48
As It = I1 + I2 + I3
then It = 14.38 + 9.58 + 4.79 = 28.75 A
17
Calculations with series and parallel circuits
This type of circuit combines both
series and parallel circuits as shown.
To calculate the total resistance in
a combined circuit, we must first
calculate the resistance of the
parallel group. Then, having found
the equivalent value for the parallel
group, we simply treat the circuit as
being made up of series connected
resistors.
R1 = 10 Ω
R2 = 20 Ω
R3 = 30 Ω
110 V supply
Figure 1.22 A series and parallel
circuit
Calculate the total resistance of this
circuit and the current flowing through
the circuit, when the applied voltage is 110 V.
Worked example 5
Step 1
Find the equivalent resistance of the parallel group (Rp):
1
1
1
1
=
+
+
Rp R1 R2 R3
1
1
1
+
+
10 20 30
1 6+3+2
=
Rp
60
=
=
11
60
Rp
1
=
60
11
therefore
Rp = 5.45
Step 2
Add the equivalent resistor to the series resistor R4:
Rt = Rp + R4
= 5.45 + 10
= 15.45 Ω
Step 3
Now, using Ohm’s law, we can calculate the current in the circuit:
I=
18
110
15.45
I = 7.12 A
R4 = 10 Ω
Questions
1
Two resistors of equal value are connected to three other resistors of value 33 Ω,
47 Ω, and 52 Ω to form a series group of resistors with a combined resistance of
160 Ω. What is the resistance of the unknown resistors? Choose from:
a) 7 Ω
b) 14 Ω
c) 28 Ω
d) 44 Ω
2
The four field coils of a motor are connected in series. Each has a resistance of
33.4 Ω.
a) Calculate the total resistance.
b) Determine the value of an additional series resistance which will give a total
resistance of 164 Ω.
3
Four resistors of values 23 Ω, 27 Ω, 33 Ω, and 44 Ω are connected in series.
It is required to modify their combined resistance to 140 Ω, by replacing one of the
existing resistors by a new resistor of value 40 Ω. Which of the resistors should be
replaced?
4
A circuit has three resistors all connected in series. The resistance values are
10 Ω, 20 Ω, and 25 Ω. The resistors are connected to a 110 V supply. Calculate the
following:
a) the total circuit resistance
b) the total circuit current flow
c) the p.d. (potential difference) across each resistor
d) the power that each resistor uses and the total power.
5
The following groups of resistors are connected in parallel. In each case calculate
the equivalent resistance. Where necessary, make the answers correct to two
significant figures (all values are in ohms).
a) 2, 3, 6
b) 3, 10, 5
c) 9, 7
d) 4, 6, 9
e) 7, 5, 10
f)
14, 70
g) 12, 12
h) 15, 15, 15
i)
40, 40, 40, 40
6
A cable carries a current of 65 A with a voltage drop of 13 V. What must be the
resistance of a cable which, when connected in parallel with the first cable, will
reduce the voltage drop to 5 V?
7
Resistors of 24 Ω and 30 Ω are connected in parallel. What would be the value of a
third resistor to reduce the combined resistance to 6 Ω?
19
8
Three PVC-insulated cables are connected in parallel, and their resistances are
0.012 Ω, 0.015 Ω, and 0.008 Ω. With a total current of 500 A flowing on a 110 V
supply, calculate:
a) the current in each cable
b) the combined voltage drop over the three cables in parallel
c) the individual voltage drop over each cable in a parallel circuit.
9
An electric fire of resistance 24.8 Ω, an immersion heater of resistance 34.8 Ω,
a microwave oven of resistance 45.9 Ω, and a toaster of resistance 120 Ω are
connected to a 230 V power circuit. Calculate the current taken by each appliance
and the total current drawn from the supply.
10
A 230 V electric kettle has a resistance of 88 Ω and is connected to a socket outlet
by a twin cable, each conductor of which has a resistance of 0.1 Ω. The total
resistance of the cable from fuse board to the socket is 0.8 Ω. Calculate the total
resistance of the whole circuit.
11
Calculate the resistance and the current drawn from the supply by the following
equipment:
a) a 4 kW 230 V immersion heater
b) a 600 W 230 V microwave oven
c) a 1 kW 230 V electric fire
d) a 750 W 230 V stereo system.
12
For the circuit shown below, calculate:
a) the resistance of the parallel group
b) the total resistance.
R1 = 9
R3 = 2.3
R2 = 4
50 V
13
For the circuit shown below calculate:
a) the total resistance
b) the supply voltage.
R1 = 0.3
R3 = 6
R2 = 2
6A
20
1.6 Resistivity
So far when we have looked at Ohm’s law, we have always had circuits where we
introduced a known resistance (R) into them. However, even if we removed this resistor
from the circuit, we would still have some resistance within the circuit, caused by the
actual conductor or cable.
The factors that influence cable resistance are:
●
The material used
Each material has its own resistance to the electron flow, shown by the resistivity
symbol (r), the Greek symbol ‘rho’. It is measured in ohm-metres or microohm-millimetres
(μΩ mm).
●
The length
A longer cable has a greater resistance.
●
The cross-sectional area
A cable with a greater cross-sectional area has a lower resistance.
This is the formula for resistance and resistivity:
resistivity × length
cross-sectional area
ρ×I
R=
A
Resistance =
Did you know?
Worked example 1
Did you
know?
The resistivity for each material is found by measuring the resistance of a 1 m cube of
the material, then, as cable dimensions are measured in millimetres (i.e. 1.5 mm2,
2.5 mm²), this figure is divided down to give the value of a 1 mm cube.
Resistivity values: Copper is 17.8 µΩ mm.
Aluminium is 28.5 µΩ mm.
Find the resistance of the field coil of a motor where the conductor’s crosssectional area is 2.0 mm², the length of wire is 4000 m and the material
resistivity is 18 µΩ mm.
ρ×I
R=
A
4 000 000 72000 000
18
=
= 36
R=
×
1000 000
2
2000 000
The value of r is given in millionths of an ohm millimetre. If we have
18 µΩ mm, then we have 18 millionths of an ohm:
18
1000 000
In calculations, all units should be the same. Here the length is in metres, but
everything else is in millimetres. So you write 4 000 000 mm.
21
Worked example 2
A copper conductor has a resistivity of 17.8 µΩ mm and a cross-sectional area
of 2.5 mm². What is the resistance of a 30 m length of this conductor?
ρ×I
A
17.8 × 30 × 10 3
R=
2.5 × 106
R = 0.2136
R=
then
A copper conductor has a resistivity of 17.8 μΩ mm and is 1.785 mm in
diameter. What is the resistance of a 75 m length of this conductor?
Worked example 3
Step1
You need to use the following formula to convert the diameter into a crosssectional area (use p = 3.142):
A=
π d 2 3.142 × 1.785 × 1.785 10.01
=
=
= 2.5mm2
4
4
4
Step 2
Put in the correct values in the resistance equation:
ρ × I 17.8 × 10 −6 × 75 × 10 3 17.8 × 10 −3 × 75
=
=
2.5
A
2.5
17.8 × 75 1335
=
=
= 0.534
2.5 × 10 3 2500
R=
Questions
1
Calculate the resistivity of aluminium, if a 100 m length of cross-sectional area
4 mm2 has a measured resistance of 0.7 Ω.
2
Calculate the resistance per 100 m of the following sizes of copper cable:
a) 1.5 mm2
b) 6 mm2
c) 10 mm2
3
Calculate the diameter of an aluminium busbar, which is 25 m long and has a
resistance of 0.001 39 Ω.
4
Calculate the resistance of 35 m of 1 mm² copper cable.
5
Calculate the resistance of 35 m of copper busbar whose dimensions are 50 mm
by 5 mm.
The effect of temperature on resistance of a conductor
When a current is passed through a cable or conductor the temperature rises. An
example is the element of an electric kettle. The effect of this heat on the resistance of
the conductor depends on the material the conductor is made from.
22
Did you know?
●
●
●
The resistance of pure metals such as copper or aluminium increases as
temperature increases, i.e. they have a positive temperature coefficient.
The resistance of certain alloys (e.g. constantin or manganin) does
not change much with increases in temperature, i.e. they have a zero
temperature coefficient.
The resistance of carbon and electrolytes (liquids used in batteries)
decreases when the temperature increases, i.e. they have a negative
temperature coefficient.
Calculating resistance increase
There are two formulae for calculating the increase in resistance of a conductor due to
temperature change, depending on whether the temperature increases from a baseline
temperature or whether the temperature increases between two different temperatures.
Formula A
Formula B
Temperature increases from a baseline Temperature increases between two
value (e.g. 0 °C)
intermediate temperatures
Rf = R0 (1 + a t)
where R0 = resistance at 0 °C
R2 1 + αt2
=
R1 1 + αt1
Rf = final resistance
where R1 = first temperature
a = temperature coefficient
R2 = second temperature
t = rise in temperature
a = temperature coefficient
t1 = first temperature
The resistance of a coil of
copper wire at 0 °C is 100 Ω.
Calculate the resistance
of the coil at 30 °C. The
temperature coefficient of the
copper is 0.004/°C.
Rf = R0 (1 + at)
= 100 × (1 + 0.004 × 30)
= 100 × 1.12
= 112 Ω
Worked example 5
Worked example 4
t2 = second temperature
The coils in a motor have a
resistance of 200 Ω at 20 °C. Find
the resistance of the coils when the
motor temperature increases to
40 °C. The temperature coefficient
of the conductor is 0.004/°C.
R2 1 + αt 2
=
R1 1 + αt 1
R2
1 + 0.004 × 40
=
200 1 + 0.004 × 20
R2
1 + 0.16
=
200 1 + 0.08
R2
1.16
=
200 1.08
200 × 1.16
R2 =
1.08
R2 = 214.81
23
1.7 Circuit protective devices
A circuit protective device is a safety device, designed to disconnect the electrical circuit
when the conditions are faulty or dangerous to protect both equipment and people.
These devices are normally fuses or circuit breakers.
Figure 1.23 Cartridge fuses
Cartridge fuses
The cartridge fuse consists of a porcelain tube with metal end-caps to which the element
is attached. It is filled with granulated silica. This type of fuse is generally found in plug
tops used with standard socket outlets. There are two main fuse ratings available, the
3 A, which are used with appliances up to 720 W (e.g. radios, table lamps), and the 13 A
fuse used for appliances rated over 720 W (e.g. irons, kettles, fan heaters, electric fires,
lawnmowers, toasters, refrigerators, washing machines, and vacuum cleaners).
High breaking capacity (HBC) fuses
The HBC fuse is a sophisticated variation of the cartridge fuse, and is normally found
protecting motor circuits and industrial installations. It consists of a porcelain body filled
with silica with silver element, and lug-type end caps. Another feature is the indicating
bead, which shows when the element has blown. It is a very fast acting fuse and can
discriminate between a starting surge and an overload. This type of fuse is normally
found in distribution boards and at the mains intake positions. These types of fuse would
be used when an abnormally high prospective short circuit current exists.
Porcelain body
End caps
Indicating bead
Figure 1.24 A sectional view of a typical HBC fuse
24
Miniature circuit breakers (MCBs)
The circuit breaker is an automatic
switch, which opens when there is
excess current. The switch can be
closed again when the current returns
to normal, because the device does not
damage itself during normal operation.
The contacts of a circuit breaker are
closed against spring pressure, and held
closed by a latch arrangement. A small
movement of the latch will release the
contacts, which will open quickly under
spring pressure to break the circuit.
Compression spring
Main contacts
Trip leaver
Pivot
Re-set facility
Figure 1.25 A miniature circuit breaker
Normal currents will not affect the latch,
whereas excessive currents will move it to operate the breaker. There are two basic
methods by which over current can operate or ‘trip’ the latch: thermal and magnetic
tripping.
Thermal tripping
The load current is passed through a
small heater, the temperature of which
increases as the current it carries
increases. This heater is arranged to
warm a bimetallic strip either directly,
i.e. the current passes through the
bimetallic strip, which in effect is part
of the electrical circuit, or indirectly,
i.e. the current passes through a coil
wound around the bimetallic strip. The
bimetallic strip is made of two different
metals, normally brass and steel, brass
expanding more than steel. These two
dissimilar metals are securely riveted or
welded together along their length. The
rate of expansion of the two metals is
different so that as the strip is warmed,
it will bend and will trip the latch. The
bimetallic strips are arranged so that
normal currents will not heat the strip
to tripping point. If the current flow
increases beyond the rated value, the
bimetallic strip is raised in temperature,
bends and trips the latch.
Current in
Trip
lever
Contacts closed
Directly heated
bimetallic strip
Current
out
Contacts open
Figure 1.26 Thermal tripping
25
Magnetic tripping
Contacts
The principle used here is the force of
attraction, which can be set up by the
magnetic field of a coil carrying the load
current. At normal currents the magnetic field
is not strong enough to attract the latch, but
overload currents operate the latch and trip
the main contacts.
Coil carrying
current
A simple attraction type – The magnetic
field is set up by a current in the flexible strip
attracting the strip to the iron, and releasing
the latch. This is often used in miniature
circuit breakers combined with a thermal trip.
Magnetic
plunger
Oil-filled
dashpot
Oil-escape
hole
The oil dashpot solenoid type – This is used
Figure 1.27 Magnetic tripping
on larger circuit breakers. The time lag is
adjustable by varying the size of oil-escape
hole in dashpot position. Current rating is
adjustable by vertical movement of the plunger.
Motor starters
A motor starter can provide several functions, such as:
switching of supply to the motor
● protecting the motor from overload
● preventing automatic re-starting after a power failure
● provide means of stopping a motor safely in an emergency
● reversing of motor direction
● auto connecting in either star or delta to reduce high starting currents
● direct on line starter (DOL).
●
Did you know?
This type of starter basically switches a three-phase supply directly onto the stator
windings. Start-up current is about 6 to 10 times full load current and start-up torque is
about 150% of full load. This is usually limited to motors up to 5 kW.
26
Normally open contact in a control circuit means that the contact is normally
open when the circuit is not energised. When the circuit is energised then
the normally open contacts become closed and any normally closed become
opened.
Retaining contact is when the start button is released (take your finger off) then
the supply is maintained to the coil and hence the three-phase supply remains
connected to the motor.
Direct on line starter (DOL)
When a supply is connected to the coil of a contactor via a start button, this causes the
coil to magnetise the former inside the contactor and subsequently attract the top former
holding the three contacts causing them to close and supply three phases to the stator
windings.
Three-phase and neutral supply
N
L1
L2
L3
Start
Stop
Signal
lamp
ptc
Thermistor
Motor
Ph
Transformer
Key terms
Figure 1.28 Three-switch relay with ‘normally open’ start
button
Fuse – also known as ‘overcurrent protection’, disconnects a circuit when an
overload or short circuit is present.
Circuit breaker – a device which automatically disconnects the supply from a
circuit in the event of excessive current flowing in the circuit.
27
The stop button is needed to de-energise the contactor coil and thus open the supply to
the motor. Normally closed contacts are connected in series with the stop button, which
will open the circuit should an overload condition occur and hence de-energise the coil
and switch off the supply to the motor. All stop buttons (if there are more than one) are
always connected in series with each other and also in series with other safety switches
such as overloads and interlocks. They are also connected in series with any start
buttons and contactor coils so that the control circuit supply can be interrupted.
Start buttons (if there are more than one) are always connected in parallel with each
other but in series with the coil and stop button.
Three-phase and neutral
supply connections
N
L1
L2
U
V
W
Three-phase
motor connection
L3
230 V
Coil
Additional
start buttons
Start
Stop
O/L
Additional
stop buttons
Figure 1.29 Remote stop/start control
28
1.8 Principles of a.c. and d.c. current flow
Power factor
Power factor is a number less than 1, which is used to represent the relationship
between the apparent power of a circuit and the true power of that circuit.
Power factor (p.f.) =
power (watts)
voltage (volts) × current (amps)
In order to understand power factor, we need to look at alternating current (a.c.).
What is alternating current?
Alternating current (a.c.) is a flow of electrons which rises to a maximum value in one
direction and then falls back to zero before repeating the process in the opposite
direction. In other words, the electrons within the conductor do not drift (flow) in one
direction, but actually move backwards and forwards.
The journey taken, i.e. starting at zero, flowing in both directions and then returning to
zero, is called a cycle. The number of cycles that occur every second is said to be the
frequency and this is measured in hertz.
Because an alternating current is continuously changing direction, the following are
important:
●
Instantaneous – the value at a specific instant in time
●
Maximum/Peak – the highest value obtained in the cycle
●
Average – the average value obtained across one half of the cycle
●
r.m.s. (the root mean square) – produced to give a comparable value to those of a d.c.
circuit and used for descriptive and calculation purposes.
For calculation, we take the average value of a sinusoidal voltage or current to be 0.637
times the maximum value and we take the r.m.s. value to be 0.707 times the maximum
value.
Sinusoidal waveforms
Steady rate of rotation
Meter
pointer
swings
from side
to side
Coil
Commutator
rings
Brushes
Alternating voltage
Figure 1.30 A simple loop a.c. generator
29
When the loop in the generator
is rotated within the magnetic
field, an e.m.f. will be induced,
at the rate of one cycle per
rotation. The e.m.f. induced in
the loop at any instant in time
will depend upon the rate of
‘cutting’ through the lines of
magnetic flux.
+
e.m.f.
1 cycle
0
Time
–
X
Y
X
Y
When the coil is vertical it
X
Y
N
S N
S N
S N
S
does not cut field lines and no
Y
X
e.m.f. is induced. Maximum
generation occurs when the coil
Figure 1.31 The e.m.f. produced by an a.c. generator
is horizontal and the maximum
(a sine wave)
amount of field lines are being
cut. This induced e.m.f. will also
cause an induced current (flow of electrons) within the circuit.
Resistance (R) and phasor representation
We can also represent a.c. by the use of
phasors. A phasor is a straight line whose
Lamp
V
length is a scaled representation of the size
of the a.c. quantity and whose direction
I
represents the relationship between the
360°
voltage and current, this relationship being
known as the phase angle. Figure 1.32 shows
the sine wave and Figure 1.33 is the phasor
diagram for a filament lamp.
Figure 1.32 The waveform for a
Circuits like this are said to be resistive, and
tungsten filament lamp (a.c.)
in this type of circuit the values of e.m.f.
(voltage) and current actually pass through the
same instants in time together. In other words,
as voltage reaches its maximum value, so does the current.
I
V
This happens with all resistive components connected to
Figure 1.33 A phasor
an a.c. supply, and the voltage and current are said to be
diagram for a tungsten
‘in phase’ with each other, or possess a zero phase angle.
filament lamp (a.c.)
Voltage and current are perfectly linked and are therefore
said to possess unity power factor. This would be given the
value of 1.0.
A resistive component will consume power and the calculations are the same as for a
d.c. circuit, i.e.
P=V×I
Resistive equipment (filament lamps, fires, water heaters, etc.) use this power to create
heat, but such a feature in long cable runs, windings, etc. causes unsuitable power loss
in the circuit.
30
Inductance (L)
If a motor or transformer (something possessing windings) is used then the load is
inductive. With an inductive load the voltage and current become ‘out of phase’ with
each other. This is because the windings of the equipment set up their own induced
e.m.f., which opposes the flow of the applied voltage and thus forces the flow of
electrons (current) to fall behind the force pushing them (voltage). However, over one full
cycle, we would see that no power is consumed. This is known as possessing a lagging
phase angle or power factor.
The voltage and current are no longer perfectly linked, so the circuit would be given a
power factor of less than 1.0 (perfection), for example, 0.8. The sine wave and phasor
diagram are shown in Figure 1.34, and the current is lagging the applied voltage by 90°.
+
I
90°
360°
V
V
I
–
Figure 1.34 A sine wave and phasor diagram for an
inductive circuit
We assumed that the above circuit is purely inductive, however, in reality, this is
not possible as every coil is made of wire and that wire will have a resistance. The
opposition to current flow in a resistive circuit is resistance.
The limiting effect to the current flow in an inductor is the inductive reactance which we
are able to calculate with the following formula:
XL = 2pfL
where XL = inductive reactance (in ohms, Ω), f = supply frequency (in hertz, Hz), L = circuit
inductance (in henrys, H).
Power given as the result of the voltage and the current (P = V × I), is known as the
apparent power, called the unity power factor given the value one (1.0).
Depending upon the equipment, the true power (actual) in the circuit must take into
account the phase angle (P = V × I × cos ∅) and will often be less than the apparent
power but never greater.
31
If we have an inductive load, consuming 3 kW of power from a 230 V supply, with a power
factor of 0.7 lagging, then the current (amount of electrons flowing) required to supply
the load is:
P = V × I × cos ∅
therefore by transposition:
I=
P
V × cos ∅
and
I=
P
V × p.f.
therefore
I=
3000
= 18.6 A
230 × 0.7
However, if the same size of load was resistive, then the power factor would be 1.0, and
thus:
P=V×I
therefore by transposition
I=
P
V
therefore
I=
3000
= 13 A
230
In other words, the lower the power factor of a circuit, then the higher the current will need
to be to supply the load’s power requirement. It therefore follows that if the power factor
is low, then it will be necessary to install larger cables, switchgear, etc. to be capable of
handling the larger currents. There will also be the possibility of a higher voltage drop due
to the increased current in the supply cables, with which a capacitor helps.
Capacitance (C)
A capacitor is a component which stores an electric charge. If a potential difference is
applied across it, this energy can be returned back to the circuit. When a capacitor is
connected to an a.c. supply, it is continuously storing the charge and then discharging as
the supply moves through its positive and negative cycles. But, as with the inductor, no
power is consumed.
This means that in a capacitive circuit, we have a leading phase angle or power factor.
The sine wave and phasors used to represent this are shown, where the current leads
the voltage by 90°.
+
V
I
360°
90°
I
V
–
Figure 1.35 A sine wave and phasor diagram for a capacitive circuit
32
As the capacitor provides a leading power factor, if connected in parallel across the load,
it can help neutralise the effect of a lagging power factor.
