Electrical Trade Theory
N2
Module 1: Conductors and cables
METHODS OF INSTALLING CABLES
There are three general methods of installing armoured cables:
• Open (in free air);
• In ducts; and
• Buried.
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Module 1: Conductors and cables (continued)
INDUCTANCE – SELF INDUCTION
When there is a change of current flowing through a coil, there will also be a
change of flux around the coil. Whenever there is a change in flux in a coil,
an emf is induced in that coil. Since the emf is induced in the same coil
through which the current responsible for setting up the flux flows, it is known
as an emf of self induction.
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Module 1: Conductors and cables (continued)
TRUE POWER, APPARENT POWER AND REACTIVE POWER
• This product of the voltage and the current is known as apparent power.
• The true power or active power is a product of the voltage and the
in-phase component.
• The component of the current does no useful work and causes energy
loss in the form of heat. The reactive component is said to produce
reactive power.
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Module 1: Conductors and cables (continued)
CABLE SELECTION
Following are a number of variables to consider when selecting a cable for a
specific application:
• Load to be supplied.
• Permissible volt drop.
• Prospective fault (short-circuit) current.
• The circuit protection that is available.
• Environmental conditions.
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Module 2: Cable joints and switchgear
JOINING CONDUCTORS
Joints are made in conduit boxes, cable jointing boxes, appliances and light
fittings in many smaller applications.
When large diameter copper conductors are joined, crimping ferrules are
almost always used.
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Module 2: Cable joints and switchgear (continued)
CABLE JOINTS (SPLICES)
There are five basic steps in making a joint in a cable:
1. Prepare the ends (make off the ends and prepare the surfaces).
2. Join the conductors.
3. Insulate.
4. Reshield (earth sock, armouring, screening, metal sheaths).
5. Rejacket.
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Module 2: Cable joints and switchgear (continued)
TYPES OF JOINTS
Low voltage joints exist, such as:
High voltage joints exist, such as:
• Acceptable cable couplers;
• Resin joint with taped conductors;
• Resin joint;
• Hot shrink joint;
• Taped joint;
• Cold shrink joint.
• Screw connectors in joint box;
• Hot or cold shrink joint with resin; and
• Strip connectors in joint box; and
• Resin pressure joints.
• Resin pressure joints.
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Module 2: Cable joints and switchgear (continued)
INSULATORS AND INSULATION
The main function of insulating materials is to support and/or electrically
separate conductors in order to prevent the flow of an electric current where
a potential difference exists between two or more points.
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Module 2: Cable joints and switchgear (continued)
RETICULATION NETWORK
Power stations are often situated at considerable
distances from the consumers. In order to transmit
electrical energy economically over long distances, it is
necessary to do so at high voltages and by means of
overhead conductors.
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Module 2: Cable joints and switchgear (continued)
DISCONNECTORS, RELAYS AND CONTACTORS
A disconnector is a mechanical switching device that provides an isolating
distance in accordance with specified requirements, and is capable of
opening and closing a circuit either when negligible current is present.
Relays are small electromagnetic switches. They are used to relay electrical
signals from one circuit to another.
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Module 2: Cable joints and switchgear (continued)
MINIATURE CIRCUIT BREAKERS (MCB)
Low tension circuit-breakers of the domestic and small industrial type may be
of the single-phase or three-phase type. Miniature circuit-breakers can be of
the magnetic-hydraulic type or of the thermalmagnetic type.
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Module 3: Direct current motors
CONSTRUCTION
The general construction of a four-pole dc motor is shown below. The main
frame is referred to as the yoke and is made of a magnetic material. Field
coils which produce the main magnetic field are wound around soft iron pole
cores.
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Module 3: Direct current motors (continued)
INTERNAL CONNECTIONS (TYPES OF MOTORS)
DC machines are named according to the method used to connect the field
coils relative to the armature. There are two main types of internal
connections: seperately excited and self excited.
Self excitation can be subdivided into three categories; shunt, series and
compound connected.
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Module 3: Direct current motors (continued)
THE NEED FOR MOTOR STARTERS
The armature current of a motor is limited by the back emf and armature
circuit resistance. In order to limit this high starting current to a safe value,
the resistance of the armature circuit must be increased. This is done by
adding a starting resistor to the armature circuit while the motor is being
started.
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Module 3: Direct current motors (continued)
FACE PLATE STARTER FOR A SHUNT MOTOR
A face plate starter is manually operated. It has a spring loaded contact arm
which returns automatically to the off position when the no-volt release coil
(NVR) de-energises.