The opposition to the flow of a.c. to a capacitor is termed capacitive reactance, which,
like inductive reactance, is measured in ohms and calculated using the following formula:
XC =
1
2π fC
where XC = capacitive reactance (ohms, Ω), f = supply frequency (hertz, Hz), and
C = circuit capacitance (farads, F).
Since in this type of circuit we have voltage and current but no power, the formula of
P = V × I is no longer accurate. Instead, we say that the result of the voltage and current
is reactive power, which is measured in reactive volt amperes (VAr). The current to the
capacitor, which doesn’t contain resistance or consume power, is called reactive current.
Phasors
We can start from zero on the wave diagram with either the voltage or the current. In
electrical science we often need to add together alternating values. If they were ‘in
phase’ with each other, then we would simply add the values together. However, when
they are not ‘in phase’ we need to use phasor diagrams. The chosen alternating quantity
is drawn horizontally and is known as the reference.
When choosing the reference phasor, it makes
sense to use a quantity that has the same
value at all parts of the circuit. For example,
in a series circuit, the same current flows in
each part of the circuit – therefore use current
as the reference phasor. In a parallel circuit,
the voltage is the same through each branch
of the circuit and therefore we use voltage as
the reference phasor. We can now measure
all phase angles from this reference phasor,
and the resultant is found by completing a
parallelogram. In Figure 1.36, the result of
adding A and B together will be phasor C.
A
C
B
Figure 1.36 The resultant of a phasor
Impedance
Components within an a.c. circuit are in opposition to the flow of current. To summarise:
●
The opposition to current in a resistive circuit is called resistance (R), is measured in
ohms and the voltage and current are in phase with each other.
●
The opposition to current in an inductive circuit is called inductive reactance (XL), is
measured in ohms and the current lags the voltage by 90°.
●
The opposition to current in a capacitive circuit is called capacitive reactance (XC), is
measured in ohms and the current leads the voltage by 90°.
Circuits will contain a combination of these components. The total opposition to current
is called the impedance (Z) of that circuit.
33
Questions
1
A coil of 0.25 H is connected in series with a 75 Ω resistor across a 100 V 50 Hz
supply. Calculate the following:
a) the inductive reactance of the coil
b) the impedance of the circuit
c) the circuit current.
2
A coil of 0.5 H is connected in series with a 50 Ω resistor across a 100 V 50 Hz
supply. Calculate the following:
a) the inductive reactance of the coil
b) the impedance of the circuit
c) the circuit current.
3
A coil of 0.75 H is connected in series with a 25 Ω resistor across a 100 V 50 Hz
supply Calculate the following:
a) the inductive reactance of the coil
b) the impedance of the circuit
c) the circuit current.
4
A coil of 0.145 H is connected in series with a 50 Ω resistor across a 230 V 50 Hz
supply. Calculate the following:
a) the inductive reactance of the coil
b) the impedance of the circuit
c) the circuit current
d) the p.d. across each component
e) the circuit phase angle.
5
A coil of 0.25 H is connected in series with a 70 Ω resistor across a 230 V 50 Hz
supply. Calculate the following:
a) the inductive reactance of the coil
b) the impedance of the circuit
c) the circuit current
d) the p.d. across each component
Key terms
e) the circuit phase angle.
34
Frequency – the number of oscillations of alternating current in an electric
system transmitted from a power station to the end-user per second. It is
normally 50 Hz.
Unity power factor – when the voltage and current in a circuit are ‘in phase’
with each other.
Capacitor – a component which stores an electric charge if a voltage is applied
across it.
1.9 Resistance, inductance, and capacitance
Resistance (R) and inductance (L) in series
I
R
L
VR
VL
+
I
Ø
Supply (Vs)
V
–
Figure 1.37 A resistor connected in series with an inductor (a.c.)
In a series circuit, the current (I) will
be common to both the resistor and
the inductor, causing voltage drop
VR across the resistor and VL across
the inductor. The sum of these
voltages must equal the supply
voltage. But, because this is an
a.c. circuit, we have to use a phasor
diagram to work this out.
VL
VS
Ø
Our reference phasor
VR
I
In a series circuit, we know that
VS
VL
current will be common to both the
Ø
resistor and the inductor, therefore
current is our reference phasor. We
VR
also know that voltage and current
Figure 1.38 Phasor diagrams for a resistor
will be in phase for a resistor.
connected in series with an inductor (a.c.)
Therefore the voltage drop (p.d.)
across the resistor (VR), must be
in phase with the current. In an inductive circuit, the current lags the voltage by 90°, so
voltage is leading current. This means that the voltage drop across the inductor (VL) will
lead the current by 90°. We can then find the value of the supply voltage (Vs), by completing
the parallelogram.
When we draw phasors, we always assume that they rotate anti-clockwise and that the
symbol ∅ represents the phase angle. Figure 1.38 shows two ways of drawing this.
In example b) the phasors produce a right-angled triangle. We can therefore use
Pythagoras’ theorem to give us the formula:
Vs2 = VR2 + VL2
We can then use trigonometry to give us the different formulae, depending on the values
that we have been given:
cos ∅ =
VR
Vs
sin ∅ =
VL
Vs
tan ∅ =
VL
VR
35
Here are two examples:
A coil of 0.15 H is connected in series with a 50 Ω resistor across a 100 V
50 Hz supply.
Calculate the following:
a) the inductive reactance of the coil
b) the impedance of the circuit
c) the circuit current.
a) For inductive reactance, we use the formula XL = 2pfL (Ω)
Worked example 1
XL = 2 × 3.142 × 50 × 0.15
= 47.1 Ω
b) When we have resistance and inductance in series, we calculate the
impedance using the following formula:
Z 2 = R2 + XL2 which becomes Z = R2 + XL2
Z
The first formula is the same as Pythagoras’
theorem for a right-angled triangle (A2 = B2 + C2).
This is the impedance triangle and is drawn for this
type of circuit as shown.
Here, the angle (∅) between sides R and Z, is the
same as the phase angle between current and
voltage. If we therefore apply some trigonometry, the
following applies:
cos ∅ =
R
Z
sin ∅ =
XL
Z
tan ∅ =
XL
Ø
R
Figure 1.39 The
impedance triangle
XL
R
From Z = R2 + XL2 , Z = 502 + 47.12 = 68.69
c) As we are referring to the total opposition to current, we use the formula:
I=
36
100
V
=
= 1.46 A
Z 68.69
A coil of 0.159 H is connected in series with a 100 Ω resistor across a 230 V
50 Hz supply. Calculate the following:
a) the inductive reactance of the coil
b) the circuit impedance
c) the circuit current
d) the p.d. across each component
Worked example 2
e) the circuit phase angle.
a) XL = 2pfL,
therefore XL = 2 × 3.142 × 50 × 0.159 = 50 Ω
2
b) Z = R + XL2 , therefore
Z = 1002 + 502 = 111.8
V
230
= 2.06 A
therefore I =
Z
111.8
d) VR = I × R therefore V = 2.06 × 100 = 206 V
c)
I=
VL = I × XL, therefore V = 2.06 × 50 = 103 V
e) Using the right-angled triangle:
tan ∅ =
VL 103
=
= 0.5
VR 206
On a calculator enter 0.5, then press the INV key then press TAN, you should
see the number 26.6. In other words, current is lagging voltage by 26.6°.
Resistance (R) and capacitance (C) in series
+
V
C
R
VR
VC
I
I
Ø
Supply (VS)
–
Figure 1.40 Resistance (R) and capacitance (C) in series
In a series circuit the current (I) will be common to both the resistor and the capacitor,
causing voltage drop (p.d.), Vr across the resistor and Vc across the capacitor.
As with the resistance/inductance (RL) circuit previously, we can take current as the
reference phasor. Similarly, the voltage across the resistor will be in phase with that
current. Remember, in a capacitive circuit the current leads the voltage by 90º. Therefore,
we can say that the voltage across the capacitor will be lagging the current.
37
As before we can now calculate the supply voltage (Vs) by completion of the
parallelograms as follows:
As with the inductor, we can apply Pythagoras’ theorem and trigonometry to give us the
following formulae:
Vs2 = VR2 + Vc2
cos ∅ =
VR
Vs
sin ∅ =
Vc
Vs
tan ∅ =
Vc
VR
A capacitor of 15.9 µF and a 100 Ω resistor are connected in series across a
230 V and 50 Hz supply. Calculate the following:
a) the impedance of the circuit
b) the circuit current
c) the p.d. across each component
Worked example 3
d) the circuit phase angle.
a) To be able to find the impedance, we must first find the capacitive
reactance.
106
1
Xc =
however, as the capacitor value is given in μF, we use X c =
2π fC
2π fC
106
106
=
= 200
This gives us: X c =
2 × 3.142 × 50 × 15.9 4995.78
When we have resistance and capacitance in series, we use the following
formula:
Z 2 = R2 + X c2 which becomes Z = R2 + X c2
Therefore Z = 1002 + 2002 = 50 000 = 224
b)
I=
V 230
=
= 1.03 A
Z 224
c) VR = I × R = 1.03 × 100 = 103 V
Vc = I × Xc = 1.03 × 200 = 206 V
d) Using our right-angled triangle:
tan ∅ =
Vc 206
=
=2
VR 103
If we then take our calculator and enter 2, then press the INV key, then press
TAN, we should see the number 63.4. In other words, current is leading voltage
by 63.4º.
38
Resistance, inductance, and capacitance in series (RLC)
Consider Figure 1.41:
Here we have a resistor connected in series with an inductor and a capacitor and then
fed from an a.c. supply. Such an arrangement is often referred to as an RLC circuit or as
a general series circuit.
I
R
L
C
VR
VL
VC
Supply (Vs)
Figure 1.41 An RLC series circuit
Again, as we have a series circuit, the current (I) will be common to all three components,
causing voltage drop (p.d.) VR across the resistor, VL across the inductor, and VC across
the capacitor.
Here, VR will be in phase with the current, VL will lead the current by 90º (because the current
lags the voltage), and VC will lag the current by 90º (because current leads the voltage in a
capacitive circuit). Because VL and VC are in opposition to each other (one leads and one
lags), the actual effect will be the difference between their values (see Figure 1.42).
VL
Our reference
phasor
VS
VL – Vc
VR
I
Vc
Figure 1.42 An RLC circuit phasor diagram
We calculate Vs by completing the parallelogram as follows:
We apply Pythagoras’ theorem and trigonometry to give us the following formulae:
VS2 = VR2 + (VL − VC )2
Z = R2 + ( XL − X C )2
And finally: cos ∅ =
VR
Vs
sin ∅ =
VL − VC
Vs
tan ∅ =
VL − VC
VR
39
A resistor of 5 Ω is connected in series with an inductor of 0.02 H and a
capacitor of 150 μF across a 230 V and 50 Hz supply. Calculate the following:
a) the impedance
b) the supply current
c) the power factor.
a) In order to find the impedance, we must first find out the relevant values of
reactance. Therefore:
XL = 2pfL = 2 × 3.142 × 50 × 0.02 = 6.28 Ω
Worked example 4
Xc =
106
1
= 21.2
allowing for microfarads =
2 × 3.142 × 50 × 150
2π fC
The effect of inductance and capacitance together in series will be the
difference between their values. Consequently, this means that the resulting
reactance (X) will be found as follows:
X = Xc - XL = 21.2 - 6.28 = 14.92 Ω
We can now use the impedance formula as follows:
2
2
Z = R2 + ( X c − XL )2 , which gives us Z = 5 + 14.92 = 15.75
Note:
XC is greater than XL. Therefore we subtract XL from XC. Also, as capacitive
reactance is higher, the circuit current will lead the voltage.
V
230
b) I = =
= 14.6 A
Z 15.75
R
5
= 0.32 and therefore p.f. = 0.32 leading
c) cos ∅ = =
Z 15.75
A resistor of 10 Ω is connected in series with an inductor of 0.5 H and a
capacitor of 75 µF across a 230 V 50 Hz supply. Calculate the following:
a) the impedance
b) the supply current
c) the power factor.
a) We must first find out the relevant values of reactance.
Worked example 5
XL = 2pfL = 2 × 3.142 × 50 × 0.5 = 157.1 Ω
Xc =
106
1
= 42.43
allowing for microfarads =
2 × 3.142 × 50 × 75
2π fC
The resulting reactance (X) will be found as follows:
X = XL - Xc = 157.1 - 42.43 = 114.67 Ω
We can now use the impedance formula as follows:
Z = R2 + ( XL − X c )2 , which gives us Z =
b) I =
c)
40
52 + 114.672 = 114.78
V
230
=
= 2.0 A
Z 114.78
cos ∅ =
R
5
=
= 0.043 and therefore p.f. = 0.043 leading
Z 114.78
Resistance, inductance and
capacitance in parallel
60 µF
There can obviously be any combination of
4.0 H
the above components in parallel. However, to
demonstrate the principles involved, we will look
at all three connected across an a.c. supply.
50 Ω
As we have a parallel circuit, the voltage (Vs)
will be common to all branches of the circuit.
Consequently, when we draw our parallelogram
Supply (VS)
we will use voltage as the reference phasor.
250 V 50 Hz
In this type of circuit, the current through the
Figure 1.43 An RLC parallel circuit
resistor will be in phase with the voltage, the
current through the inductor will lag the voltage
by 90º and the current through the capacitor will lead the voltage by 90º. Normally when
we carry out calculations for parallel circuits, it is easier to treat each branch as being
a separate series circuit. We then draw to scale each of the respective currents and
their relationship to our reference phasor, which is voltage. As with voltages VL and VC in
the RLC series circuit, the current through the inductor (IL) and the current through the
capacitor (IC) are in complete opposition to each other. Therefore, the actual effect will
be the difference between their two values. We calculate this value by the completion of
our parallelogram. The bigger value (IC or IL) will determine whether the current ends up
leading or lagging.
First find the current through each branch:
V 230
IR = =
= 4.6 A
50
R
XL = 2p fL = 2 × 3.142 × 50 × 0.4
= 126 Ω
therefore IL =
V
230
=
= 1.8 A
XL 126
106
106
=
2π fC 2 × 3.142 × 50 × 60
V
230
= 53
=
= 4.4 A
therefore IC =
Xc
53
XC =
The actual effect will be IC - IL which gives
us 4.4 - 1.8 = 2.6 A
5
4
Ic
3
2
(Ic – IL)
IT = 5.2 A
1
28°
0
1
VS
IR
2
3
4
5
6
1
IL
2
We now add this to IR by completing our
scale drawing as shown.
3
This gives us a current of 5.2 A which is
leading voltage by an angle of 28º.
Figure 1.44 Scale drawing to find IR
41
Measuring current in an a.c. parallel circuit
●
●
Why don’t you?
●
VR
Connect up the components of the
circuit as shown.
L
Ensure the capacitor is not in
circuit (i.e. switch ‘S’ is in the OPEN
position).
C
Switch on the supply and record the
current taken by the lamp in amps.
Ac
S
A
●
Close switch ‘S’ and record the
current taken by the capacitor.
●
Also record the new circuit current.
●
Then, produce a phasor diagram
(not to scale) showing all three
currents recorded, using the supply voltage as your reference.
●
Account for the difference in circuit current when the capacitor is removed.
●
State the two functions performed by the choke (the inductor) and the
function performed by the capacitor.
●
How is the assumed current demand of a discharge lamp calculated, when
only the lamp wattage and circuit voltage are known?
Figure 1.45 Measuring current in a
parallel circuit
Measuring voltage in an a.c. series circuit
Why don’t you?
●
42
Connect up the components of the
circuit as shown.
●
Switch on the supply.
●
Using approved test probes and
leads measure and record the
following voltages:
a) the voltage across the supply
in volts
b) the voltage across the choke
VR
L
V
V
V
Figure 1.46 Measuring voltage in a
parallel circuit
c) the voltage across the lamp.
●
Switch off the supply and isolate the circuit.
●
Produce a phasor diagram (not to scale) showing all three voltages recorded,
using the circuit current as your reference.
●
Account for the fact that the voltages across the series components do not
add up to the supply voltage.
●
Explain why special precautions are necessary when installing this type of
luminaire in a workshop containing rotating machinery.
●
What precautions should be taken with the disposal of this type of lamp?
Questions
1
A small industrial unit is fed by a 400 V, three-phase, four-wire supply system. On the
estate are three factories connected to the system as follows:
a) Factory A taking 50 kW at unity power factor.
b) Factory B taking 80 kVA at 0.6 lagging power factor.
c) Factory C taking 40 kVA at 0.7 leading power factor.
Calculate the overall kW, kVA, kVAR and power factor for the system. To clarify, you
are trying to find the values of P (true power), S (apparent power) and Q (reactive
power). First you need to work out the situations for each factory.
2
Three coils of resistance 40 ohms and inductive reactance 30 ohms are connected
in delta to a 400 V, 50 Hz, three-phase supply. Calculate the following:
a) the line current in each coil
b) line current
c) total power.
3
A balanced load of 10 ohms per phase is star connected and supplied with 400 V
50 Hz at unity power factor. Calculate the following:
a) phase voltage
b) line current
c) total power consumed.
4
A resistor of 25 Ω is connected in series with an inductor of 0.2 H and a capacitor of
750 µF across a 230 V 50 Hz supply. Calculate the following:
a) the impedance
b) the supply current
c) the power factor.
5
A capacitor of 15 µF and a 150 Ω resistor are connected in series across a 230 V
50 Hz supply. Calculate the following:
a) the impedance of the circuit
b) the circuit current
c) the p.d. across each component
d) the circuit phase angle.
43
1.10 Cells and batteries
Primary cells are batteries that are not easily recharged and are therefore discarded
after a single use, e.g. an AA battery used to power a clock or TV remote control. Primary
cells are made from an absorbent material (but not a liquid), so are known as ‘dry cells’.
Primary cells are cheap to produce and have low maintenance issues.
Secondary cells are batteries which can be recharged and used many times, e.g. a car
battery or back-up emergency lighting. They contain a liquid electrolyte (so are known as
‘wet cells’), are larger than primary cells, and require more maintenance.
Table 1.4 Comparing primary and secondary cells
Secondary cell
Low initial cost to make / buy
Higher initial manufacturing / purchase cost
Disposable
Regular maintenance required
Replacement readily available
Not as easily available
Light and small, used in portable devices
Less suitable for portable devices
Good charge retention
Poor charge retention
Not good for heavy electrical loads
Good for heavy electrical loads (starting vehicles)
Key
term
Did you know?
Why don’t
you?
Primary cell
44
Research and draw pictures of the different types of battery, including
Leclanché, mercury and nickel–cadmium.
Have you ever noticed signs of corrosion on a battery? This is known as ‘local
action’, and is a result of a slow discharge of the battery when not connected to
a circuit. The electrolyte attacks the impurities in the zinc case, forming small
cells on the surface of the case, and eroding the case.
The lifetime of a battery can be reduced due to an effect called polarisation.
This is a chemical reaction inside the battery which reduces the effectiveness
of the cell. Bubbles of hydrogen gas (an insulator) gather around the carbon
electrode when the cell is in use and resist the flow of current.
Polarisation – the shift of positive and negative electric charge in opposite
directions within an insulator, or dielectric, induced by an external e.m.f.
2. Electrical and electronic drafting
2.1 Drawings and circuit diagrams
People working in the electrical industry need to communicate complex technical
information to others. A technical diagram is a means of conveying this information
easily and clearly. In the electrical industry diagrams are used in different forms, for
example:
●
block diagrams
●
layout diagrams
●
circuit diagrams
●
electrical symbols
●
wiring diagrams
●
scale drawings also known as site plans.
●
schematic diagrams
Block diagrams
A block diagram can be used to relate information about a circuit without giving details
of components or the manner in which they are connected. The components are
represented by a square or rectangle.
Gate
circuit
Reference
Comparator
Triac
Motor
Load
Tachogenerator
Figure 2.1 A block diagram
Circuit diagrams
A circuit diagram uses symbols to represent all circuit components and shows how these
are connected, following a logical progression route from supply to output. The symbols
used are internationally recognised circuit diagram symbols.
N/C contact
Reset
ELV Supply
Call point
1
3
Latching-on
contact
N/O contact
2
Relay coil
7
6
8
Alarm bell
Figure 2.2 An alarm system
45
Wiring diagrams
In a wiring diagram the
physical layout is taken
into consideration. The
components and connections
carry information of a specific
nature regarding the wiring or
connection of components.
Relay
Reset
4
3
5
6
2
1
8
7
Alarm bell
Call point
ELV supply
Figure 2.3 Wiring diagram for an alarm system
Schematic diagrams
Schematic diagrams are similar to
circuit diagrams, in that they do not
show how to wire components but they
do show a clear and logical progression
route from supply to output. They tend
to be used for larger, more complicated
electrical diagrams such as control
systems for motor starters and heating
systems.
(L3)
(L1)
`E´
`D´
ET
Start
13 Δ14
a
b
15 Δ 16 7
Stop
Link
b
Δ a
b
M
14 13
8
O/L
15 16
L
Y
11 12
M
a
15 16
Figure 2.4 Schematic diagram for a full
control circuit of an automatic star/delta
starter
Layout diagrams
Layout diagrams are scale drawings prepared by
an electrical consultant or engineer responsible for
a particular installation and are based upon the
architect’s drawings of the building. These drawings
show the required position of all equipment,
metering and control gear to be installed. They
normally show the plan view of the installation,
and internationally recognised location symbols
are used. They are used to show the sequence of
control of large installations.
4
Legend
4
65 W
Scale 1:50
50 mm × 50 mm trunking run
4 Gang 1 way switch
Lighting distribution board
Main control
Fluorescent luminaire
Figure 2.5 Layout diagram for a
warehouse (note the legend)
46
9
2.2 Electrical symbols
Different countries have different voltages, electrical equipment ratings, and sockets.
Over time the Caribbean will standardise these across the islands.
Conductors
Conductor: general symbol
Flexible conductor
Conductors in conduit
Two conductors
Alternative symbol:
N = number of conductors
"
Connections
Terminal or tag: general symbol
(further detail may have to be
given as shown below)
Hinged or bolted contacts
Link normally closed,
with two bolted contacts
Earth
Earth connection
Conductors & connection
Crossing of conductors
(no electrical connection)
Junction of conductors
Double junction of
conductors
Cells and batteries
Symbol for primary or secondary
cell (long line represents the
positive pole)
A number of cells in series
Alternative symbol showing a
battery of primary or secondary cells
47
Switches
Single-pole, one-way switch
Two-pole, one-way switch
Cord-operated single-pole,
one-way switch
Two-way switch
Intermediate switch
Time switch
Dimmer switch
Normally closed
Normally open
Make contact (normally open)
-1 way
Two-pole switch mechanical link
- neon switch
Three-pole switch mechanical link
- rotary
Changeover contact
-2 way
48
Lamps
Lighting point or lamp:
general symbol
Wall lamp
Emergency (safety)
lightning point
Spotlight
Fluorescent lamps single
Fluorescent lamps groups
Miscellaneous
Thermostat:
block symbol
t
Restricted access push
button for fire alarm
Bell
Indicator panel
N = number of ways
N
Clock
Fan
Speaker
Microphone
49
Rotating machines
Alternating current generator
G
Direct current generator
G
Motor: general symbol
M
Socket outlets
Socket outlet:
General symbol
Switched socket outlet
Multiple socket outlet
example: for 2 plugs
2
or
Multiple switched
socket outlet
Example: for 2 plugs
Figure 2.6 Common electrical symbols using international standards
50
2.3 Scale drawings
Working drawings are the documents referred to when
installing electrical circuits. For an installation to be
completed accurately there needs to be plans or
drawings of where everything should go. They are drawn
to scale and contain internationally recognised symbols.
When we draw to scale, to retain accuracy, everything
needs to be made smaller by the same amount. The
most common scales in electrical installation are 1:20,
1:50, 1:100, meaning everything is a 20th, a 50th or a
100th of its normal size, respectively.
A materials list gives the electrical items that are needed
in the installation. An architect or the customer will
have specific requirements about the equipment to be
installed, these are listed on specification sheets or
‘specs’ which give the technical details.
1:10
1:100
1:5
1:50
0
1
2
3
4
5 cm
0
10
20
30
40
50 cm
0
1
2
3
4
5m
0
100 mm
200 mm
0
1m
2m
Why don’t you?
Figure 2.8 Different scales
(a)
(b)
(c)
Figure 2.7 The location
of several electrical
components in a room;
(a) the room, (b) the
appropriate location of
various components, (c) the
position of the components
on a plan of the room
Draw the plans for a new car park lighting installation at Bay View Hotel.
Guests have complained about security and poor visibility in the car park and
the management want to install lighting. They want 12 lights equally spread
around the car park, which measures 200 metres by 400 metres. The cable run
from the distribution board to the first light is 16 metres and an allowance for
termination of 1 metre for each of the 12 lights is needed.
1
Make a scale drawing of the car park and show the positions of all 12 lights.
The first light is located in a corner position. Use the appropriate electrical
symbols.
2
What is the total length of cable required and the total area of the car park?
3
Using information from local suppliers/internet, work out a materials list for
the complete installation (lights, cables, lamps, cable clips, etc.) with prices
to present to the management.
51
3. Electrical power, power and machines
In this section we are going to look at how electricity is produced in large quantities to
meet the needs of both households and industry.
3.1 How is electricity produced?
Electricity is generated in power stations, by turning the shaft of a three-phase alternator.
For example, water is heated until it becomes high-pressure steam and then this is
forced onto the vanes of a steam turbine, which in turn rotates the alternator.
In order to create the steam we use different primary sources of fuel. Traditionally,
within the Caribbean islands, the majority of electricity is produced from oil-fuelled power
stations; a smaller proportion comes from gas (especially in Trinidad and Tobago), and
then coal. Burning these fossil fuels to produce electricity creates pollution and has a
negative impact on the environment. The Caribbean islands are aiming to produce their
electricity from other, renewable sources such as solar, wind and tidal power.
Table 3.1 Sources of energy
Primary source of energy
Pros
Fossil fuels – e.g. oil, gas,
coal
●
Produce vast quantities
●
Established technology
●
Relatively cheap to
produce
Mineral fuel (nuclear
power)
Did you
know?
Renewable – solar
(photovoltaic), wind, tidal
and geothermal
52
Cons
●
Loss of electricity
during production and
distribution
●
Not an infinite resource
●
Negative impact on
environment, e.g.
greenhouse gases
●
Risk of contamination
●
Cheap to produce
(although expensive
start-up cost)
●
Quiet
●
Less wastage
●
Smaller power output
●
Using natural resources
●
●
No greenhouse gases
Not constant power
generation
Power plants create a lot of electricity, but a lot is wasted in the production and
distribution of the electricity. Renewable sources are more effective as they are
smaller, and can be sited closer to where the electricity is to be used.
Step-up transformer
switchyard
Fossil fuel
Hydro-plant
Wind farm
Low voltage
transmission
lines
Residential
customer
High voltage
transmission
lines
Business
customer
Residential
transformer
Transmission
tower
Industrial
step-down
transformer
Figure 3.1 The power production process
Electricity goes from the alternator to a transformer because the output of most
alternators is about 25 000 V (25 kV) and we need to transform it to:
●
132 kV for the island grid system
●
66 kV and 33 kV for secondary transmission to towns
●
11 kV for high voltage distribution for industrial requirements
●
415/400 V for commercial properties
●
230/110 V for household supplies.
Transmission at very high voltage results in a reduction in current and therefore a
reduction in power line loss due to the heating effect. Without this, we would otherwise
need to install very large cables and switchgear which would be very expensive.
Why don’t
you?
At this point, the electricity is fed into the island’s distribution system – a network
of overhead and underground power lines that link power stations together and are
interconnected throughout the island. In Jamaica, the eight generation stations transmit
at 138 kV to local distribution points, and 69 kV to 24 kV to feeders which also form part
of the distribution network.
Check out the interactive map showing Jamaica’s electrical distribution on:
http://www.myjpsco.com/about-us/jamaicas-electricity-grid/
What new power plants are available?
If a fault develops in any one of the contributing power stations or transmission lines,
then electricity can be requested from another station on the system. Electricity is
transmitted around the grid, mainly by the use of steel-cored aluminium conductors,
which are suspended from steel pylons.
We use overhead cables for three main reasons:
●
The cost of installing cables underground is excessive.
●
Air is a very cheap and readily available insulator.
●
Air also acts as a coolant for the heat being generated in the cable.
53
Electricity is then ‘taken’ from the island’s grid via a series of appropriately located
transformer sub-stations. These will eventually transform the grid supply back down to
11 kV and then distribute electricity at a local level. The 11 kV supply is transformed
down to 400 V and distributed to the customer via a network of underground radial
circuits or, in some rural areas, using overhead lines.
Distribution to the customer
Once the electricity has left the local sub-station, it will eventually arrive at the customer
at the main intake position where these will be found:
●
a sealed over-current device that protects the supply company’s cable
●
an energy metering system to determine the customer’s electricity usage.
It is after this point that the consumer’s installation is reached. This must be controlled
by a main switch, which is located as close as possible to the supply company
equipment and is capable of isolating all live conductors. In the average domestic
installation, this device is merged with the means of distributing and protecting the final
circuits in the consumer unit.
Did you know?
Electricity can be generated through different methods:
●
Friction, e.g. production of static electricity when a balloon is rubbed against
clothing
●
Pressure, e.g. some cigarette lighters
●
Heat, e.g. boiling water to generate steam to turn a turbine
●
Light, e.g. photovoltaic (solar) light generates electricity
●
Chemical action, e.g. in a primary or secondary cell (battery)
●
Electromagnetic induction, e.g. an alternator in a power station.
Renewable energy sources
Key terms
Renewable energy sources produce electricity without being used up. The sources are
either quickly replaceable or have effectively unlimited lifespans. Several examples are
shown in Table 3.2.
54
Fossil fuels – formed by a natural process over millions of years and include
petroleum, coal, and natural gas.
Transformer – an electrical machine with no moving parts which is used to
change the value of an alternating voltage.
Renewable energy – energy which is naturally replenished on a human
timescale, such as sunlight, wind, rain, tides, waves, and geothermal heat.
Table 3.2 A summary of some renewable energy sources in the Caribbean
Energy source
Example
How it operates
Key advantages
Key disadvantages
Solar power
There are no
large-scale solar
power plants in the
Caribbean but many
new buildings have
panels installed. Most
homes in Barbados
have solar water
heaters on their roofs.
The energy
from sunlight
is converted
directly to
electricity using
semiconductor
cells.
No fuel required.
Plenty of sunlight
in the Caribbean.
Electricity can be
produced on the
building in which it
is to be used.
Can’t produce electricity
at night so battery
storage required.
Quite expensive to build.
Hydroelectricity
The Dominican
Republic has 20 dams
providing 10% of the
electricity needs.
Water is trapped
behind a dam
and used to
drive turbines as
it is allowed to
escape.
No fuel required.
Can produce
large amounts
of electricity very
quickly.
Floods large amounts of
land which is a precious
resource on islands.
Only small- and mediumscale systems can be
used on Caribbean
islands as there are few
large valleys or rivers.
Tidal power
Tidal flows around
the Caribbean could
provide energy but are
not used currently.
Water is trapped
in estuaries and
drives turbines.
No fuel required.
Predictable energy
output.
There are very few
suitable estuaries in
the Caribbean islands.
Alters habitats and
wildlife.
Wind turbines
The Wigton wind farm
on Jamaica produces
20 MW of electricity.
The wind spins
turbine blades
mounted on
towers.
No fuel required.
Plenty of wind
around island
systems. Largescale wind farms
or small-scale
local generation
possible.
Offshore turbines
could affect tourism
by spoiling views.
Some noise pollution
is produced, especially
from large wind farms.
Geothermal
energy
Beneath the volcanic
islands of St Kitts
and Nevis there are
vast reserves of
geothermal energy.
Research projects are
underway to develop a
power station.
Thermal energy
released by
radioactive decay
within the Earth
is used to heat
water into steam
and the steam
drives turbines.
No pollution
is caused and
sources are very
reliable.
Only a very few locations
are suitable.
Biofuels
Sugar cane is grown
on many islands and
some could be used
to produce oils or
ethanol to replace
crude oil.
Biological
material, such
as wood, sugar
cane, or ethanol,
is burnt.
New supplies can
be grown fairly
quickly. Does not
add extra carbon
dioxide to the
atmosphere.
The land used to grow
fuel crops may be better
used to grow food for
increasing populations.
(photovoltaic
cells)
55
3.2 Transformers
The transformer is used in electricity distribution, construction work and electronic
equipment. It transforms voltage, which can enter the transformer at one level (input)
and leave at another (output). When the output voltage is higher than the input voltage
we say that we have a step-up transformer, and when the output voltage is lower than the
input, we say that we have a step-down transformer.
Transformers make use of an action known as mutual inductance. For example, two coils,
A and B, are placed side by side, but not touching each other. Coil A has been connected
to a d.c. supply via a switch, and coil B is connected to a meter. If the switch is in the
open position, no current will flow in coil A; if the switch is closed, current will start to
flow in coil A. Current flow through a coil will create a magnetic field, therefore, as the
current in coil A increases up to its maximum value, it creates a changing magnetic flux.
As long as there is a changing magnetic flux, there will be an e.m.f. ‘induced’ into coil B
and this would be detected on the meter. The meter would briefly indicate a value of
e.m.f. when the switch is closed, and then quickly return to zero.
This is because it is the rising (changing) current that causes the change of magnetic
flux. Once the current is at its maximum value, it has stopped rising (changing) and
therefore there is no change of magnetic flux. If there were no changing magnetic flux
there can be no e.m.f. induced in coil B.
If the two coils were now wound on an iron core, we would find that the level of magnetic
flux is increased, and the level of mutual inductance is also increased. If an a.c. supply
instead of d.c. is used, the magnetic flux produced in coil A would be constantly changing
with the frequency of the supply, and therefore constantly producing an e.m.f. in coil B.
An a.c. supply is needed to allow transformers to operate correctly.
There are two main types of transformer construction available: core and shell. A third
type of transformer construction, toroidal, is also available.
Core-type transformers
The transformer in Figure
3.2 is double wound:
●
the input is wound on one
side of the iron core, the
primary winding
●
the output is wound
on the other side, the
secondary winding.
Primary winding
P
Ip
Vp
Secondary winding
Np
Ns
Is
Vs Load
N
Figure 3.2 A single phase, double wound, core-type
transformer
The number of turns in each winding, the turns ratio, will affect the induced e.m.f.
Np is the number of turns in the primary winding, and Ns is the number in the secondary.
When voltage (Vp) is applied to the primary winding, it will cause a changing magnetic flux
to circulate in the core. This changing flux will cause an e.m.f. (Vs) to be induced in the
secondary winding. Assuming that we have no losses or leakage (i.e. the transformer
is 100% efficient), then power input will equal power output and the ratio between the
primary and secondary sides of the transformer can be expressed as follows:
Vp
Vs
56
=
Np
Ns
I
= s
Ip
A transformer has no moving parts, so if the following apply, it is a very efficient piece of
equipment:
●
Use laminated (layered) steel cores, not solid metal. In a solid metal core ‘eddy
currents’ are induced which cause heating and power losses. Laminated cores, where
each lamination is insulated, help to reduce this effect.
●
Use soft iron with high magnetic properties for the core.
●
Windings should be made from insulated, low resistance conductors. This will prevent
short circuits occurring either within the windings, or to the core.
Copper losses
The resistance of the windings will cause the currents passing through them to cause a
heating effect and subsequent power loss. This power loss (in watts) can be calculated
using the formula:
Pc = I2 × R
Iron losses
Losses take place in the magnetic core of the transformer, normally caused by eddy
currents (small currents which circulate inside the laminated core of the transformer), and
hysteresis (the force necessary to overcome magnetic flux density that remains in the iron
core). Eddy currents are reduced by using a laminated core construction. Hysteresis is
reduced by adding silicon to the iron from which the transformer core is made.
Reducing leakage
Some of the magnetic flux being produced by the primary winding
will not react with the secondary winding and is often referred to
as ‘leakage’. Splitting each winding across the sides of the core
reduces this leakage, see Figure 3.3.
Shell-type transformers
In the shell-type transformer, both windings are wound onto the
central leg of the transformer and the two outer legs are then
used to provide parallel paths for the magnetic flux.
Vp
Vs
Figure 3.3 Splitting
the winding
The autotransformer
The autotransformer uses the principle of ‘tapped’
windings in its operation. Some devices are supplied P
with the capability of providing more than one
output, such as small transformers for calculators,
Vp
musical instruments or doorbell systems. Tapped
VS
connections are the normal means by which this is
achieved. A tapped winding means that a connection N
to the winding has been brought out to a terminal.
By connecting between the different terminals, we
Figure 3.4 An autotransformer
can control the number of turns that will appear in
that winding and we can provide a range of output
voltages. An autotransformer has only one tapped
winding, and the position of the tapping on that
winding will dictate the output voltage.
57
One of the advantages of the autotransformer is that, because it only has one winding,
it is more economical to manufacture. However, if the winding ever became broken
between the two tapping points, then the transformer wouldn’t work and the input
voltage would appear on the output terminals.
Instrument transformers
Instrument transformers are used in conjunction with measuring instruments, because it
would be very difficult and expensive to design normal instruments to measure the high
currents and voltages that we find in certain power systems. There are two types of instrument
transformer, both being double wound: the current transformer, and the voltage transformer.
The current transformer
The current transformer (c.t.) has very few turns on its
primary winding so that it doesn’t affect the circuit to be
measured, with the actual meter connected across the
secondary winding. Care must be taken when using a c.t.
never to open the secondary winding while the primary is
‘carrying’ the main current. If this happened, a high voltage
would be induced into the secondary winding. Apart from the
obvious danger, this heat build up could cause the insulation
on the c.t. to break down. A c.t. could be used inside a
switchgear panel, where the busbar would pass through the
centre of the c.t. and the meter would be connected across
the two terminals at the top of the c.t.
Figure 3.5 A current
transformer
The voltage transformer
This is very similar to the standard power transformer, in that it is used to reduce the
system voltage. The primary winding is connected across the voltage that we want to
measure and the meter is connected across the secondary winding.
Three-phase transformers
A three-phase transformer can be best thought of as three single-phase transformers,
which have then been connected together in a three-phase arrangement. Like their
single-phase counterparts, three-phase transformers may be step-up or step-down.
The arrangement of the
windings (3 × primary and
3 × secondary) can then
follow one of the following
four patterns, where the
windings are given as
primary then secondary:
●
Star – Star
●
Delta – Delta
●
Star – Delta
●
Delta – Star
R
Supply Y
B
N
N
R Load
Y
B
Figure 3.6 A three-phase transformer
The electricity supply companies normally use Delta in high voltage transmission and
then use a Delta–Star transformer in the local sub-stations.
58
A transformer having a turns ratio of 2 : 7 is connected to a 230 V supply.
Calculate the output voltage.
Transformer ratios are given in the order primary then secondary. Therefore in
this example for every 2 windings on the primary winding, there are 7 on the
secondary.
Worked Example 1
We use the formula:
Vp
Vs
=
We transpose this to get: Vs =
Np
Ns
Vp × Ns
Np
We insert the ratio into the formula:
Vs =
Vp × Ns
Np
=
230 × 7
= 805 V
2
Lets say that we know the number of turns in the windings to be 6 in the
primary and 21 in the secondary. If we now apply this to the formula we get:
Vs =
Vp × Ns
Np
=
230 × 21
= 805 V
6
A single-phase transformer, with 2000 primary turns and 500 secondary turns,
is fed from a 230 V a.c. supply. Find:
a) the secondary voltage
b) the volts per turn.
Worked Example 2
a)
Vp
Vs
=
Np
Ns
Using transposition, re-arrange the formula to give Vs
Vs =
Vs =
Vp × Ns
Np
230 × 500 115 000
therefore Vs = 57.5 V
=
2000
2000
b) In the primary
In the secondary
Vp
Np
=
230
= 0.115 volts per turn
2000
Vs
57.5
=
= 0.115 volts per turn
Ns
500
59
A single-phase transformer is being used to supply a trace heating system.
The transformer is fed from a 230 V 50 Hz a.c. supply and needs to provide an
output voltage of 25 V.
If the secondary current is 150 A and the secondary winding has 50 turns, find:
a) the output kV A of the transformer
b) the number of primary turns
Worked Example 3
c) the primary current
d) the volts per turn.
a) kV A =
b) If
c) If
Vp
Vs
Vp
Vs
=
volts × amperes Vs × Is 25 × 150
=
=
= 3.75kV A
1000
1000
1000
Np
Ns
then by transposition Np =
Vs
=
230 × 50
= 460 turns
25
I
V ×I
25 × 150
= s then, by transposition Ip = s s =
= 16.3 A
Ip
Vp
230
d) In the primary
Vp
Np
In the secondary
60
Vp × Ns
=
230
= 0.5 volts per turn
460
Vs
25
=
= 0.5 volts per turn
Ns 50
A step-down transformer, having a ratio of 2 : 1, has an 800 turn primary winding
and is fed from a 400 V a.c. supply. The output from the secondary is 200 V
and this feeds a load of 20 Ω resistance. Calculate the power in the secondary
winding, and the power in the primary winding.
Remember: The formula for power is P = V × I, and we can use Ohm’s law to find
V
current, I = .
R
Therefore, the current in the secondary winding:
Worked Example 4
Is =
Vs 200
=
= 10 A
Rs
20
The power generated in the secondary winding:
P = V × I = 200 × 10 = 2000 W = 2 kW
To find the current in the primary winding we transform:
Vp
Vs
I
= s
Ip
l × Vs
Ip = s
Vp
=
10 × 200 2000
=
= 5A
Vp
400
Power generated in the primary winding:
P = V × I = 400 × 5 = 2000 W = 2 kW
(This proves that power input equals power output.)
61
4. Electrical installations
4.1 Introduction
The Institute of Engineering and Technology (IET)
Link
The Institute of Engineering and Technology (IET) was previously known as the Institute of
Electrical Engineers (IEE).
This website gives the wiring regulations:
http://electrical.theie.org/wiring-regulations/
Founded in 1871, the IEE is the largest professional engineering society in Europe and
has a worldwide membership of just under 130 000.
As well as setting standards of qualifications for professional electrical, electronics,
software, systems and manufacturing engineers, the IET prepares regulations for the
safety of electrical installations for buildings. The IET Wiring Regulations (BS 7671) have
now become the standard for the UK and many other countries.
It also recommends the requirements for ships and offshore installations internationally.
The National Electrical Code (NEC)
Link
The National Electrical Code (NEC) is a regionally adoptable standard for the safe
installation of electrical installations and equipment in the US.
This website shows the National Electric Code:
http://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-ofcodes-and-standards?mode=code&code=70
It is part of the published National Fire Codes series. In some cases, the NEC is
amended, altered and may even be rejected in lieu of regional regulations as voted on by
local governing bodies.
The NEC deals with the installation of these, in commercial, residential, and industrial
premises:
●
electrical conductors
●
equipment
●
signalling and communications conductors
●
equipment
●
fibre optical cables.
62
JS 21 Standards
Link
This code incorporates key technical content and requirements from the latest versions of
ASHRAE and other standards, which are complex second or third generation standards.
This website shows the JS 21 Standards:
https://law.resource.org/pub/jm/ibr/js.217.1994.pdf
The standards cover the following areas:
●
energy
●
comfort
●
productivity.
It encourages the design of new buildings and the retrofit of existing buildings so that
they may be constructed, operated and maintained in a manner that reduces the use of
energy without constraining the building function, or the comfort or productivity of the
occupants, and with appropriate regard for cost considerations.
Minimum standards and criteria are provided to conserve energy when designing new
buildings and major retrofits.
Recommended practices provide guidance for energy conserving design that
demonstrates good professional judgment and exceeds minimum standards and criteria.
Calculation methods and tools provide methods, compliance forms, and tools for
determining compliance with these criteria and minimum standards. The tools include
both manual and microcomputer-based calculation procedures.