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Module 3: Direct current motors (continued)
REVERSING THE DIRECTION OF ROTATION
The direction of rotation of a dc motor may be reversed by reversing the
direction of the current through the field windings or through the armature
windings, but not both.
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Module 4: Alternating current motors
THE INDUCTION MOTOR
In the induction motor there is no electrical connection between the stator
and the rotor. As in a transformer, the energy is transferred entirely
magnetically by means of the emf induced in the rotor conductors by the
rotating field set up by the stator windings.
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Module 4: Alternating current motors (continued)
CONSTRUCTION
The induction motor consists essentially of two main parts namely:
• A stator, and
• A rotor.
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Module 4: Alternating current motors (continued)
ROTATING MAGNETIC FIELD
A rotating magnetic field is produced:
The currents through each of the three stator windings in a three-phase
motor differ in phase by a third of a cycle (120°). A pulsating magnetic field is
set up by each of these windings. Since each magnetic field follows after the
next, they combine to set up a resultant field which moves around the stator
surface at a constant speed.
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Module 4: Alternating current motors (continued)
PRINCIPLE OF OPERATION
When the stator winding (primary) is energised from a three-phase supply,
a rotating magnetic field, as described above, is established, which rotates at
synchronous speed.
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Module 4: Alternating current motors (continued)
SPEED AND SLIP
The rotor speed, even at no-load, will always be slightly less than
synchronous speed in order that current may be induced in the rotor which
will produce a torque large enough to overcome friction and wind resistance.
This difference between rotor speed and synchronous speed is called slip.
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Module 4: Alternating current motors (continued)
STAR AND DELTA CONNECTIONS
Three-phase induction motors run with their stator windings connected in
delta. For the starting of large squirrel-cage induction motors, the stator
winding is often connected in star (which reduces the voltage across each
phase winding) for starting and delta for running. This is accomplished by
means of a star-delta starter.
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Module 4: Alternating current motors (continued)
SINGLE-PHASE INDUCTION MOTORS
A single-phase motor is not self-starting and requires special means to
get it started. In order to start a single-phase induction motor, the phase is
split by means of an auxiliary (starting) winding. Because of this they are also
referred to as split-phase motors.
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Module 4: Alternating current motors (continued)
OVERCURRENT PROTECTION OF MOTORS
Overcurrent protection for induction motors are often set in the region of 20%
above the full load current rating. The use of a starter reduces the starting
current but increases the accelerating time because of the reduced value of
the starting torque.
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Module 4: Alternating current motors (continued)
STARTING OF INDUCTION MOTORS
The purpose of a motor starter is to start a motor manually or automatically
from a position close to the motor or remotely. The function of a motor starter,
other than a direct-on-line starter, is to reduce the current drawn by the motor
while the rotor accelerates.
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Module 4: Alternating current motors (continued)
ELECTRICAL TESTS ON THE STATOR
There are three electrical tests that are carried out on the stator windings of a
three-phase motor:
• Insulation resistance test between windings;
• Insulation resistance to earth test; and
• Short-circuit and open-circuit tests.
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Module 5: Earthing
EARTHING
The purpose of earthing is to guard every electrical installation, machine,
appliance or apparatus against the effects of leakage currents, static charges
and lightning discharges.
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Module 5: Earthing (continued)
EARTHING OF UNDERGROUND CABLES
The wire armouring of an armoured cable may be used as an earth continuity
conductor.
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Module 5: Earthing (continued)
OVERHEAD LINES
Earth continuity conductors are erected
above overhead lines in order to protect
them from lightning. It is also a useful
means of earthing the pylons and to supply
the consumer with a reliable earth.
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Module 5: Earthing (continued)
THE EARTHING CHAIN
Earthing is a serious matter and has many sections from the
point which requires to be earthed up to the final connection
with general mass of the earth. These can be compared to
the links of a chain and just as with a chain; an earthing chain
is as strong as its weakest link.
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Module 5: Earthing (continued)
SYSTEM EARTHING
System earthing relates to the earthing of the power system such as earthing
the windings of transformers. The same applies to the star point of an
alternator. Proper system earthing is required to ensure proper surge
protection and the proper functioning of other protective relays and devices.
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Module 5: Earthing (continued)
EQUIPMENT EARTHING
Equipment earthing relates to an earthing connection to the metal frames of
electrical machines, the metal housings that contain switching equipment or
other electrical apparatus, metal raceways containing electrical conductors,
closely adjacent conducting structures and any other extraneous conductive
parts judged to be vulnerable to contact by an energised conductor.