International codes
Link
The code provides minimum standards to safeguard life and property by regulating
and controlling the design, construction and installation, quality of materials location
operation and maintenance, and use of electrical installations and equipment.
This website shows the international electric code:
http://www.slideshare.net/gueste69b7bd/2003-international-electrical-code
63
4.2 Circuits
Ring circuits
In a ring circuit the phase, neutral and circuit protective conductors are connected to
their respective terminals at the consumer unit and looped into each socket outlet in
turn, and then returned to their respective terminals in the consumer unit. Each socket
outlet has two connections back to the mains supply.
For a standard domestic ring circuit:
●
An unspecified number of socket outlets may be provided. Each socket outlet of twin
or multiple sockets is regarded as one socket outlet.
●
The floor area served by a single 30 A or 32 A ring final circuit must not exceed 100 m².
Ring conductor
rating not less
than 0.67 times
fuse rating
Cable size not
less than ring
cable
Non-fused spur
P
Cable size not
less than ring
cable
E
Spur box
N
Fused spur
Fuse to suit
cable size
Stationary
appliance
Figure 4.1 A domestic ring circuit
●
Consideration must be given to the loading of the circuit, especially kitchens which
may require a separate circuit.
●
When more than one ring circuit is installed in the same premises, the socket outlets
installed should be reasonably shared amongst the ring circuits so that the assessed
load is balanced.
Permanently connected equipment
Permanently connected equipment, e.g. cookers and water heaters, should be
individually protected by a fuse and controlled by a switch or protected by a circuit
breaker.
64
Fused and non-fused spurs
Spurs may be installed on a ring circuit. These may
Ring circuit
be fused, but it is more common to install non-fused
Single socket
spurs connected to a circuit at the terminals of socket
outlet
outlets, at junction boxes, or at the origin of the circuit
Ring circuit
in the distribution board. A non-fused spur may supply
only one single or one twin socket outlet, or one item
Double socket
of permanently connected equipment, e.g. a water
outlet
heater. The total number of non-fused spurs should not
Figure 4.2 The connection
exceed the total number of socket outlets and items
of spurs
of stationary equipment connected directly to the ring
circuit. The size of conductor for a non-fused spur must
be the same as the size of conductor used on the ring circuit.
A fused spur is connected to a circuit through a fused connection unit. The fuse should
be related to the current carrying capacity of the cable used for the spur. The total
number of fused spurs is unlimited.
Fuse connection unit
(not exceeding 13 A)
Appliance
Ring
circuit
Figure 4.3 A fused connection unit
Radial circuits
In a radial circuit the conductors do not form a loop but finish at the last outlet. There is no
maximum or specified number of sockets in a radial circuit, but this will be determined by
the estimated load and safety requirements. Local wiring regulations must be adhered to.
A2 radial circuit
The current rating of the cables is determined by the rating of the overcurrent protection
device, i.e. 30 or 32 A amp fuse or circuit breaker. This means that copper cables of not less
than 4.0 mm² may be used and the floor area served may not exceed 75 m². See Figure 4.4.
A3 radial circuit
In domestic premises an A3 radial circuit is wired in 2.5 mm² copper cable and protected
by a 20 A fuse or circuit breaker. Local regulations will specify the floor area within which
any number of socket outlets may be installed. See Figure 4.5.
A2 type circuit
Floor area
maximum 75 m2
A3 Type circuit
Floor area
maximum 75 m2
4 mm2
30 A
Figure 4.4 A2 radial circuit
2.5 mm2
20 A
Figure 4.5 A3 radial circuit
65
Emergency lighting
Emergency lighting should be planned, installed and maintained to the highest standards
of reliability, so that it will operate when called into action.
Emergency lighting is not required in private homes where occupants are familiar with
their surroundings, but in public buildings where people are in unfamiliar surroundings
and need a well illuminated and easily identified exit route in an emergency.
Emergency lighting is provided for two reasons:
●
to illuminate escape routes, called ‘escape’ lighting
●
to enable a process or activity to continue after a normal light failure, called ‘standby’
lighting.
Escape lighting schemes should be planned to provide light for escape routes from
a point within a building to a final exit. Obstructions should be visible in lower levels
of illumination. The level of illumination required is 0.2 lux, which is similar to the
brightness of the full Moon.
Standby lighting is required in buildings where an operation or process once started must
continue, even if the main lighting fails, e.g. hospital operating theatres and in industry.
Maintained emergency lighting
In this type of system the luminaire is supplied with a single light source which may be
switched on and off as required. The emergency lamps are continuously lit using the
normal supply when it is available, and change over to an alternative supply when the
mains supply fails.
The advantage of this system is that the lamps are continuously proven healthy and
any fault is immediately obvious. Maintained emergency lighting is normally installed in
places of entertainment where the normal lighting may be dimmed or turned off whilst
the building is occupied, e.g. in theatres, cinemas, nightclubs. The emergency lamps are
wired in parallel from a low voltage supply.
Normally
open relay
Emergency supply
Mains
supply
Transformer
Rectifier
Alarm bell
Emergency lamps
Figure 4.6 Maintained emergency lighting
66
Non-maintained emergency lighting
In this type of circuit the emergency lamps are only illuminated if the normal mains
supply fails. Failure of the main supply de-energises a solenoid and a relay connects
the emergency lamps to a battery supply, which is maintained in a state of readiness
by a trickle charger from the normal main supply. When the normal supply is restored,
the relay solenoid is energised, breaking the relay contacts, which disconnects the
emergency lamps, and the charger re-charges the battery.
The disadvantage with this type of installation is that broken lamps are not detected
until they are called into operation in an emergency, unless they are regularly maintained.
A battery contained within the luminaire, together with a charger usually provides the
emergency supply and relay making the unit self-contained.
Normally
open relay
Mains
supply
Transformer
Rectifier
Emergency lamps
Emergency supply
Did you
know?
Figure 4.7 Non-maintained emergency lighting
The lux (symbol: lx) is the SI unit of illuminance and luminous emittance,
measuring luminous flux per unit area. It is equal to one lumen per square
metre.
67
Fire alarm systems
A correctly installed fire alarm system installation is of paramount importance. Life
could be lost and property damaged if fire detection and alarm equipment have been
incorrectly connected.
Life protection
Alarms are required in areas where a fire could lead to a high risk to life, e.g. sleeping
areas, store rooms, kitchens, plant rooms, etc., and places where the occupants are
especially vulnerable due to age, illness, or unfamiliarity with the premises.
Property protection
A satisfactory fire alarm system for the protection of property will automatically detect
a fire at an early stage, indicate its location and raise an effective alarm in time to
summon the fire fighting forces (both trained staff and professional fire brigade).
Automatic detectors
When choosing the type of detector to be used in a particular area it is important
to remember that the detector has to discriminate between a fire and the normal
environment existing within the building, i.e. smoking in hotel bedrooms, fumes from
forklift trucks in warehouses, steam from kitchens and bathrooms. There are several
automatic detectors available. See Table 4.1.
Table 4.1 Automatic detectors
Heat detectors
(fixed
temperature type)
●
●
●
Heat detector
(rate of rise type)
●
●
Smoke detector
●
●
Alarm sounders
●
●
68
A simple device designed to activate the alarm circuit once a
predetermined temperature is reached
Operates at either 60 °C or 90 °C
Suitable for rooms where fluctuations in ambient temperature are
commonplace, e.g. kitchen or boiler room
Responds to rapid rises in temperature by sampling
the temperature difference between two heat-sensitive
thermocouples or thermistors mounted in a single housing
Not suitable for boiler room or kitchen where fluctuations
in ambient temperature occur regularly as this can result in
nuisance alarms
The optical smoke or photoelectric smoke detector operates by
means of the light-scattering principle. A pulsed infrared light
is targeted at a photo-receiver, but separated by an angled nonreflective baffle positioned across the inner chamber. When smoke
and combustion particles enter the chamber, light is scattered and
reflected onto the sensitive photo receiver and triggers the alarm.
Detector heads for fire alarm systems should only be fitted
after all work that could create dust is finished.
Normally a bell or electronic sounder, which must be audible
throughout the building in order to alert (and/or evacuate) the
occupants of the building
The alarm sounders should be loud enough to be heard above
normal background noise, but not so high to cause permanent
damage to hearing
Key
term
Thermistor – a device whose resistance quickly decreases with an increase in
temperature.
Wiring systems for fire alarms
Fire detectors and sounders must be wired in a continuous parallel formation
arrangement as shown in Figure 4.8. No spurs are permitted, as this would prove difficult
to monitor for breaks and short circuits. Circuits from the control panel should be as
short as possible as shown in Figure 4.9.
In order to continually monitor both cables and terminations for open and short circuit
conditions, an end of line resistor should be connected across the terminals of the
last sounder. It is wise to leave the wiring of the fire alarm system until most of the
constructional work has been completed. This will help avoid accidental damage
occurring to the cables. Keep the control panel in the packing carton and only remove
when building work has been completed in the area where it is to be mounted, thus
avoiding possible contamination to the unit.
End of line resistors
Smoke
detectors
Sounders
Loop resistance
must be less
than 10 Ω
Smoke
detectors
+
–
Zone 1
Control panel
Figure 4.8 A wiring system for fire alarms
Zone 1
Control panel
Figure 4.9 Circuits for fire alarms
69
4.3 Wiring methods and terminations
PVC-insulated (polyvinylchloride) and sheathed cables are used extensively for lighting
and heating installations in domestic dwellings, being generally the most economical
method of wiring for this work.
Table 4.2 Types of cable and cord
Polyvinylchloride (PVC):
●
Tough, cheap, and easy to work with and install; PVC-insulated/sheathed cable is
the most popular type of cable in current use
●
PVC insulation has its limitations in conditions of excessive heat and cold
●
Can be subject to mechanical damage unless additional mechanical protection is
used in certain situations (e.g. installed in trunking)
●
The most versatile of all the wiring systems
Single-core PVC-insulated unsheathed cable:
●
Made from PVC-insulated solid or stranded copper conductor, coloured red or black;
other colours include blue, green, yellow, white, and green/yellow stripes for use as
the earth
●
A flexible and thin cable; it is generally installed into trunking or conduit for extra
protection
Single-core PVC-insulated and sheathed cable:
●
Suitable for surface wiring where there is little risk of mechanical damage
●
Normally used as ‘meter tails’ for connecting the consumer unit/distribution board
to the supply company’s meter
●
The construction of this cable is PVC-insulated and PVC-sheathed solid or stranded
plain copper conductor
●
The core colours are normally black or red, sheath colours are normally black, red,
or grey; other colours are available
Single-core PVC-insulated and sheathed cable with a circuit protective conductor (CPC):
Used for domestic and general wiring where a (CPC) is required for all circuits
●
Made from PVC-insulated copper conductor laid parallel with a plain copper circuit
protective conductor and PVC-sheathed overall
●
Core colours are red or black, the CPC is plain copper, and sheath colour is normally
white or grey
Key terms
●
70
Polyvinylchloride (PVC) – a material that acts as an insulator, normally the
covering for electrical cables.
Conduit – a tube made from metal or PVC in which insulated conductors are
contained.
PVC-insulated and sheathed flat wiring cables:
●
Used for domestic and industrial wiring where there is little risk of mechanical damage
●
Made from two or three plain copper, solid or stranded conductors, insulated with
PVC and sheathed overall with PVC
●
Core colours for two cores are brown and blue
●
Three cores are brown, black, and grey
●
The sheath colours are normally grey or white
●
The construction of three core cables are exactly as mentioned above with the
inclusion of an uninsulated plain copper earth between the cores of twin cables and
between the grey and black cores of three core cables
Heat-resisting PVC-insulated and sheathed flexible cords:
●
Suitable for use in ambient temperature up to 85 °C; not suitable for use with
heating appliances; construction is plain copper flexible conductors insulated with
heat-resisting (HR) PVC and (HR) PVC-sheathed
●
Core colours are single core brown or blue encased in a white sheath
PVC-insulated and sheathed flat twin flexible cord:
●
Intended for internal, light duty usage, e.g. table lamps, radios and TV sets where
the cable may lie on the floor; should not be used with heating appliances
●
Made from plain copper flexible conductor PVC, insulated two cores are laid parallel
and sheathed overall with PVC
●
Core colours are brown and blue; the sheath colour is white
PVC-insulated bell wire:
●
Used for wiring bells, alarms and other indicators, which operate at extra low voltage
●
Made from one single-core plain soft copper conductor insulated with PVC; twin-core
wire is produced from two single-core wires laid parallel and insulated overall with
PVC compound to form a figure 8 section
●
Standard colour of this wire is white; core identification is by a red or coloured
stripe formed on one core
Cables with thermosetting insulation (XLPE):
●
Thermosetting insulation is used when higher operating temperatures are needed
●
Maximum continuous operating temperature for XLPE is 90 °C compared with 70 °C
for PVC insulation; increased temperature permits a reduction in conductor size if
XLPE insulated cables are used in preference to cables having PVC insulation
●
Used mostly for mains distribution
71
Installing PVC cables
Cables are fixed at intervals using plastic clips, which
incorporate a masonry-type nail. When bending PVC cable
around corners, the radius of the bend should be such
that the cable or conductor does not suffer damage.
Where PVC cables are installed on the surface the cable
should be run directly into the electrical accessory
ensuring that the outer sheathing of the cable is taken
inside the accessory to a minimum of 10 mm. If the cable
is to be concealed, a flush box is usually provided at each
control or outlet position.
Figure 4.10 Cable clips
Cable runs
Cable runs should be planned to avoid cables having to cross one another, which would
result in an unsightly and unprofessional finish. When cables are to be installed in
cement or plaster they should be protected against damage. They should be covered with
metal or plastic channel, or by installing them in oval PVC conduit. Care must be taken
when installing PVC cables to ensure that they are not allowed to come into contact with
gas pipes and water pipes and any other non-earthed metal work. PVC-sheathed cables
should also not come into contact with polystyrene insulation, as a chemical reaction
takes place between the PVC sheath and the polystyrene resulting in migration of
polymers with the cable known as ‘marring’.
Terminating cables and flexible cords
The entry of the cable end into an accessory is known as a termination. In the case
of a stranded conductor the strands should be twisted together with pliers before
terminating. Care must be taken not to damage the wires, and to ensure the terminal
or socket is correctly connected. When current flows in a conductor, heat is generated;
the consequent expansion and contraction may be sufficient to allow a poorly connected
conductor to be pulled out of the terminal or socket. This could lead to an electrically
dangerous situation. One or more strands of wire (known as a whisker) left out of the
terminal or socket will reduce the effective cross-sectional area of the conductor at that
point. This may result in increased resistance and probable overheating, potentially
leading to a fire.
Pillar terminals
A pillar terminal has a hole through its side into which the
conductor is inserted and secured by a set-screw. If the
conductor is small in relation to the hole it should be doubled
back. When two or more conductors are to go into the same
terminal they should be twisted together. Care should be taken
not to damage the conductor by excessive tightening.
Screwhead, nut and washer terminals
Figure 4.11 A pillar
terminal
When fastening conductors under screwheads or nuts, it is best to form a conductor
end into an eye using round-nosed pliers that should be slightly larger than the screw
shank but smaller than the outside diameter of the screwhead, nut or washer. The eye
should be placed in such a way that rotation of the screwhead or nut tends to close the
72
joint in the eye. If the eye is put in the opposite way round the motion of the screw or nut
will tend to untwist the eye and will probably result in an imperfect contact.
Figure 4.12 Screwhead, nut and washer
terminals
Claw washers
In order to get a better connection, claw washes can be used. Lay the looped conductor
in the pressing. Place a plain washer on top of the loop and squeeze the metal points
flat using the correct tool.
Figure 4.13 Claw washers
Strip connectors
The conductors to be terminated are clamped by means of grub screws in connectors,
which are usually made of brass and mounted in a moulded insulated block. The
conductors should be inserted as far as possible into the connector so that the pinchscrew clamps the conductor. A clean, tight termination is essential in order to avoid high
resistance contacts resulting in overheating of the joint.
Figure 4.14
Solderless lugs
Solderless lugs are used extensively for terminating smaller-sized cables. Lugs of this
type are made from solid copper and are tinned. They are fastened to cable ends by
crimping.
73
4.4 Lighting circuits
There are numerous switching arrangements which make up a lighting circuit. These
circuits can be installed using either multi-core or single-core cables. If using single-core
cables, this should be installed in conduit or trunking for mechanical protection. Here are
the most common wiring arrangements using multi-core cables.
Multi-core cables (referred to as twin and earth) are basically a three-core cable – a
phase, a neutral and an earth conductor. Both the phase and neutral conductors consist
of copper conductors insulated with coloured insulation (the earth is a non-insulated
copper conductor sandwiched in between the phase and neutral). The outer white or grey
covering is the sheath, and its main purpose is to prevent mechanical damage occurring
to the insulation of the conductors. This cable is often used for wiring domestic and
commercial lighting circuits, normally using 1.0 mm² or 1.5 mm² cable. Using this type of
cable is slightly more complicated because the cable will always consist of a brown, blue,
and earth conductor.
Loop in ceiling rose
Loop in ceiling rose is the most common method used to install lighting circuits. The
power comes into the ceiling rose from either the consumer unit or the last lamp in the
circuit. The power is then sent to the next lamp via the connecting blocks in the ceiling
rose. The live wire coming in is connected to the live wire going out. This is the same for
the neutral and earth wires.
From previous light
Earth terminal
To next light
To switch
From
switch
(Live)
Switch
terminal
(Live)
Neutral
terminal
Live terminal
To light
Figure 4.15 Loop in ceiling rose
In the simplest situation, one switch controls one light. However, you may want lights to
be controlled by one or more switches in different locations around the room. Figure 4.16
shows a simple loop system in a three-room property, and Figure 4.17 shows a one-way
switching arrangement.
74
Ceiling rose C
Ceiling rose A
Ceiling rose B
Consumer unit
(fuse board)
Lamp
Ro
om
3
Light switch
2
om
Ro
Figure 4.16 A loop system
1
om
Ro
Figure 4.17 A one-way switching arrangement
Two-way switching
Where more than one switch position is required for one or more lights a different
switching arrangement must be used. The most common is the two-way switch circuit
which allows lights to be switched on or off from two positions. This arrangement is most
commonly used on stairways and corridors. The switch feed feeds one two-way switch
and the switch wire goes from the other two-way switch to the lights. Two wires known as
‘strappers’ join the two-way switch.
This method of switching using multi-core cables requires the use of four-core cable,
which has three coloured (brown, grey and black) and insulated conductors and a bare
earth conductor. This type of cable is normally only stocked in 1 mm² and used in a
lighting circuit where its application is for switching that requires more than one switch
position, i.e. two-way and intermediate switching. It is also used for converting one-way
switching into two-way switching.
Figure 4.18 Two-way switching using multi-core cables
75
Intermediate switching
If more than two switch positions are required, then intermediate switches must be
used. The switches are wired in the ‘strappers’ between the two-way switches. The
action of the intermediate switch is to cross connect the ‘strapping’ wires. Any number
of intermediate switches may be used and they are all wired into the ‘strappers’. For
example, you may find this switching arrangement in long corridors in hotels.
A
Power feed in from fuse board or previous light
Twin and earth switch drop from
C ceiling rose or juction box
Switch
back box
Switch
face plate
C
See Note A about this earth loop
3 core and earth between light switches
L2 L2
Terminal
block
L1 L1
E
Switch
back box
3 core and earth between light switches
Switch face plate
COM
L1 L2
Figure 4.19 Intermediate switching
76
9
-2 -3 -4 -5 -6 -7
COM
L1 L2
D
B
Feed out to next light
in the radial circuit
-1
-8
Ceiling rose
Live and neutral
L to lamp holder N
Notes
Note A – if you are using metal light switches you should
include this additional earth loop from the switch back boxes
to the switch plates on both switches
Wiring with a junction or joint box
Wiring with a junction box has now been superseded by the loop in method, but older
properties may still have a junction box installed. Care should be taken when wiring
junction boxes:
●
The protective outer sheath should be taken inside the junction box entries, to a
minimum of 10 mm.
●
Where terminations are made into a connector only sufficient insulation should be
removed.
●
Sufficient slack should be left inside the joint box to prevent excess tension on
conductors.
●
Cables should be inserted so that they are not crossing, they should be neat and
fitted so that the lid fits without causing damage.
●
Correct size joint boxes should be used.
●
Joint boxes should be secured to a platform fitted between the floor and ceiling joists.
Switch 1
Switch feed
Switch wire
Lamp 1
Phase
Switch wire
Neutral
Neutral
CPC
Joint box
Figure 4.20 The joint box method of control for one light
77
Did you know?
4.5 Lamps
Illumination using electricity has been available for over 100 years, though it
has changed in many ways. The first type of electric lamp was the ‘arc lamp’
which used electrodes to draw an electrode through the air. This was quite an
unsophisticated use of electricity and resulted in many accidents and fires.
The first lamp that was developed for indoor use was the carbon filament lamp.
Although this was a dim lamp by modern standards, it was cleaner and far less
dangerous than the exposed ‘arc lamp’.
Incandescent lamps
The General Lighting Service (GLS) lamp is commonly referred to
as the ‘light bulb’. At its ‘core’ is a very thin tungsten wire, which
is formed into a small coil and then coiled again. A fine filament of
wire is connected across an electrical supply and is made to heat
up until it is white-hot and gives off light. The filament wire reaches
a temperature of about 2500–2900 °C. These lamps are very
inefficient as only a small proportion of the available electricity is
converted into light. Most of the electricity is converted into heat as
infrared energy. The light bulb has many advantages, including:
●
comparatively low initial costs
●
immediate light when switched on
●
no control gear
●
can be easily dimmed.
Figure 4.21 The
light bulb
Tungsten halogen lamps
Tungsten halogen lamps were introduced in the 1950s. The tungsten filament is enclosed
in a gas-filled quartz tube together with a carefully controlled amount of halogen such as
iodine. When an electric current heats the filament the tungsten is evaporated from the
surface of the filament and is carried by convection currents to the comparatively cool walls
of the lamp. Here it combines with the halogen, which has vaporised and forms a tungsten
halide. This compound returns to the filament where the high temperatures convert it back
to tungsten and the halogen gas that is left sinks to be drawn again through the filament
in a continuing cycle. The quartz glass bulbs must not be handled, as contamination from
the skin will form opaque patches on the glass when it becomes hot. This can cause
premature failure of the lamp, so when fitting these types of lamps it is advisable to leave
the wrapping around the lamp until it is fitted in place, or alternatively handle only by the
ends. Accidental contact with the glass bulb should be cleaned with a solvent such as
carbon tetrachloride. The linear type of lamp must be operated within 4 ° of the horizontal
to prevent the halogen vapour migrating to one end of the tube causing early failure. These
types of lamps have many advantages, including:
●
increased lamp life
●
100% lumen output
●
increase in efficacy (up to 23 lumens per watt)
●
reduction in lamp size.
78
Figure 4.22 The linear tungsten
halogen lamp
Low-pressure mercury vapour lamps
The fluorescent lamp, or low-pressure mercury vapour
lamp, consists of a glass tube filled with a gas such as
Figure 4.23 The
krypton or argon and a measured amount of mercury
fluorescent lamp
vapour. Coated on the inside of the glass tube is a
phosphor and at each end there is a set of oxide-coated
electrodes, cathodes. When a voltage is applied across the ends of a fluorescent tube the
cathodes at the ends of the tube heat up and this forms a cloud of electrons, which ionise
the gas in their immediate vicinity. The voltage to carry out this ionisation must be much
higher than the voltage required to maintain the actual discharge across the lamp. Several
methods are used to achieve this high voltage, usually based on a transformer or choke.
This ionisation is then extended to the whole length of the tube so that the arc strikes and
is then maintained in the mercury, which evaporates and takes over the discharge. The
mercury arc being at low pressure emits little visible light but a great deal of ultraviolet,
which is absorbed by the phosphor coating and transformed into visible light. The cathodes
are sealed into each end of the tube and consist of tungsten filaments, which are coated
with an electron emitting material. Larger tubes incorporate cathode shields. The gas in
standard tubes is a mercury and argon mix, although some lamps (the smaller ones and
the new, slim energy-saving lamps) have krypton gas in them. The phosphor coating is a
very important factor affecting the quantity and quality of light output.
When choosing different lamps there are three main areas to be considered:
●
lamp efficacy
●
colour rendering
●
colour appearance.
The glow starter circuit
In the glow starter, normally open contacts are mounted on bimetal strips and are
enclosed in an atmosphere of helium gas. When switched on, a glow discharge takes
place around the open contacts in the starter, which heats up the bimetal strips
causing them to bend and touch each other. This puts the electrodes at either end
of the fluorescent tube in circuit and they warm up, giving off a cloud of electrons;
simultaneously an intense magnetic field is building up in the choke, which is also
in circuit. The glow in the starter ceases once the contacts are touching so that the
bimetal strips now cool down and they spring apart again. This momentarily breaks the
circuit causing the magnetic field in the choke to collapse, and provide the high voltage
required for ionisation of the gas and enabling the main discharge across the lamp
to take place. The voltage across
Choke
the tube under running conditions
Lamp
is not sufficient to operate the
starter, and so the contacts
remain open. The resistance of
PF
the mercury gets less and less
correction
as it warms up and conducts
capacitor
Starter
more current. This could lead to
switch
disintegration of the tube; however,
Figure 4.24 Glow starter switch circuit
the choke has a secondary
79
function that of a current limiting impedance. Limiting the current across the lamp and
keeping it in balance is one reason why it is often referred to as ballast. The capacitor
shown connected across the supply terminals is to correct for the poor power factor that
has been created by the inductor. This type of starter may not succeed first time and can
result in the characteristic flashing when switching on.
Stroboscopic effect
A simple illustration of the stroboscopic effect is watching the wheels on a horse-drawn
cart on television. You may have noticed the wheels appearing stationary, or even going
backwards. This phenomenon is brought about by the fact that the spokes on the wheels
are being rotated at about the same revolutions per second as the frames per second
of the film being shot. This effect is known as the ‘stroboscopic effect’ and can also be
produced by fluorescent lighting. The discharge across the electrodes is extinguished
100 times per second producing a flicker effect. This flicker is not normally observable,
but can cause this stroboscopic effect which can be dangerous. For instance, rotating
machinery illuminated from a single source will appear to have slowed down, changed
direction of rotation or even stopped. This is a potentially dangerous situation if the
phenomenon is not understood or recognised. Also certain frequencies can induce
degrees of drowsiness, headaches, eye fatigue and, in extreme cases, disorientation.
However, this stroboscopic effect can be harnessed to check the speed of a CD player
and the speed of a motor vehicle for calibration purposes.
Utilising one of the following methods can counteract the stroboscopic effect:
1. Tungsten filament lamps can be fitted locally to the lathe, pillar drilling machines, etc.
This will lessen the effect, but will not eliminate it completely.
2. Adjacent fluorescent fittings can be connected to different phases of the supply.
Because in a three-phase supply the phases are 120 ° out of phase with each other,
the light falling on the machine will arrive from two different sources. Each of these
will be flickering at a different time, which will interfere with each other and reduce
the stroboscopic effect.
3. The use of high-frequency
fluorescent lighting reduces
the effect by about 60%.
4. Twin lamps can be wired
on lead-lag circuits, thus
counteracting each other.
The lead-lag circuit, as the
name implies, is a circuit
that contains one lamp
in which the power factor
leads the other – hence
the other lags. Using the
leading current effect of a
capacitor and the lagging
current effect of an inductor
produces the lead-lag
effect. The lagging effect is
80
Lead circuit lamp
Lag circuit lamp
Series
Capacitor
P
Figure 4.25 Lead-lag circuit
N
produced naturally when an inductor is used in the circuit. The leading effect uses
a series capacitor, which has a greater effect than the inductor in the circuit. When
these two circuits are combined as shown in Figure 4.25, there is no need for
further power factor correction as one circuit will correct the other.
The inverse square law
The inverse square law states that the further the distance a surface is from the light
source, then the illumination falling on that surface will reduce inversely as the square of
the distance increases. Figure 4.26 illustrates this law.
ity
Light intens
¼
1
1
Dista
nc
e from
light
sourc
e
2
3
Figure 4.26 The inverse square law
The cosine law
The cosine law states that the luminous intensity of a surface is proportional to the
cosine of the angle between the direction and the normal to the surface. The surface will
then appear equally bright from all directions.
Cosine law: Eθ = E * cos(θ)
0°
30°
100%
87%
60°
50%
85°
9%
Figure 4.27 Lambert’s cosine law
81
4.6 Conduits
Steel conduit
Annealed mild steel tubing known as conduit is widely used as a commercial and
industrial wiring system. Single core cables are run inside the steel tubing. Conduit can
be bent without splitting, breaking or kinking, provided the correct methods are used. It
offers excellent mechanical protection to the wiring and in certain conditions may also
provide the means of earth continuity. The two types of commonly used steel conduit
are known as black enamel conduit which is used indoors where there is no likelihood of
dampness, and galvanised conduit which is used in damp situations or outdoors.
Screwed conduit
Screwed steel conduit can be either seam welded or solid drawn. Solid drawn is stronger
but much more expensive. The thread used on steel conduit is not used on any other
pipe, so special conduit dies are therefore required.
Bending machines
Bending machines give consistent results every time
and require the minimum of practice. To position the
stand, as shown in Figure 4.28, swing the rear leg (E)
to its maximum. Place the safety pin (D) through the
hole beneath where the pin hangs, locking the rear
leg in place. The machine should now be standing
with the swivel arm (A) hanging downwards. C is
the conduit guide and B is the adjusting arm for the
conduit guide.
Types of bend
C
B
A
D
E
Figure 4.28 Positioning the
stand
Sharp bends must be avoided. The minimum radius
of steel conduit is laid down as 2.5 times the outside
diameter of the conduit. See Figure 4.29.
D
X
The right-angled bend
The right-angled bend is used to go around a corner or
change direction by 90º. When bending, measurements
may be taken from the back, centre, or front of the bend.
Allowance should be made for the depth of the fixing
saddle bases.
Must be a minimum of 2.5 × D
Figure 4.29 The minimum
bending radius allowed
The set
The set is used when surface levels
change, or when terminating into a
box entry. Sets should be parallel
and square, not too long and not too
short. Where there are numerous sets
together all sets must be of the same Figure 4.30 The set
length.
82
The double set
The double set is used when passing
girders or obstacles.
The kick
The kick is used when a conduit run
changes direction by less than 90º.
Figure 4.31 The double set
Figure 4.32 The kick
The bubble or saddle set
The bubble set or saddle set is used when passing obstructions, especially pipes or roof
trusses, etc. The centre of the obstruction should be central to the set as shown below.
Making a 90° set from a fixed point
Fixed point
Required set
200 mm
Figure 4.33 Making a 90° set
1. Mark the conduit 200 mm from the fixed point, as shown in Figure 4.34. If the
distance is given to the inside or centre of the tube, simply add on either the diameter
or half the diameter respectively to give the back bend measurement and follow the
same procedure as for the outside measurement.
Fixed point
Fixed point
Mark 200 mm from fixed point
Figure 4.34 Step 1
83
2. Place the tube in the ‘former’ with the fixed point to the rear. Position the tube so that
a square held against the tube at the fixed point touches and forms a tangent to the
leading edge of the former.
Fixed point
Too long to down bend
Inital mark
200 mm
Required distance from fixed
point to back of bend
Figure 4.35 Step 2
3. Where the remaining length of tube from the measured point is too long to down bend
and where it is not convenient or possible to up bend, deduct three times the outside
diameter of the tube from the initial mark.
New
mark
Fixed point
Initial mark
20 mm O.D
60 mm
(20 mm x 3)
Figure 4.36 Step 3
4. Place the tube in the former with a fixed point to the front with the mark at 90 ° to the
edge of the former. This will give a 90 ° bend at the required distance from the fixed
point to the back of the bend as shown in Figure 4.35.
Fixed point
Down bend
to 90°
Fixed point
New mark
Down bend
to 90°
Figure 4.37 Step 4
84
Conduit fixings
Table 4.3
Type of fixing
Illustration
●
‘strap saddle’ – used for fixing conduit to cable tray or steel
framework
●
‘half saddle’ – used for fixing to steel framework or cable tray
●
‘spacer bar saddle’ – used when fixing to an even surface, gives a
clearance of 2 mm
●
‘distance saddle’ – used if the surface is uneven and where brick
on concrete can give rise to heavy condensation
●
‘hospital saddle’ – used where it is necessary to clean around the
conduit fixing
●
‘multiple saddle strip’ – used to fasten multiple runs of conduit
together
●
‘girder clamp’ – will fix conduit to girders and I beams without
having to drill a hole in the girder
●
‘pipe hook or crampet’ – used when conduits are secured to a wall
or cast in concrete
Cutting and screwing conduit
Conduit should be cut with
a hacksaw with either a
fine-toothed 12 teeth per
centimetre or 32 teeth per
inch progressive blade.
The cut should be square
and the full length of a
hacksaw blade should
be used taking steady
strokes. The conduit
should be held in a pipe
vice and not a bench
vice. The vice should be
secured but not so tight
that it cuts into the pipe.
Figure 4.38 Using a pipe vice for cutting the conduit
85
Before threading, the conduit should be chamfered with a file to help the die start.
Screwing is carried out using stocks and dies, the guide ensuring the screw cut is
square. Stocks and dies should be kept clean and any lubricant or steel shavings should
be removed after cutting. The cut is made by placing the stock and die on the conduit
and then turning clockwise while applying forward pressure, sometimes a great deal of
pressure may be required. Once the cut is started the stock and die are removed so that
a cutting agent can be applied. Having applied the cutting agent the stock is placed on
the conduit again and the threading begins. The stock and die is turned back every turn
to clean out the cuttings.
Figure 4.39 Using stock and die
When the thread is finished the stock and die is
removed and the inside of the conduit is cleaned and
reamed. This removes all burrs, which would cut the
cables when they were installed. Reaming can be
carried out with a reamer or round file. The standard
length of thread for a normal joint is half a coupling
length (coupling of the same size conduit).
All couplings, bushes and conduit boxes must be
fully tightened before installation. Where possible,
couplings, bushes and boxes should be tightened
while the conduit is held firmly in a pipe vice.
Figure 4.40 Tightening conduits
Running coupling
Sometimes two conduits must be joined together and neither
can be turned. This may be due to one conduit coming through a
wall or ceiling or long runs combined with bends making turning
impossible. In these cases a running coupling must be used.
Running couplings are made by having one thread a normal half
coupling length and the other thread the length of a coupling
plus locking ring.
86
Conduct Coupling
Locknut
Figure 4.41 Running
coupling
The coupling and locking ring are fixed on the long thread side and the two conduits are then
butted together. The coupling is then removed from the long thread to the shorter thread and
finally rests across the two sides. After tightening, the coupling is locked with the locking ring.
Because the coupling is transversing two threads simultaneously, the thread must be very
clean and well cut. Reversing the dies and running them over the thread can help this. This is
particularly important where the running coupling is in an awkward position (as it often is). A
locking ring must be used because lock nuts get caught on the ceiling in tight situations.
Figure 4.42
Termination of conduit
There are our several methods available for terminating conduit, for example, at a box
using a conduit coupling and brass male bush.
Use of non-inspection elbows and tees
Non-inspection elbows are only used adjacent to an outlet box or inspection type fitting.
One solid elbow may be used if positioned less than 500 mm from an accessible outlet
in a conduit run of less than 10 m that has other bends which are not more than the
equivalent to one right angle.
500 mm
max
Solid
elbow
One rightangle bend
Tee or
elbow
beside
inspection
outlet
Switchgear
10 m max
Figure 4.43
Wiring conduit
Cables should not be drawn into a conduit system until the conduit system is complete.
When drawing in cables they must first be run off the reels or drum. If the cables are
allowed to spiral off the reels they will become twisted and this would cause damage to the
insulation. If a large number of cables are to be drawn into a conduit system at the same
time, the cable reels should be arranged on a stand or support so as to permit them to
revolve freely. In new buildings, cables should not be drawn in until the conduit is dry and
87
free from moisture. If there is any doubt, a draw tape with a swab at the end should be drawn
through the conduit so as to remove any moisture that may have accumulated. It is usual
to commence drawing in cables from a midpoint in the conduit system so as to minimise
the length of cable that has to be drawn in. A steel tape should be used from one draw-in
point to another. The draw tape should not be used for drawing in cables as it may become
damaged – a steel tape should only be used to pull through a draw wire. The ends of the
cables must be paired for a distance of approximately 75 mm and threaded through a loop in
the draw wire.
When drawing in a number of cables, they must be fed in very carefully at the delivery
end while someone pulls them at the receiving end. Care should be taken to feed into
the conduit in such a manner as to prevent any cables crossing. Always leave some
slack cable in all draw-in boxes and make sure that cables are fed into the conduit so as
not to finish up with twisted cables at the draw-in point. This operation needs care and
there must be synchronisation between the person who is feeding and the person who is
pulling. If in sight of each other, this can be achieved by some pre-arranged signal or, if
within speaking distance, by word given by the person feeding the cables.
Plastic conduit
Plastic conduit is made from polyvinylchloride (PVC), which is produced in both flexible
and rigid forms. It is impervious to acids, alkalis, oil, aggressive soils, fungi and bacteria,
and is unaffected by sea, air and atmospheric conditions. It withstands all pests and
does not attract rodents. PVC conduit is preferable for use in areas such as farm milking
parlours. It may be buried in lime, concrete, or plaster without harmful effects.
Advantages
●
Lightweight
●
Easy to handle
●
Easy to saw, cut and clean
●
Simple to bend
●
Does not require painting
●
Minimum condensation due to low thermal conductivity in walls
●
Quick to install.
Disadvantages
●
Care must be taken when gluing joints to avoid forming a barrier across the inside of
the conduit.
●
If insufficient adhesive is used, the joints may not be waterproof.
●
PVC expands around five times as much as steel and this expansion must be allowed for.
Working with PVC conduit
PVC conduit is easily cut using a junior hacksaw. Any roughness of cut and burrs should
be removed by simply wiping with a cloth. The most common jointing procedure uses
a PVC solvent adhesive. Generally the joint is solid enough for use after two minutes,
although complete adhesion takes several hours. In order to ensure a sound joint the
tube and fittings must be clean and free from dust and oil. Where expansion is likely and
adjustment is necessary, a mastic adhesive should be used. This is a flexible adhesive,
88
which makes a weatherproof joint, ideal for surface installations and in conditions of
wide temperature variation. It is also advisable to use mastic adhesive where there are
straight runs on the surface exceeding 8 m in length. Care must be taken when using
these adhesives as they are volatile liquids and the lid must be replaced on the tin
immediately after use. Always read the manufacturer’s instructions.
PVC conduit expands considerably more than metal conduit with an increase in temperature.
The expansion can be ignored where the conduit is buried in concrete or plaster. In surface
work, precautions must be taken to prevent such expansion from causing the conduit to
bow. Usually where bends and sets are close together these take up any expansion. Where
longer runs of conduit occur in conditions of varying temperatures, some provision for
expansion must be made, using expansion couplers as shown in Figure 4.44. A good guide
to the use of expansion couplers is one coupler per 6 m in straight runs.
Silicon grease to keep
watertight
75 mm
19 mm
Free to slide
Slide fit
25 mm free space
Adhesive cement
Figure 4.44 An expansion coupler
PVC conduits not exceeding 25 mm diameter can be bent cold by using a spring. The
bend is then made by either the hands or across the knee. In order to achieve the angle
required, the original bend should be made at twice the angle required and the tube
allowed to return to the correct angle. Under no circumstances should an attempt be
made to force the bend back with the spring inserted, as this can damage the spring.
It is easier to withdraw the spring if it is twisted in an anti-clockwise direction. This
reduces the diameter of the spring, making it easier to withdraw. In cold weather it may
be necessary to warm the conduit slightly at the point where the bend is required. One of
the simplest ways is to rub the conduit with the hand or a cloth. The PVC will retain the
heat long enough for the bend to be made. In order that the bend is maintained at the
correct angle the conduit should be saddled as soon as possible.
General points
●
Ample capacity must be provided at junctions employed for cable connections.
●
Where a steel conduit forms the protective conductor, a separate conductor must be
used to connect from any socket outlet to its back box.
●
Where flush switch boxes and switch grids are used, a circuit protective conductor (CPC)
is required from an earthing terminal in the box to the earthing terminal in the switch grid.
●
Where conduits pass through walls, the hole will have a fire-resistant material.
89
4.7 Testing and commissioning
Tests should be carried out in a set sequence, and repeated if failed. Initial tests should
be carried out in the following sequence before the supply is connected or with the
supply disconnected as appropriate:
1. Continuity of protective conductors, including main and supplementary bonding
conductors
2. Continuity of ring final circuit conductors
3. Insulation resistance
4. Polarity
With the electrical supply connected re-test polarity before further testing:
5. Earth fault loop impedance
6. Residual current operated devices.
Forms of completion or periodic inspection, inspection, test and an installation schedule
(including test results) should be provided to the person ordering the work.
1. Continuity of protective conductors
Every earth conductor including each bonding conductor needs to be tested to verify that
it is electrically sound and correctly connected, using a low reading ohmmeter.
Test method 1
Before carrying out this test the leads should be ‘nulled out’. This means the resistance
of the leads should be measured and deducted from the readings taken. The phase
conductor and the earth conductor are linked together at the consumer unit or
distribution board. The ohmmeter is used to test between the phase and earth terminals
at each outlet in the circuit.
Link
C.P.C
P
E
N P Socket outlet
C.C.U
Low reading
Ohmmeter
Figure 4.45 Test method 1
Test method 2
One lead of the continuity tester is connected to the consumer’s main earth terminals.
The other lead is connected to a trailing lead, which is used to make contact with earth
conductors at light fittings, switches, spur outlets, etc. The resistance of the test leads
will be included in the result, therefore the resistance of the test leads must be
measured and subtracted from the reading obtained if the instrument does not have a
nulling facility. In this method the earth conductor only is tested and this reading is
recorded on the installation schedule.
90
Long ‘Wander’ lead
N
E
Socket outlet
P
C.P.C
C.C.U
Low reading
Ohmmeter
N.B neutral and phase
omitted for clarity
Figure 4.46 Test method 2
Test of the continuity of supplementary bonding conductors
Test method 2 is used with the ohmmeter leads connected between the points being
tested, i.e. pipe work, sinks, etc. or between conductive parts and exposed conductive
parts (metal parts of the installation). This test will verify that the conductor is sound.
2. Continuity of ring final circuit conductors
The test results should ensure that the ring is complete and has no interconnections, and
that the ring is not broken. The phase, neutral and protective conductors are identified and
their resistances are measured separately (see Figure 4.47).
N1
P1
P2
cpc 1
N2
cpc 2
Figure 4.47
The phase and neutral conductors are then connected together so that the outgoing phase
conductor is connected to the returning neutral conductor and vice versa (see Figure 4.48).
The resistance between phase and neutral conductors is then measured at each socket
outlet. The readings obtained from those sockets wired into the ring will be substantially
the same and the value will be approximately half the resistance of the phase or the neutral
loop resistance. Any sockets wired as spurs will have a proportionally higher resistance
value corresponding to the length of the spur cable.
N1
cpc 1
P1
Link
P2
Link
N2
cpc 2
Figure 4.48
91
The previous test is then repeated with the phase and CPC cross-connected as before
(Figure 4.49). The resistance between phase and earth is then measured at each socket.
N1 Link
cpc 1
Link
P2
P2
N2
cpc 2
Figure 4.49
3. Insulation resistance
Insulation resistance tests are to check that the insulation of conductors, electrical
accessories and equipment is sound and not damaged, and that electrical conductors and
protective conductors are not short-circuited, or do not have a low insulation resistance.
Before testing ensure that:
●
Pilot or indicator lamps and capacitors are disconnected from circuits to avoid an
inaccurate test value being obtained
●
Voltage-sensitive electronic equipment such as dimmer switches, delay timers, power
controllers, electronic starters for fluorescent lamps, emergency lighting, Residual
Current Devices, etc. are disconnected so that they are not subjected to the test
voltage which could damage them
●
There is no electrical connection between any phase or neutral conductor (e.g. lamps
left in).
Insulation resistance tests should be carried out using the appropriate d.c. test voltage. The
installation will conform with the regulations if the main switchboard and each distribution
circuit tested separately, with all its final circuits connected but with current using equipment
disconnected, has an insulation resistance not less than that specified in Table 4.4.
Table 4.4 Minimum values of insulation resistance
Circuit nominal voltage
Test voltage Minimum insulation
d.c. (V)
resistance (M ohms)
SELV and PELV
250
0.25
Up to and including 500 V with the exception of
the above systems
500
0.5
Above 500 V
1000
1.0
The tests should be carried out with the main switch off, all fuses in place, switches and
circuit breakers closed, lamps removed, and fluorescent and discharge luminaires and
other equipment disconnected. Where the removal of lamps and/or the disconnection of
current using equipment is impracticable, the local switches controlling such lamps and/
or equipment should be open.
92
Simple installations that contain no distribution circuits should be tested as a whole,
however, to perform the test in a complex installation it may need to be sub-divided into
its component parts.
Test 1: Insulation resistance between live conductors
For single-phase circuits, test between the phase and neutral conductors at the
appropriate switchboard.
NP
C.P.C
Single pole switch
Lamp holder
Lamp out
M
High reading
ohmmeter
Figure 4.50 Testing insulation resistance
on a single-phase lighting circuit
For three-phase circuits, make a series of tests between live conductors in turn at
the appropriate switchboard as follows (ensuring that the incoming neutral has been
disconnected so there is no connection with earth):
1. Between red phase (and yellow phase, blue phase, and neutral) grouped
2. Between yellow phase (and blue phase, and neutral) grouped
3. Between blue phase and neutral.
Test 2: Insulation resistance from earth to phase and neutral connected together
For single-phase circuits, test between the phase and neutral conductors and earth at the
appropriate distribution board. Where any circuits contain any two-way switching, the twoway switches will require to be operated and another insulation resistance test carried out
including the two-way strapping wire, which was not previously included in the test.
For three-phase circuits, measure between all phase conductors and neutral bunched
together, and earth.
C.P.C
N P
Phase
Temporary link
M
Insulation resistance
tester
Figure 4.51 Testing insulation
resistance from earth to phase
to neutral
93
4. Polarity
A test needs to be performed to check the polarity of all circuits. This must be done
before connection to the supply, with either an ohmmeter, or the continuity range of an
insulation and continuity tester. The live connection in socket outlets and the centre
contact of screw-type lampholders must all be connected to the phase conductor. See
Figure 4.52. Also a check should be made to ensure that the polarity of the incoming
supply is correct, otherwise the whole installation would have the wrong polarity.
S P switch
C.P.C
Neutral
Phase
Temporary link
Edison screw
lamp holder
Switch closed
Low reading
ohmmeter
Figure 4.52 Testing polarity
4.8 Fault diagnosis
Electricians should have the ability to recognise when something is not up to standard or
is not functioning correctly. For example, if a metal-clad switch is in a damp environment,
the moisture would lead to the corrosion of the metal, thus reducing the integrity of the
switch and making it unsafe to use. An example of a circuit not operating correctly could
be a two-way lighting circuit only operating from one switch; this type of fault should be
found before energising the circuit. Not all faults are easily visible, some are concealed
and may develop over a period of time. Regular testing and inspection are not only needed
at the completion of works, but should also take place during the installation process of
the wiring system. Regular inspection, tests and maintenance checks should be used over
periods of time to confirm the quality, and to extend the life of the electrical installation.
Safe working procedures for fault diagnosis
Remember!
Before discussing symptoms, types of faults and the various problems which occur in
electrical apparatus, circuits and systems, it is important to be aware of the safe
isolation procedures and safe working practices.
Before beginning work on any piece of electrical equipment or circuit you should
make sure that it is completely isolated from the supply by following recognised
procedures.
Isolation
In order to carry out work safely on electrical systems it is important to:
●
identify sources of supply
●
isolate
94
●
secure isolation
●
test the equipment/system is dead.
Test instruments
All test equipment must be regularly checked to make sure it is in good and safe working
order and has a current calibration certificate indicating that the instrument is working
properly and providing accurate readings. When checking the equipment the following
points should be noted:
●
Check the equipment for any damage. Check to see if the case is cracked or broken.
This could indicate a recent impact, which could result in false readings.
●
Check that the batteries are in good condition and have not leaked (local action) and
check that they are all of the same type.
●
Check that the insulation on the leads and the probes is not damaged. Check that the
insulation is complete and secure.
●
Check the operation of the meter with the leads both open and short-circuited.
●
Then zero the instrument on the ohm scale.
●
If you have any doubt about an instrument or its accuracy ask for assistance.
Test probes and leads used in conjunction with a voltmeter, multimeter, electrician’s test
lamp or voltage indicator should be selected to prevent danger. Good test probes will
have the following:
●
Finger barriers, or shaped to guard against inadvertent hand contact with the live
conductors under test
●
Insulation to leave an exposed metal tip not exceeding 2 mm measured across any
surface of the tip. Where practicable, it is strongly recommended that this is reduced
to 1 mm or less or that spring-loaded retractable-screened probes are used
●
Suitable high-breaking capacity fuse or fuses with a low current rating, usually not
exceeding 500 mA or a current-limiting resistor and a fuse.
Check the leads:
●
are adequately insulated
●
are coloured so that one lead can be easily distinguished from the other, are flexible
and of sufficient capacity for the duty expected of them
●
are sheathed to protect against mechanical damage
●
are long enough for the purpose, while not too long
so that they are clumsy or unwieldy
●
do not have accessible exposed conductors other
than the probes or tips, or have live conductors
accessible to the person’s finger if a lead becomes
detached from a probe
●
are held captive and sealed into the body of the
voltage detector.
Fuses and/or
current limitation
Barrier to prevent
access to live
terminals
Robust, flexible well
insulated leads
Minimum of
exposed metal Shrouded or
firmly attached
connectors
Figure 4.53 Testing polarity
95
Minimum of
exposed metal
Voltage indicating devices
Instruments used solely for detecting a voltage fall into
two categories:
●
●
Detectors, which rely on an illuminated lamp (test
lamp) or a meter scale (test meter). Test lamps are
fitted with a 15 W lamp that has a guard to protect it.
Lamp
Minimum of
exposed metal
Robust, flexible
well insulated leads
Fuse and/or
current limitation
Detectors, which use two or more independent
indicating systems (one of which may be audible) and Figure 4.54 A typical test lamp
limit energy input to the detector. An example is a
two-pole voltage detector, i.e. a detector unit with an integral
test probe, an interconnecting lead and a second test probe.
These detectors are designed and constructed to limit
the current and energy that can flow into the detector. The
limitation is usually provided by a combination of the circuit
design using the concept of protective impedance and currentlimiting resistors built into the test probes. They have in-built
test features to check the functioning of the detector before
and after use. The interconnecting lead and second test
probes are not detectable components, and they do not require
additional current-limiting resistors or fuses to be fitted.
Figure 4.55 A typical
two-pole voltage
indicator
Test lamps and voltage indicators need to be clearly marked with the maximum voltage
which may be tested by the device and any short time rating for the device if applicable.
This rating is the recommended maximum current that should pass through the device
for a few seconds. These devices are generally not designed to be connected for more
than a few seconds.
Warning notices
It is important that once a circuit or item of electrical equipment has been isolated, it
cannot be inadvertently switched back on. A good method of providing full electrical and
mechanical isolation is to lock off the device or distribution board containing the device
with a padlock. The person working on the isolated equipment keeps the key. A skilled
person should be the only one allowed to carry out this, or similar responsible tasks
concerning the electrical installation wiring. Many items of electrical equipment when
installed are provided with a local means of switching or isolation. A good example of local
isolation is when a motor is provided with a control unit, which can include the starter,
switch and lock-off facility. Warning notices stating the maximum voltage present should
be fixed to every item of equipment (or enclosure), which
contains circuits operating at voltages in excess of 250 V
and where the presence of such a voltage would not
ON
normally be expected. Where accessories, control gear
or switch gear are wired on different phases of a threeOFF
phase supply, but can only be reached simultaneously,
a notice must be placed in a position where anyone
ER
NG
DA NOT
removing an accessory, or gaining access to the terminals
DO H ON
ITC
of control gear, switchgear, etc., is warned of the maximum
SW
voltage which exists between parts. Warning notices
should be displayed at the point of isolation of the circuit.
Figure 4.56 A warning notice
96
Model question
and answer 1
Model question
and answer 2
What would limit the need for rectification of faults?
Model question
and answer 6
Model question
and answer 5
Model question
and answer 4
Model question
and answer 3
What type of switch accessory is suitable for damp environments?
The designer’s knowledge of electrical installation systems and practices should
ensure that a polycarbonate type accessory is specified, which has special
qualities including being waterproof and durable.
Quality checks and tests at all stages of the installation process and regular
maintenance.
Why are neon type terminal screwdrivers unsuitable and dangerous?
The principle of operation of this device relies on the user making contact with
earth at zero volts and the screwdriver metal whose voltage is only limited by
the use of a resistor. If this resistor breaks down and contact is made with live
parts, the touch voltage to the user could reach 250 V, which could be fatal.
Furthermore the terminal screwdriver has none of the qualities of GS38 type
testers.
Why is it not enough just to simply isolate a circuit before working on it? Which
other major precautions should you take?
The circuit could be inadvertently switched back on.
The circuit should be checked for voltages present, and then the board or device
should be locked off and warning notices posted.
State three examples of situations where local isolators are used.
●
Machine or motor
●
Immersion heater
●
Cooker, etc.
State the basic difference between a switch and an isolator.
A switch is an on-load device, whereas an isolator is primarily an off-load device.
Although some switches can be used as isolators, an isolator should not be
used as a switch.
97
Types of fault
There are different types of faults, the consequence of the fault depending on its location
within the installation or in a specific circuit. Faults can occur when installing electrical
systems and from poor, or lack of, maintenance.
As technology advances and as the systems we install are being constantly
improved, then the faults developed become more complicated to solve. Therefore
an understanding of the electrical installation and the equipment we install is really
important. There are occasions when it may be impractical to rectify a fault, for example,
the cost of the repair may be more than the cost of replacement. It is part of the
electrician’s job to build into the design of the electrical installation fault protection and
damage limitation. For example:
●
Installing more than one circuit, when a fault occurs on one of the two circuits the
fault can be limited to that circuit
●
Installing fuses and circuit breakers to disable the fault and limit its effect
●
Ability to access, maintain and repair, where access to the installation allows for
maintenance and repair.
It is easier to find faults on installations where there are plenty of circuits. Indeed it is
a requirement of The IEE Wiring Regulations that every installation shall be divided into
circuits as necessary to:
●
Avoid danger and minimise inconvenience in the event of a fault
●
Facilitate safe operation, inspection, testing and maintenance.
This regulation will allow the electrician to locate faults easily, usually by process of
elimination (operating each fuse individually), or simply looking at the device to see
which one has operated. Protective devices and simple fuses are designed to operate
when they detect large currents due to excess temperature. In the case of a short-circuit
fault, high levels of fault current can develop causing high temperatures and breakdown
of insulation. Such faults can cause fires.
Position of faults
The location of the fault can limit its severity with regard to disruption and inconvenience.
For example, if the supply company’s protective device or fuses were rated at 100 A per
phase and the designer loaded each phase up at 200 A, this would result in a 100%
overload. The consequence of this would be the operation of the mains protective
devices and the whole installation being without power.
Operation of overload and fault current devices
When a fault is highlighted, it is usually because a circuit or piece of equipment has
stopped working. This is usually because the protective device has done its job and
operated. The rating of a protective device should be greater or at least equal to the
rating of the cable, circuit or equipment it is protecting, for example:
●
10 × 100 W lamps equate to a total current use of 4.35 A. Therefore a device rated at
5 or 6 A could protect this circuit.
●
A portable domestic appliance which has a label rating of 2.7 kW equates to a total
current of 11.74 A. Therefore a fuse rated at 13 A should be fitted in the plug.
98
Protective devices are designed to operate when an excess of current (greater than
the design current of the circuit) passes through it. The fault current’s excess heat can
cause a fuse element to rupture or the device mechanism to trip, dependent on which
type of device is installed. These currents may not necessarily be circuit faults, but shortlived overloads specific to a piece of equipment or outlet.
The regulations define these as:
●
overload current
●
over-current
●
earth fault current
●
short-circuit current.
Whichever fault occurs, the electrician should take account of its effect on the
installation wiring and choose a device suitable to disconnect the fault quickly and
safely. The fundamental effect of any fault is a rise in current and therefore a rise in
temperature. High temperature destroys the insulation, which in itself could lead to
a short circuit. High currents damage equipment, and earth fault currents can cause
dangerous or even fatal electric shocks.
Typical overload faults:
●
Adaptors used in socket outlets exceeding the rated load of circuit
●
Extra load being added to an existing circuit or installation
●
Not accounting for starting current on a motor circuit.
Typical earth faults:
●
Insulation breakdown
●
Incorrect polarity
●
Poor termination of conductors.
Typical short-circuit faults:
●
Insulation breakdown
●
Severing of live circuit conductors
●
Wrong termination of conductors, energised before being tested.
Insulation failure
Insulation is designed primarily to separate conductors and to ensure their integrity
throughout their life. It is also used to protect against electric shock by protecting people
against direct contact, and is often used as a secondary protection against mechanical
damage, i.e. in the case of PVC/PVC twin and CPC cables.
When insulation of conductors and cables fail, it is usually due to one or more of the
following:
●
Poor installation methods
●
Poor maintenance
●
Excessive ambient temperatures
●
High fault current levels
●
Damage by others.
99
Plant equipment and component failure
There will be some faults that the electrician will attend which will be caused by
breakdown simply due to wear and tear. Planned maintenance systems and regular
testing and inspections can extend the life of equipment. Here are some common
failures on installations and plant:
●
Switches not operating due to age
●
Motors not running because new brushes are required
●
Lighting not working because a lamp’s life has expired
●
Fluorescent luminaries not working because new lamps or starters needed
●
Outside passive infrared sensor (PIR) not switching because of water
●
Corridor socket outlet not working because of poor contacts due to excessive
use/age.
Faults due to misuse, abuse, and negligence
A common reason for faults on any electrical system or equipment is due to misuse
where the system or the equipment is simply not being used in relation to its design.
Every item of electrical equipment comes with user instructions that usually cover
procedures and precautions. Such instructions should be read, as misuse usually leads
to invalidating the guarantee. When an electrical installation is completed, a manual is
handed over to the customer that includes all product data and installation schedules
and test results. This data will help the client when additions to the installation are
made, inspections and tests are carried out, or to assist in maintaining the installation.
Some faults are caused by carelessness during the installation process. Simple faults
due to poor termination and stripping of conductors can lead to serious short circuits or
overheating. Good mechanical and electrical processes should be carried out and every
installation should be tested and inspected prior to being energised. Here are some
examples of faults arising from misuse:
●
Using a miniature circuit breaker (MCB) as a switch, where constant use could lead to
breakdown
●
Unplugging on-load portable appliances, damaging socket terminals
●
Damp accessories, due to hosing walls in dry areas
●
MCB’s nuisance tripping from connecting extra load to circuits.
Here are some examples of installer misuse:
●
Poor termination of conductors, causing overheating
●
Loose bushes and couplings with no earth continuity, causing a risk of electric shock
●
Wrong size conductors used, causing excessive voltage drop and excess current
which could lead to an inefficient circuit and overheating of conductors
●
Not protecting cables when drawing in to enclosures, causing damaged insulation
●
Overloading conduit and trunking capacities, causing overheating and insulation
breakdown.
100
Model question
and answer 7
State the consequence when the protective device operates from a short-circuit
at the following locations:
a) in the supply company’s mains supply cable
b) within a final circuit
c) in a three-phase mains switch.
a) There will be a total loss of supply within the installation.
b) There will be a loss of supply in the circuit only.
Model question
and answer 8
c) All sub-circuits that are fed by the operated device will be dead.
Briefly explain how protective devices such as fuses and circuit breakers
operate.
Such devices operate when heat is produced from increased levels of current
that appear in the circuit. Such currents exist from overloads and faults from
short circuits and earth faults. The device detects the heat and operates either
by tripping, or by the fuse element rupturing.
Model question
and answer 9
Give three examples of plant, machine, or component failure.
Any three from:
●
●
●
●
●
●
Switches not operating – due to age.
Motors not running – new brushes required.
Lighting not working – lamp’s life expired.
Fluorescent luminaire not working – new lamp or starter needed.
Outside PIR not switching – ingress of water causing failure.
Corridor socket outlet not working – poor contacts due to excessive use/age.
State how misuse or negligence causes installation faults.
Any explanation for:
User misuse
●
Model question
and answer 10
●
●
●
Using an MCB as a switch – where constant use could lead to breakdown.
Unplugging on-load portable appliances – damaging socket terminals.
Damp accessory – due to hosing walls in dry areas.
MCB’s nuisance tripping – connecting extra load to circuits.
Installer misuse
●
●
●
●
●
Poor termination of conductors – overheating poor electrical contact.
Loose bushes and couplings – no earth continuity, electric shock risk.
Wrong size conductors used – excessive voltage drop and excess current,
which could lead to an inefficient circuit and overheating of conductors.
Not protecting cables when drawing in to enclosures – damaged insulation.
Overloading conduit and trunking capacities – overheating and insulation
breakdown.
101
4.9 Fuses and circuit breakers
Fuses are slower to operate and less accurate than circuit breakers. In order to work
out how effective these fuses are we need to have some way of knowing their circuit
breaking and ‘fusing’ performance. We can work this out for fuses by the use of a
fusing factor formula:
Fusing factor =
fusing current
current rating
The fusing current is the minimum current causing the fuse to blow, and the current
rating is the maximum current which the fuse can sustain without blowing. Fusing
currents can be found in the IET wiring regulations. These tables are logarithmic so the
scales increase by factors of 10, not uniformly as may be expected. The rating of the
fuse is the current it will carry continuously without deterioration.
Table 4.5 Fuses
Type of device and rating
Fusing current
Fusing factor
BS 3036 20 A
40 A
40
=2
20
BS 1361 20 A
38 A
38
= 1.9
20
BS 88 20 A
34 A
34
= 1.7
20
Type 1 20 A MCB
26 A
26
= 1.3
20
BS 3036 45 A
100 A
100
= 2.22
45
BS 1361 45 A
88 A
88
= 1.95
45
BS 88 45 A
66 A
66
= 1.47
45
Type 1 45 A MCB
52 A
52
= 1.15
45
Fusing factors for the following devices can be grouped as follows:
102
BS 3036
1.8–2
BS 1361
1.6–1.9
BS 88
1.25–1.7
MCBs
up to 1.5
The number in kA stamped onto the end cap of an HBC fuse, or printed onto the body
of a BS 1361 fuse, is known as the breaking capacity. When a short circuit occurs, the
current may, for a fraction of a second, reach hundreds or even thousands of amperes.
The fuse or circuit breaker must be able to break such a current or make it harmless
to its surroundings by arcing, overheating or scattering hot particles. The breaking
capacities of circuit breakers are indicated by a ‘M’ number, e.g. M6. This means that
the breaking capacity is 6 kA or 6000 A. The breaking capacity will be related to the
prospective short-circuit current.
Miniature Circuit Breakers (MCBs)
The modern MCB forms an essential part of the majority of installations at the final
distribution level.
The circuit breaker is an automatic switch, which opens in the event of carrying excess
current. The switch can be closed again when the current returns to normal, because the
device does not damage itself during normal operation. The contacts of a circuit breaker
are closed against spring pressure, and held closed by a latch arrangement. A small
movement of the latch will release the contacts, which will open quickly under spring
pressure to break the circuit.
Thermal tripping
The load current is passed through a small heater, the temperature of which depends
on the current it carries. This heater is arranged to warm a bimetal strip either directly,
i.e. the current passes through the bimetal strip, which in effect is part of the electrical
circuit, or indirectly, i.e. a coil of current carrying conductor is wound around the bimetal;
excess current warms the bimetal. The bimetal strip is made of two different metals
normally brass and steel, brass expanding more than steel. These two dissimilar metals
are securely riveted or welded together along their length. The rate of expansion of the
two metals is different so that as the strip is warmed, it will bend and will trip the latch.
The bimetal strips are so arranged that normal currents will not heat the strip to tripping
point. If the current increases beyond the rated value, the heater dissipates power and
the bimetal strip is raised in temperature to trip the latch.
Magnetic tripping
The principle used here is the force of attraction, which can be set up by the magnetic
field of a coil carrying the load current. At normal currents the magnetic field is not
strong enough to attract the latch, but overload currents operate the latch and trip the
main contacts.
Combined tripping
There is always some time delay in the operation of a thermal trip, since the heat
produced by load current must be transferred to the bimetal strip. Thermal tripping
is thus best suited to small overloads of comparatively long duration. Magnetic trips
are fast-acting for heavy overloads. The two methods are therefore combined to take
advantage of the best characteristics of each.
103
5. Electronics
Devices such as security alarms, telephones, dimmers, boiler controls, and speed
controllers have brought electronics into general electrical installation work.
5.1 Resistors
There are two basic types of resistor: fixed and variable. The resistance value of a
fixed resistor cannot be changed by mechanical means (though its normal value can
be affected by temperature or other effects). Variable resistors have some means of
adjustment (usually a spindle or slider).
Fixed resistors
Making a resistor involves taking some material of a known
resistivity and making the dimensions (cross-sectional area
(CSA) and length) of a piece of that material such that
the resistance between the two points at which leads are
attached (for connecting into a circuit) is the value required.
Most of the very earliest resistors were made by taking a
length of resistance wire (wire made from a metal with a
relatively high resistivity) and winding this onto a support rod
of insulating material. They are relatively difficult to mass
produce, which makes them expensive.
Techniques for making resistors from materials
other than wire have now been developed for
low power applications, for example, coating an
insulating rod (usually ceramic or glass) with
a thin film of resistive material. The resistive
materials in common use today are carbon and
metal oxides. Metal end caps fitted with leads
are pushed over the ends of the coated rod
and the whole assembly is coated with several
layers of very tough varnish or similar material to
protect the film from the atmosphere and from
knocks during handling. These resistors can be
mass-produced with great precision at very low cost.
Metal
oxide
resistor
End cap
Vitreous enamel
coating
Resistance winding
on ceramic former
Figure 5.1 A typical wire
wound resistor
Silicon laquer or
paint coating
Resistive
carbon
compound
Embedded
connection
leads
Figure 5.2 A coated rod resistor
Variable resistors
Key
term
Variable resistors require some sort of sliding contact together with a fixed resistor
element. Wire wound variable resistors are often made by winding resistance wire onto
a flat strip of insulating material, which is then wrapped into a nearly complete circle.
A sliding contact arm is made to run in contact with the turns of wire as they wrap over
the edge of the wire strip. Straight versions are also possible, where a straight former is
used and the wiper travels in a straight line along it.
104
Resistor – a component that limits the electrical current that flows through a
circuit.
Resistor coding
Standard colour code
Many resistors are so small that it is impractical to print their value on them. Instead, they
are marked with a code that uses bands of colour at one end of the component. Most
general resistors have four bands of colour, but high precision resistors are often marked
with a five-colour band system.
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
Gold
Silver
0
1
2
3
4
5
6
7
8
9
Brown
Red
Gold
Silver
None
0.1
0.01
1
2
5
10
20
What this means
Band 1 First figure of value
Band 2 Second figure of value
Band 3 Number of zeros/multiplier
Band 4 Tolerance (+
_%) see below
Note how the bands are closer to
one end than the other
Worked example 1
Did you
know?
Figure 5.3 Resistor coding
Before you read a resistor, turn it so that the end with bands is on the left-hand
side. Now you read the bands from left to right.
A resistor is colour coded red, yellow, orange, gold. Determine the value of the
resistor.
First band red (First digit) 2
Second band yellow (Second digit) 4
Third band orange (No. of zeros) 3
Fourth band gold (Tolerance) 5%
The value is 24 000 ±5%
105
Worked example 2
A resistor is colour coded yellow, yellow, blue, silver. Determine the value of the
resistor.
First band yellow (First digit) 4
Second band yellow (Second digit) 4
Third band blue (No. of zeros) 6
Fourth band silver (Tolerance) 10%
The value is 44 000 000 ±10%
Did you know?
This rhyme helps to remember the resistor codings:
Billy
Black 0
Brown
Brown 1
Runs
Red 2
Over
Orange 3
Your
Yellow 4
Garden
Green 5
But
Blue 6
Violet
Violet 7
Grey
Grey 8
Won’t
White 9
Resistance markings
Resistance values are generally given in either W, kW,or MW using numbers from 1–999
as a prefix (e.g. 10 W, 567 kW etc.). In the code system we replace W, kW and MW with
the following letters:
W=R
kW = K
MW = M
These letters are now inserted wherever the decimal point would have been in the value.
So, for example, a resistor of value 10W would now be shown as 10R, and a resistor of
value 567 kW would become 567 K.
Power ratings
Resistors often have to carry comparatively large values of current, so they must be
capable of doing this without overheating and causing damage. As the current has to be
related to the voltage, it is the power rating of the resistor that needs to be identified.
The power rating of a resistor is a convenient way of stating the maximum temperature at
which the resistor is designed to operate without damage to itself.
106
Thermistors
A thermistor is a temperature-sensitive resistor.
Electrode plating
Laser trim area
Ceramic material
Thermistor without
encapsulation
Thermistor with
encapsulation
Figure 5.4 Thermistors
They can be supplied in various shapes and are used for the measurement and
control of temperature up to their maximum useful temperature limit of about 300 °C.
They are very sensitive and, because of their small construction, they are useful for
measuring temperatures in inaccessible places. Thermistors are used for measuring the
temperature of motor windings and sensing overloads. The thermistor can be wired into
the control circuit so that it automatically cuts the supply to the motor when the motor
windings overheat, thus preventing damage to the windings. Thermistors are also used
for monitoring the temperature of the water in a motor car.
Light-dependent resistors
Cadmium sulphide
track
Circuit symbol
Figure 5.5 A light-dependent
resistor
Light-dependent resistors are sensitive to light. They
consist of a clear window with a cadmium sulphide
film under it. When light shines onto the film its
resistance varies; as the light increases the resistance
reduces. These resistors are commonly found in street
lighting. Sometimes street lights switch on during a
thunderstorm in the daytime, because the sunlight is
obscured by the dark thunderclouds, thus increasing the
resistance which in turn controls the light ‘on’ circuit.
Questions
1
A resistor is colour coded violet, orange, brown, gold. Determine the value of the resistor.
2
A resistor is colour coded green, red, yellow, silver. Determine the value of the resistor.
3
Many resistors use the four banding colour code system.
a) State the four band coding for a 27 kΩ ± 10% resistor.
b) State the value and tolerance of a blue, grey, yellow, gold resistor.
4
Calculate the resistance and the current drawn from the supply, by the following
equipment connected to a 230 V supply:
5
What value of resistance is required to have a current of 12 A, if the supply voltage
is 415 volts?
6
Calculate the resistance of a resistor which absorbs 5000 W when a current of 15 A
passes through it.
a) a 4 kW 230 V immersion heater
b) a 600 W 230 V microwave oven
107
Remember!
5.2 Capacitors
Never pick a capacitor up by the terminals as it may still be charged and you
will receive a shock. Always ensure the capacitor has been discharged before
handling. Some capacitors have a discharge resistor connected in the circuit for
this reason.
A capacitor is basically two metallic surfaces, referred to as plates, separated by an
insulator commonly known as the dielectric. The plates are usually metal and the dielectric
is any insulating material such as air, glass, ceramic, mica, paper, oils and waxes.
Dielectric
Metal
plates
Lead to plates
Lead to plates
+
Fixed (non-polarised) Fixed (polarised) Variable
Preset
Figure 5.6 Common symbols used for capacitors
The two plates are not in contact with each other and as such they do not form a circuit
in the same way that conductors with resistors do. However, the capacitor stores a small
amount of electric charge so it can be thought of as a small rechargeable battery, which
can be quickly recharged.
The capacitance of any capacitor depends on:
●
the working area of the plates, i.e. the area of the conducting surfaces facing each
other
●
the thickness of the dielectric between the plates
●
the nature of the dielectric or spacing material used.
Capacitor types
There are two major types of capacitors, fixed and variable, both of which are used in
a wide range of electronic devices. Fixed capacitors can be further sub-divided into
electrolytic and non-electrolytic types. All capacitors possess some resistance and
inductance because of the nature of their construction, resulting in limitations in their
applications.
108
Electrolytic capacitors
Electrolytic capacitors have a much higher capacitance,
volume for volume, than any other type. This is achieved
by making the plate separation extremely small by using
a very thin dielectric (insulator), often mica or paper.
The main disadvantage of an electrolytic capacitor is
that they are polarised and must be connected to the
correct polarity in a circuit, otherwise a short circuit and
destruction of the capacitor will result. They look like a
raindrop with two leads protruding from the bottom. The
polarity and values may be marked on the capacitor or the
colour code, shown later, can be used.
Figure 5.7 The outside
view of a typical electrolytic
capacitor
Capacitor coding
The capacitance, working voltage, type of construction, and polarity need to be known
to identify a capacitor. The identification of capacitors is not easy because of the wide
variation in shapes and sizes. In the majority of cases the capacitance will be printed on
the body of the capacitor, in farads (F) or the following:
1 microfarad = 1 µF = 1 × 10-6 F
1 nanofarad = 1 nF = 1 × 10-9 F
1 picofarad = 1 pF = 1 × 10-12 F
Figure 5.8 A newer type of electrolytic capacitor using tantalum and tantalum oxide to
give a further capacitance/size advantage
The power factor correction capacitor found in fluorescent luminaries would have a value
typically of 8 µF at a working voltage of 400 V. The working voltage of a capacitor is
the maximum voltage that can be applied between the plates of the capacitor without
breaking down the dielectric insulating material.
It was quite common for capacitors to be marked with colour codes, but today relatively
few capacitors are colour coded. This method is based on the standard four band resistor
colour coding. The first three bands indicate the
Value in picofarads
value in normal resistor fashion, but the value is in
1st digit
2nd digit
picofarad. To convert this into a value in nanofarad
Tolerance
Number of zeros
green 5%
simply divide by 1000. Divide the marked value by
white
10%
1000 000 if a value in microfarads is required. The
Maximum volts
red = 250 V
fourth band indicates the tolerance, but the colour
yellow = 400 V
coding is different to the resistor equivalent. The fifth
band shows the maximum working voltage of the
Figure 5.9 Capacitor colour
component.
bands
109
Worked example 1
Bands are then read from top to bottom. Digit 1 gives the first number of the component
value; the second digit gives the second number. The third band gives the number
of zeros to be added after the first two numbers, and the fourth band indicates the
capacitor tolerance, which is normally black 20%, white 10% and green 5%.
A plastic film capacitor is colour coded from top to bottom brown, red, yellow,
black, red. Determine the value of the capacitor, its tolerance and working
voltage.
Brown = 1
Red = 2
Yellow = 4, multiply by 10 000
Black = 20% tolerance
Red = 250 V
Worked example 2
The capacitor has a value of 120 000 pF or 0.12 µF, with a tolerance of 20% and
a maximum working voltage of 250 V.
A plastic film capacitor is colour coded from top to bottom orange, orange,
yellow, green, yellow. Determine the value of the capacitor, its tolerance and
working voltage.
Orange = 3
Orange = 3
Yellow = 4, multiply by 10 000
Green = 5% tolerance
Yellow = 400 V
The capacitor has a value of 330 000 pF, or 0.33 μF, with a tolerance of 5% and
a minimum working voltage of 400 V.
Polarity
Key terms
Some capacitors are constructed in such a way that if the component is operated with
the wrong polarity its properties will be destroyed – this is especially true for electrolytic
capacitors. Polarity may be indicated by a + or - as appropriate.
110
Capacitor – a component which stores an electric charge if a voltage is applied
across it.
Working voltage – the maximum voltage that can be applied between the plates
of the capacitor without breaking down the dielectric insulating material.
5.3 Semi-conductor devices
Key
term
Semi-conductors are materials that have an electrical quality
somewhere between a conductor and an insulator, in that they
are neither a good conductor nor a good insulator. Typically,
we use semi-conducting materials such as silicon or
germanium, materials in which the atoms are arranged in a
‘lattice’ structure. The lattice has atoms at regular distances
from each other with each atom ‘linked’ to the four atoms
surrounding it. Each atom then has four valence electrons.
Figure 5.10
Valence electrons – the electrons in an atom’s outermost orbit.
Atoms of pure silicon or germanium have no free electrons, so no conduction is possible.
To allow conduction to take place, we add an impurity to the material via a process
known as doping. When we have an extra electron, we have a surplus of negative charge
and call this type of material ‘n-type’. When we have removed an electron we have a
surplus of positive charge, and call this material ‘p-type’.
The p–n junction
A semi-conductor diode is basically created
when we bring together an ‘n-type’ material
and a ‘p-type’ material to form a p–n junction.
The two materials form a barrier where they
meet which we call the depletion layer. In this
barrier, the coming together of unlike charges
causes a small internal p.d. to exist.
Depletion
layer
Anode
p
n
Cathode
We now need to connect a battery across the
Figure 5.11 A p–n junction
ends of the two materials, where we call the
end of the p-type material the anode, and the end of the n-type material the cathode.
If the anode is positive and the battery voltage is big enough, it will overcome the effect
of the internal p.d. and push charges (both positive and negative) over the junction.
In other words, the junction has a low resistance and current can flow. This type of
connection is known as being forward biased. If the battery connections are reversed
so that the anode is now negative and the junction becomes high resistance, no current
can flow. This type of connection is known as being reverse biased. When the junction is
forward biased, it only takes a small voltage (0.7 V for silicon) to overcome the internal
barrier p.d. When reverse biased, it takes a large voltage (1200 V for silicon) to overcome
the barrier and thus destroy the diode, effectively allowing current to flow in both
directions.
111
A diode allows current to flow through it in one direction
only. The symbol is shown in Figure 5.12, where the
direction of the arrow can be taken to represent the
direction of current flow.
Zener diode
A Zener diode is a silicon junction
diode (not germanium) with a predetermined breakdown at reverse
voltage. The breakdown conduction
mechanism is very tightly controlled
and, once breakdown occurs,
the voltage across the diode
changes very little over a wide
range of currents. The stable
reverse breakdown behaviour of
these diodes accounts for their
alternative and more accurate
name: voltage regulator diodes.
Anode
Figure 5.12 The symbol for
a diode
Band
(a)
Cathode
Anode
IF /mA
Cathode
(b)
–20
VZ
–15
Forward
bias
–10
In its forward bias mode, when the
anode is positive and the cathode
–5
negative, the Zener will conduct at
VF /V
about 0.6 V, just like an ordinary
0
+2
+4
–6
–4
–2
diode. However, it is in the reverse
A
–5
mode that the Zener diode is
normally used. When connected
Reverse
–10
with the anode made negative and
bias
B
–15
the cathode positive, the reverse
current is zero until the reverse
voltage reaches a pre-determined
Figure 5.13 A Zener diode
value (VZ) when the diode switches
on as shown in Figure 5.13. As
current actually starts to flow slightly before the applied reverse voltage reaches 5.1 V,
the Zener voltage (VZ) increases slightly above 5.1 V as the current through the Zener
diode increases towards the maximum.
Because of these two features of the curve, the nominal Zener voltage is always quoted
on data sheets with reference to a stated limit of reverse current at breakdown and, to
prevent overheating, the power rating of the diode should not be exceeded.
The point at which the Zener curve turns down is usually referred to as the Zener knee.
Ideally, as with the forward characteristic of diodes, once the voltage exceeds the value
at the knee, the characteristic should become a vertical line, but it leans. As with any
characteristic curve, the voltage at any given current, or the current at any given voltage
can be found from the curve of a Zener diode. The power dissipated in a Zener diode
when a current is flowing through it is invariably greater than that in an ordinary diode
with the same current through it. This is because the power dissipated in any component
is found by multiplying the voltage across the component by the current through it, and
the Zener voltage of a Zener diode is greater than the forward voltage of an ordinary
diode. To limit the current a resistor is placed in series with the Zener diode.
112
In Figure 5.14, the Zener diode is being used to stabilise a supply from a 9 V battery, so
the resistor R will have to drop a voltage of (9 - 5.1) = 3.9 V.
R
+
9V
5.1 V
500 m W
5.1 V
–
Figure 5.14
The value of R can be found using Ohm’s law, where
R=
max =
V
Imax
0.5 W
5.1 V
= 98mA
Therefore R =
3.9 V
0.098 A
= 40
Light emitting diodes (LEDs)
The light emitting diode is a p–n junction especially manufactured from a semi-conducting
material, which emits light when a current of about 10 mA flows through the junction. No light
is emitted if the diode is reversed bias, and if the voltage exceeds 5 V then the diode may be
damaged. If the voltage exceeds 2 V, then a series connected resistor may be required.
Photocell and light-dependent resistor
The photocell changes light (also infrared and ultraviolet radiation) into electrical
signals and is useful in burglar and fire alarms as well as in counting and automatic
control systems. Photoconductive cells or light-dependent resistors make use of
semi-conductors whose resistance decreases as the intensity of light falling on them
increases. The energy of the light sets electrons free from donor atoms in the semiconductor making it more conductive. This type of device is used for outside lights along
the streets, roads and motorways. There are also smaller versions for domestic use
within homes and businesses.
Photodiode
A photodiode consists of a normal p–n junction housed in a case with a transparent
window through which light can fall onto the semi-conductor. The photodiode operates
in the reverse bias. When light falls on the junction the energy from the light breaks
down bonds in the lattice of the semi-conductor producing electrons and holes, allowing
current to flow. This type of device is used as a fast counter, or in light meters to
measure light intensity.
113
Diode testing
The p–n junction diode has a low resistance when a voltage is applied in the forward
direction and a high resistance when applied in the reverse direction. Connecting
an ohmmeter with the red positive lead to the anode of the junction diode and the
black negative lead to the cathode would give a very low reading. Reversing the lead
connections would give a high resistance reading in a good component.
When using a multimeter it needs to be put into the diode test mode, otherwise
when testing on the ohm’s range you could get a reversed reading.
Remember!
High resistance connection
100
Ω MΩ
Low resistance connection
0.01
Ω MΩ
Figure 5.15 Diode connections
5.4 Rectification
Rectification is the conversion of an a.c. supply into a d.c. supply. Many applications
such as electronic circuits and equipment require a d.c. supply.
Half-wave rectification
A diode will only allow current to
flow in one direction and it does
this when the anode is more
Supply
Load
positive than the cathode. In
the case of an a.c. circuit, this
means that only the positive
half cycles are allowed ‘through’
Figure 5. 16 The waveform for half-wave rectification
the diode and, as a result, we
end up with a signal that resembles a series of ‘pulses’. This tends to be unsuitable for
most applications, but can be used in situations such as battery charging. A transformer
is also commonly used at the supply side to ensure that the output voltage is to the
required level.
114
Full-wave rectification
We have seen that half-wave rectification occurs when one diode allows the positive half
cycles to pass through it. Two diodes can be connected together to give a more even
supply, called a biphase circuit. Anodes of the diodes are connected to the opposite ends
of the secondary winding of a centre-tapped transformer. As the anode voltages will be
180 ° out of phase with each other, one diode will effectively rectify the positive half cycle
and one will rectify the negative half cycle. The output current will still appear to be a
series of pulses, but they will be much closer together.
Load
Figure 5.17 The waveform for full-wave rectification
The full-wave bridge rectifier
This system uses four diodes connected in such a way that, at any instant in time, two
of the four will be conducting. See Figure 5.18 where the route through the network for
each half cycle is shown.
Positive
half cycle
Negative
half cycle
D4
D1
D2
D3
D4
D1
D2
D3
Figure 5.18 The full-wave bridge rectifier
The circuits that we have looked at so far convert a.c. into a supply, which, although
never going negative, is still not a true d.c. supply.
Smoothing
To make it useful for electronic circuits, we need to smooth out the ‘ripples’ in the
waveform. One way of doing this is by using filter circuits.
115
5.5 Transistors
A bipolar transistor is a semi-conductor
device, which has two p–n junctions. It is
capable of producing current amplification
and, with an added load resistor, both
a load and voltage power gain can be
achieved.
The term ‘transistor’ is derived from
the two words ‘transfer-resistor’. This is
because in a transistor, approximately the
same current is transferred from a low to a
high resistance region.
Collector
Base
Collector Collector
P
Base
N
P
Emitter
Base
Emitter
Collector
N Base
P
N
Emitter
Emitter
Figure 5.19 The two types of bipolar
transistor and their circuit symbols
The term ‘bipolar’ means both electrons and holes are involved. A bipolar transistor
consists of three separate regions or areas of doped semi-conductor material.
When the construction is such that a central n-type region is sandwiched between two
p-type outer regions, a pnp transistor is formed. If the regions are reversed, an npn
transistor is formed.
Lead-out wires from
For transistors to operate these conditions must
be met:
●
The base must be very thin.
●
Majority carriers in the base must be very few.
●
The base-emitter junction must be forward
biased and the base-collector junction reverse
biased.
Emitter
Silicon
oxide
NP
N
emitter and base
Case
Mounting
Base
Collector
Transistor
(soldered with
collector side
to gold-plated
mounting)
Figure 5.20 Transistor construction
Current amplification
The collector current is always a fixed proportion of the emitter current set by the
thinness of the base and the amount of doping. Consider, for example, a base bias of
some 630 mV has caused a base current of 0.5 mA to flow but, more importantly, has
initiated a collector current of 50 mA.
This relationship between IB and IC is termed the ‘static value of the short-circuit forward
current transfer’, normally just referred to as the gain of the transistor. It is simply a
measure of the amplification achieved, using the symbol hFE, which is the ratio between
the continuous output current (collector current) and the continuous input current (base
current). Thus when IB is 0.5 mA and IC is 50 mA, the ratio is:
I
hFE = C
IB
which is equal to 100.
Note: There are no units since this is a ratio.
It can therefore be said that a small base current initiated by the controlling forward
base-bias voltage produces a significantly higher value of collector current to flow
dependent on the value of hFE for the transistor. Thus current amplification has been
achieved.
116
Voltage amplification
The low resistive reference is
the emitter circuit and the high
resistive reference, the collector
circuit; the current in both being
almost identical.
Electrons
Holes
Reverse
biased
p–n junction
Electrons
(collector current)
Collector
(n-type)
The reason the emitter circuit
Holes
is classed as having a low
(base current)
Base
resistance is because it contains
(p-type)
the forward biased base-emitter
junction. Conversely, the collector
Emitter
+
circuit contains the reverse
(n-type)
biased base-collector junction,
Forward
which is of course in the order of
biased
Electrons
p–n junction
tens of thousand ohms (it varies
(emitter current)
with an integrated circuit). In
order to produce a voltage output
Figure 5.21 Voltage amplification
from the collector, a load resistor
(RB) is added to the collector circuit.
+
B2
There is no input across Vi, which is called the quiescent (quiet) state. For transistor
action to take place the base-emitter junction must be forward biased (and has to remain
so when Vi goes negative due to the a.c. signal input). By introducing resistor RB between
collector and base, a small current IB will flow from VCC through RB into the base and down
to VO via the emitter, thus keeping the transistor running (ticking over).
Component values are chosen so that the steady base current IB makes the quiescent
collector–emitter voltage VCE about half the power supply voltage VCC. This allows VO to
have its maximum swing capability and not interfere with the input signal Vi .
IB
+VCC
IC
RL
RB
C
Vi = Alternating input
VCE
+
0
Vi
VO
VBE
t
–
Figure 5.22 A circuit for an amplifier
When an a.c. signal is applied to the input Vi and goes positive it increases VBE slightly
to around (0.61 V). When Vi swings negative, VBE drops slightly to (0.59 V). As a result a
small alternating current is superimposed on the quiescent base current IB, which in effect
is a varying d.c. current. The collector–emitter voltage (VCE) is a varying d.c. voltage, or an
alternating voltage superimposed on a normal steady d.c. voltage. The capacitor C is there
to block the d.c. voltage, but allow the alternating voltage to pass on to the next stage.
117
So, a bipolar transistor will act as a voltage amplifier if:
●
It has a suitable collector load RL.
●
It is biased so that so that the quiescent value VCE is around the value of VCC, which is
known as the class A condition.
●
The transistor and load together bring about voltage amplification.
●
The output is 180 ° out of phase with the input signal.
●
The emitter is common to the input, output and power supply circuits and is usually
taken as the reference point for all voltages, i.e. 0 V. It is called ‘common’, ‘ground’ or
‘earth’ if connected to earth.
Transistor as a switch
Transistors can operate as
a switch. Compared with
other electrically operated
switches, transistors have many
advantages, whether in discrete
or integrated circuit form. They
are small, cheap, reliable, have
no moving parts, and can switch
millions of times per second. A
transistor is the perfect switch
that has infinite resistance when
‘off’, no resistance when ‘on’,
and changes instantaneously
from one state to another, using
up no power.
Testing transistors
1000
0.06 Ω
1000 Ω
0.06 Ω
– +
– +
– +
– +
B C
E
E C
B
E C
B
B C
E
Digital
multimeter
npn
pnp
Analogue
multimeter
As all transistors consist
1000 0
1000 0
1000 0
1000 0
of either an npn or a pnp
High
Low
High
Low
resistance
resistance
resistance
resistance
construction, the testing of
–
+
–
+
–
+
–
+
them is similar to diodes.
B
E
B
E
B
E
B
E
C
C
C
C
Special meters with three
npn
pnp
terminals for testing
transformers are available
and many testing instruments
Figure 5.23 Testing transistors
have this facility. However, an
ohmmeter can be used for testing a transistor to check if it is conducting correctly. The
following results should be obtained from a transistor assuming that the red lead of an
ohmmeter is positive.
Note: This is not always the case, with some older analogue meters, the battery
connections internally are the opposite way round, so it is always good to check both
ways across base and emitter, as shown in Figure 5.23.
118
A good npn transistor will give the following readings:
Red to base and black to collector or emitter will give a low resistance.
However, if the connections are reversed it will result in a high resistance reading.
Connections of any polarity between the collector and emitter will also give a high reading.
A good pnp transistor will give the following readings:
Black to base and red to collector or emitter will give a low resistance reading.
However, if the connections are reversed a high resistance reading will be observed.
Connections of either polarity between the collector and emitter will give a high
resistance reading.
5.6 Integrated circuits
Integrated circuits (IC) are complete electronic circuits housed within a plastic case
(known as the ‘black box’). The chip contains all the components required, which may
include diodes, resistors, capacitors, transistors, etc.
There are several categories, which include analogue, digital and memories. The basic
layout is shown in Figure 5.24.
‘Chip’
Connection
from ‘chip’ to pin
Plastic case
Notch
Small dot
Metal pin
0.1 inch
Pin 1
Figure 5.24 An operational amplifier (linear IC)
The plastic case has a notch at one end and if you look at the back of the case with the
notch at the top Pin 1 is always the first one on the left-hand side, sometimes noted with
a small dot. The other pin numbers follow down the left-hand side, 2, 3, and 4, then back
up the right-hand side from the bottom right to the top, 5, 6, 7 and 8. This is an 8-pin
chip, but you can get chips with up to 32 pins. These types of IC with the pins lined up
down each side are known as ‘Dual in line’ ICs.
5.7 Thyristors
Thyristors, also known as silicon-controlled rectifiers (SCRs), are semi-conductor devices,
which act as high-speed switches. Devices are available that can operate at potentials of
several thousand volts and which will carry currents up to hundreds of amps. Thyristors
are being used increasingly to replace mechanical switches and relays, since they offer
faster switching without arcing and offer greater reliability. This is particularly the case in
continuous a.c. power control systems such as lamp dimmers, heater control and motor
speed control.
119
Anode A
Construction of thyristors
The thyristor consists of a four-layer pnpn silicon sandwich. The
circuit symbol is that of a rectifier diode with an additional terminal
called the gate. It is this gate which enables the action of the rectifier
to be controlled.
In medium and high power devices, the case will be metal with a
screw thread suitable for mounting on a heat sink. Smaller devices
resemble transistors in physical size and packaging.
p
n
p
n
Gate
G
Cathode C
Figure 5.25
A thyristor
Principle of operation
The word ‘thyristor’ is derived from the Greek word thyra which means ‘door’. The
thyristor can open or shut like a door, thus preventing or allowing current flow through the
device. The thyristor can be made to act as an open circuit between anode and cathode
and this is called the forward blocking state. Further applying a positive pulse of low
power across the gate and cathode terminals can trigger it, so a low power signal on the
gate can switch the current through a high power load.
When forward biased, the thyristor will not conduct until a positive voltage is applied
to the gate. Conduction will continue through the thyristor when the gate voltage is
removed. The only time the thyristor will stop conduction is when the supply voltage is
turned off, reversed or the anode current falls below a certain value.
This is a simple method of controlling power in a d.c. circuit using a thyristor. The control
of a.c. power can also be achieved with the thyristor by allowing current to be supplied
to the load during part of each half cycle. If a gate pulse is applied automatically at a
certain time during each positive half cycle of the input, then the thyristor will conduct
during that period until it falls to zero for the negative half cycle.
The triac
This single component performs the same function as two thyristors
connected in inverse parallel, but with a common gate terminal. Full wave
power control can thus be achieved with the triac using a much simpler
triggering arrangement than with the two separate inverse parallel
thyristors. Triacs were originally developed for, and used extensively
in, the consumer market. They are used in many low power control
applications such as food mixers, electric drills and lamp dimmers.
The two opposing arrowheads depict the bi-directional current flow
characteristics of the component.
The diac
The diac is often used in triac triggering circuits because it, along with
a resistor–capacitor network, produces an ideal pulse-style waveform.
It does this without any sophisticated additional circuitry due to its
electrical characteristics. Also it provides a degree of protection against
spurious triggering from electrical noise (voltage spikes). The device
operates like two breakdown (Zener) diodes connected in series, back to
back. It acts as an open switch until the applied voltage reaches about
32/35 V, when it will conduct.
120
G
MT1
Figure 5.26
The circuit
symbol for
a triac
Figure 5.27
The circuit
symbol for a
diac
Lamp dimmer circuit
The GLS lamp has a tungsten filament which allows it to operate at about 2500 ° C and
is wired in series with the triac. The variable (pot R) resistor is part of a trigger network
providing a variable voltage into the gate circuit, which contains a diac connected in series.
Increasing the value of the resistor increases the time taken for the capacitor to reach its
charge level to pass current into the diac circuit. Reducing the resistance allows the triac
to switch on faster in each half cycle. By this adjustment the light output of the lamp can
be controlled from zero to full brightness. The capacitor is connected in series with the
variable resistor; this combination is designed to produce a variable phase shift into the
gate circuit of the diac. When the p.d. across the capacitor rises, enough current flows
into the diac to switch on the triac. The diac is a triggering device having a relatively high
switch on voltage (32–35 V), and acts as an open switch until the capacitor p.d. reaches
the required voltage level. The triac is a two-directional thyristor, which is triggered on both
halves of each cycle. This allows it to conduct current in either direction of the a.c. supply.
Its gate is in series with the diac, allowing it to receive positive and negative pulses.
Lamp
+
Pot
R
a.c.
supply
Diac
Triac
C
–
Figure 5.28 A triac lamp dimmer circuit
Field effect transistors (FETs)
Field effect transistor devices first appeared as separate (or discrete) transistors, but
now the field effect concept is employed in the fabrication of large-scale integration
arrays such as semi-conductor memories, microprocessors, calculators and digital
watches. There are two types of field effect transistor, the junction gate field effect
transistor, which is usually abbreviated to JUGFET, JFET or FET, and the metal oxide semiconductor field effect transistor known as the MOSFET. They differ significantly from the
bipolar transistor in their characteristics, operation and construction.
Logic gates
A logic gate is an electronic component which processes a binary input and produces a
binary output based on a set of rules.
Examples:
●
A NOT gate produces an output which is not the same as the input. This means that
when there is a logic 0 input there is a logic 1 output but when there is a logic 1 input
121
there is a logic 0 output. As the output is always the opposite of the input these gates
are sometimes called inverters.
●
An OR gate produces a logic 1 output if one or other of its inputs is logic 1.
●
An AND gate produces a logic 1 output if its first input and its second input are both
logic 1.
The complete list of the five logic gates you are required to know is shown in Table 5.1.
Truth tables
Truth tables show the output states of a logic gate for all of the possible inputs to the
gate. These allow us to easily determine what the output would be when the gates are
connected to input sources. When in doubt about what a logic gates does, you should
always consult the truth table. For example the truth table shows that an OR gate will
still produce a logic 1 output even if both of the inputs are logic 1.
Table 5.1 Logic gates and their truth tables
Truth table
Gate
OR
AND
NOR
NAND
Function (high voltage
= 1, low voltage = 0)
Symbol
A
OUTPUT
B
A
OUTPUT
B
A
OUTPUT
B
A
OUTPUT
B
OUTPUT
NOT
INPUT
OUTPUT = 1
if
A OR B = 1
OUTPUT = 1
if
A AND B = 1
OUTPUT = 0
if
A OR B = 1
OUTPUT = 0
if
A AND B = 1
OUTPUT = 1
if INPUT = 0
OUTPUT = 0
if INPUT = 1
INPUTS
A
B
OUTPUT
0
0
1
1
0
1
0
1
0
1
1
1
0
0
1
1
0
1
0
1
0
0
0
1
0
0
1
1
0
1
0
1
1
0
0
0
0
0
1
1
0
1
0
1
1
1
1
0
0
1
1
0
Combining logic gates
Logic gates have limited use individually but when they are combined together they can
be used to process information and cause actions to be taken.
122
To work out the output for any collection of gates draw up a truth table representing all
of the possible input combinations. For each set of inputs in turn, work your way through
the logic combinations and find the output. Keep on going until you have found the
outputs for all of the possible input combinations.
What are the input conditions required for the logic gate system shown in
Figure 5.29 to produce an output of 0?
A
Output
B
C
Figure 5.29
Worked example 3
Draw up the truth table for the logic gate system, working in a sensible order
and making sure all of the possible input combinations are covered (Table 5.2).
Table 5.2
Input
A
Input
B
Input
C
Output
0
0
0
1
1
0
0
0
0
1
0
1
1
1
0
1
0
0
1
1
1
0
1
1
0
1
1
1
1
1
1
1
Remember!
From the truth table we can see that the output of the logic gate system is logic
0 only when A is 1, B is 0 and C is 0.
1
Binary digits can be represented by high and low voltage levels.
2
Logic gates process binary inputs and produce binary outputs.
3
Simple logic gates can be combined to provide additional processing.
123
Glossary
Ammeter An instrument used for
measuring current.
Atom The smallest part of an element
that can take part in a chemical change.
Capacitor A component which stores
an electric charge if a voltage is applied
across it.
Circuit breaker A device which
automatically disconnects the supply from
a circuit in the event of excessive current
flowing in the circuit.
Conductor A material that conducts
electricity.
Conduit A tube made from metal or
PVC in which insulated conductors are
contained.
Electricity Movement of free electrons.
Electron A stable sub-atomic particle with
a negative charge.
Fossil fuels Formed by a natural process
over millions of years and include
petroleum, coal, and natural gas.
Frequency The number of oscillations of
alternating current in an electric system
transmitted from a power station to the
end-user per second. It is normally 50 Hz.
Fuse Also known as ‘overcurrent
protection’, disconnects a circuit when an
overload or short circuit is present.
Insulator A material that doesn’t readily
conduct electricity.
Ions Atoms or molecules that have
gained or lost electrons.
Molecule A group of atoms bonded
together.
Multimeter Combines the features and
characteristics of both an ammeter and a
voltmeter, and measures current, voltage
and resistance. Digital multimeters are the
most commonly used measuring devices.
Neutron A stable sub-atomic particle with
no charge.
124
Nucleus The positively charged central
core of an atom made up of protons and
neutrons.
Polarisation The shift of positive and
negative electric charge in opposite
directions within an insulator, or dielectric,
induced by an external e.m.f.
Polyvinylchloride (PVC) A material that
acts as an insulator, normally the covering
for electrical cables.
Proton A stable sub-atomic particle
occurring in all atoms with a positive charge.
Renewable energy Energy which is
naturally replenished on a human
timescale, such as sunlight, wind, rain,
tides, waves, and geothermal heat.
Resistor A component that limits the
electrical current that flows through a circuit.
Semi-conductor A material that can
conduct electricity under certain conditions.
Static electricity A build-up of charge
on the surface of an object. The charge
remains until it is discharged, for
example, by touching the object. This can
cause a mild electric shock.
Thermistor A device whose resistance
quickly decreases with an increase in
temperature.
Transformer An electrical machine with
no moving parts which is used to change
the value of an alternating voltage.
Unity power factor When the voltage and
current in a circuit are ‘in phase’ with
each other.
Valence electrons The electrons in an
atom’s outermost orbit.
Voltmeter An instrument used for
measuring voltage. It measures the
potential difference between two points.
Working voltage The maximum voltage
that can be applied between the plates of
the capacitor without breaking down the
dielectric insulating material.
Index
A
a.c. 29–33, 35–42
rectification 114–15
alternators 52, 53
ammeters 13
AND gates 122
atoms 2–3, 4, 5, 111
B
batteries 8, 44, 47
bending conduit 82–4, 89
biofuels 55
block diagrams 45
C
cables 70–3, 74, 75
resistivity 21–3
wiring conduit 87–8
capacitance 6, 32–3, 108
in a.c. circuits 37–42
capacitive reactance 33,
38, 40
capacitors 32–3, 108–10
cells 44, 47
charge 2–3, 4, 5, 6, 7
capacitors store 32, 108
chemical effects 8
chemical energy 2
circuit breakers 25–6, 101,
103
circuit diagrams 45, 47–50
circuits 6, 7–8, 14–18
a.c. 29–33, 35–42
in installations 64–9
faults 94, 96, 98
testing 90–4
integrated 119
lamp dimmer 121
lighting 74–7
colour codes 105–6, 109–10
conductivity 3–4
conductors 3, 4, 7, 47
resistivity 21–3
conduit 82–9
consumer units 54, 64
continuity testing 90–2
corkscrew rule 10–11
cosine law 81
CPCs 89, 90–2
see also protective devices
current 6, 7–8, 14–18
a.c. see a.c.
amplification 116
in diodes 111–13
eddy currents 57
and electromagnets 10, 11
induced current 12, 30
measuring 13, 42
in parallel 16–17, 41–2
and protective devices
102–3
in transformers 56, 57,
58, 60–1
cycle 29
D
d.c. 29, 30, 56, 114–15, 120
diacs 120
diagrams and drawings 45–51
diodes 13, 111–15
direct in line starters 27–8
doping 111
E
Earth, symbol for 47
earth conductors 90
earth faults 99, 101
eddy currents 57
electrical energy 2, 7
electricity 2–4, 5, 6
production 52–5, 56, 58
electrolytic capacitors 109,
110
electromagnetic induction 8,
12
electromagnetism 9–13
electronics 104–23
capacitors 108–10
diodes 111–15
integrated circuits 119
logic gates 121–3
rectification 114–15
resistors 104–7
thyristors 119–21
transistors 116–19
electrons 2–3, 4, 5, 6
in circuits 7, 8, 14, 29
in semi–conductors 111,
113
emergency lighting 49, 66–7
e.m.f. 6, 7, 8, 12
induced 8, 30, 31, 56
energy 2, 6, 7, 8, 32
in devices 113
and lamps 78, 79
energy sources 52, 54–5
escape lighting 66
F
fault diagnosis 94–101
field effect transistors 121
fire alarm systems 68–9
fluorescent lamps 49, 79–81
forward bias 111, 112
fossil fuels 52
frequency 6, 29
fuels 52, 55
full–wave rectification 115
fuses 24, 98–9, 101, 102–3
G
gain of transistors 116
gases 2
geothermal energy 52, 55
glow starter circuits 79–80
H
half–wave rectification 114
heat detectors 68
hydroelectricity 55
hysteresis 57
I
impedance 33, 36–7, 38
and RCL circuits 39–40
incandescent lamps 78
induced current 12, 30
induced e.m.f. 8, 30, 31, 56
inductance 6, 31–2, 35–7
mutual 56
in RLC circuits 39–42
inductive circuit 33
inductive reactance 31, 40
installations 54, 62–103
circuits 64–9
conduits 82–9
fault diagnosis 94–101
lamps 78–81
lighting circuits 74–7
scale drawings 51
standards 62–3
testing 90–4
wiring methods 70–3
insulation 92–3, 99
insulators 3, 4
integrated circuits 119
inverse square law 81
ions 3, 4
isolation 94–5, 96, 97
J
joint/junction boxes 77
125
K
kinetic energy 2, 7
L
lagging 31–2
lamps 49, 78–81, 121
layout diagrams 46
LCDs 13
LEDs 13, 113
left–hand rule, Fleming’s 11
light energy 7
light–dependent resistors
107, 113
lighting 49, 66–7, 74–7
liquids 2
logic gates 121–3
loop in ceiling rose 74–5
M
magnetic energy 2
magnetism 6, 8, 9–13, 26
and transformers 56, 57
maximum/peak value 29
MCBs 25–6, 103
meters 13, 95, 118
molecules 2, 3, 4
motors 11, 26–8, 48, 107
multimeters 13, 114, 118
N
NAND gates 122
neutrons 4
NOR gates 122
NOT gates 121–2
nuclear power 52
nuclei (nucleus) 2, 3, 4
O
Ohm’s law 14–18
OR gates 122
overloads 99, 101
P
p–n junctions 111–14, 116
parallel circuits 16–18
a.c. in 41–2
particles 2, 7
phase angle 30, 31, 32
phasors 30–3, 35, 39
photocells 113
polarity 94, 95, 109, 110
potential difference 6, 7, 8
see also voltage
power 6, 8, 15–16
and a.c. 29, 30, 31–2
and transformers 56, 57,
61
126
power factor 29, 30, 31–2, 40
power ratings, resistor 106
power stations 52–3
protective conductors 89,
90–2
protective devices 24–8,
98–9, 101
performance 102–3
protons 2–3, 4, 5
PVC 70–1, 72, 88–9
R
radial circuits 65
reactance 31, 33, 38, 40
rectification 114–15
regulations 62, 98, 102
renewable energy 52, 54–5
resistance 6, 13, 14–18
and a.c. 30, 33, 35–42
and resistivity 21–3, 104
resistive circuits 30, 32, 33
resistivity 21–3, 104
resistors 14–18, 104–7
light–dependent 107, 113
reverse bias/voltage 111,
112
right–hand grip rule 10
ring circuits 64–5, 91–2
r.m.s. value 29
S
safe working 94–5, 96, 97
scale drawings 46, 51
schematic diagrams 46
semi–conductors 3, 4
devices 111–14, 116,
119
series circuits 15–16, 18
a.c. 35–40, 42
short–circuit faults 99, 101,
103
sinusoidal waveforms 29–30,
31, 32
smoke detectors 68, 69
smoothing 115
socket outlets 50, 64, 65
solar power 2, 52, 55
solenoids 10
solids 2
spurs 65
standards 47–50, 62–3
standby lighting 66
static electricity 5
stroboscopic effects 80–1
Sun see solar power
switches 74–6, 94, 97
symbols for 48, 50
thyristors as 119
transistors as 118
symbols 47–50, 108, 112,
120
units and quantities 6
T
technical diagrams 45
temperature 22–3, 107
terminals 72–3
termination 72–3, 87
testing 90–4, 114, 118–19
equipment for 95–6
thermal effects 8
thermistors 68, 69, 107
thermocouples 8, 68
three–phase 52, 80, 93, 96
and motors 26, 27, 28
and transformers 58
thyristors 119–21
tidal power 52, 55
transformers 53, 54, 56–61
transistors 116–19, 121
triacs 120
tripping 25–6, 103
truth tables 122, 123
tungsten halogen lamps 78
turns ratio 56, 59–61
V
voltage 6, 8, 14–18
in a.c. circuits 30–3, 35–42
amplification 117–18
in diodes 112–13, 114
and electricity distribution
53, 54
measuring 13, 42, 95–6
and transformers 56, 58,
59–61
working voltage 109
see also e.m.f.
voltmeters 13, 95
W
warning notices 96, 97
wind power 52, 55
windings 56–61
wiring 70–3, 77
and conduit 82, 87–8
for fire alarms 69
regulations 62, 98
wiring diagrams 46
Z
Zener diodes 112–13
Electrical and Electronic
Technology for CSEC®
Less Stress, More Success
Written by experienced teachers and experts, this study guide focuses
on the development of skills, critical thinking and teamwork — a great
foundation for your SBA, further study and entry into the world of work.
Features include:
●
Why don’t you: an opportunity to apply the
skills you have studied by way of an activity
●
Key terms: definitions of key words and terms
highlighted for easy reference
●
Detailed diagrams and step by step instructions
aid in understanding concepts
Candidates who successfully complete the CSEC examinations in
the Industrial Technology programmes will be awarded two sets of
certificates: the CSEC Technical Proficiency and a CVQ* (Caribbean
Vocational Qualification) Statement of Competence.
The Caribbean Examinations Council (CXC®) has worked exclusively
with Oxford University Press to produce a series of CSEC® and CAPE®
Study Guides.
* CVQ is the Registered Trademark of the Caribbean Association of National Training Authorities (CANTA).
eBook
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ISBN 978-0-19-839547-8
9 780198 395478