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Module 5: Earthing (continued)
COMBINED SYSTEM AND EQUIPMENT EARTHING
Proper electrical continuity between system and equipment earthing is
accomplished by means of earth continuity conductors. An earth continuity
conductor is a conductor, which includes any clamp or terminal, that
connects the consumer’s earth terminal to the exposed conductive parts of
an installation for the purpose of earthing such parts and for carrying fault
currents.
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Module 5: Earthing (continued)
EARTHING AT POWER STATIONS AND SUBSTATIONS
Good substation earthing is very important for effective protection and
operation of protective devices. When designing substation earthing, the
safety of people is the most important consideration taken into account.
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Module 6: Protection
EARTH LEAKAGE PROTECTION
The purpose of an earth leakage unit is to detect an earth fault current and to
automatically disconnect an installation or circuit from the supply, within a
specified time, when it exceeds a specified or predetermined value.
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Module 6: Protection (continued)
CORE BALANCE EARTH LEAKAGE RELAY
Core balance earth leakage relays are basically current transformers which
operate tripping devices when they detect an earth fault current which
exceeds a predetermined value.
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Module 6: Protection (continued)
PHASE IMBALANCE
In a multiphase installation, the circuits shall be so arranged that the total
load is, as nearly as is practically possible, balanced over the three phases.
With a rigidly balanced three-phase load such as a three-phase motor, any
imbalance in the supply current will indicate a fault. Single-phasing is a
severe case of phase imbalance.
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Module 6: Protection (continued)
SURGE PROTECTION
Lightning is a major cause of short-circuits on overhead transmission and
distribution lines. Transient voltages on the lines caused by lightning strikes
may exceed the insulation level of the line. This would cause a flashover
between a phase and earth or between phases. The function of a lightning
arrester is to provide a path by which these surges are discharged to earth
before a flashover on the line can occur.
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Module 6: Protection (continued)
EARTH CURRENT COMPENSATION
If the system is earthed at more than one point, a proportion of the earth fault
current will return by a path other than that at which the relays are located.
Earth compensation relays are required to correct this.
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Module 6: Protection (continued)
OVERLOAD RELAYS
An overload (overcurrent) relay is a protective device designed to protect a
motor from overload during starting and running.
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Module 6: Protection (continued)
SEVERE STARTING
Severe starting conditions can cause the contacts of contactors to overheat
and splatter or even weld together. Fuses in the supply line could deteriorate
and even melt. Severe starting can affect time delay features which could
cause nuisance tripping. Supply cables could be damaged unless severe
starting was taken into account when the cables were rated.
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Module 6: Protection (continued)
FUSES
The function of a fuse is to detect overcurrents or short-circuit currents and to
then automatically disconnect the faulty equipment or circuit from the supply.
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Module 7: Measuring instruments
WATTMETER (ELECTRODYNAMOMETER TYPE)
It is often necessary to measure the rate at which electrical energy is being
consumed. Power is defined as the rate of expending (converting energy
from one form to another) energy and is measured by means of a wattmeter.
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Module 7: Measuring instruments (continued)
CONNECTION OF A WATTMETER OR WATT-HOUR METER IN A
SINGLE-PHASE SYSTEM
There is:
• Direct connection; and
• Indirect connection.
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Module 7: Measuring instruments (continued)
WATT-HOUR METER (ENERGY METER)
The amount of electricity consumed is referred to as electrical energy and is
measured by means of a watt-hour meter. Since energy is a product of
power and time, both quantities must be taken into account in energy
measurement.
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Module 7: Measuring instruments (continued)
FREQUENCY METER
A frequency meter is used to measure the frequency of ac cycles per second
in an alternating current circuit. The unit in which frequency is measured, is
hertz (Hz).
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Module 7: Measuring instruments (continued)
POWER FACTOR METER
Power factor is the ratio of the true power to apparent power of an alternating
current circuit. Power factor meters are used to measure the power factor of
an alternating current circuit.
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Module 7: Measuring instruments (continued)
MAXIMUM DEMAND
Electrical demand comes at different times for entities such as consumers
and industries. In order to measure the above-mentioned energy
consumption, a maximum demand meter is used. Maximum demand meters
register not only the effective power consumption in kWh, but also the
highest average value of power in kW during the time period of metering.
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Module 8: Transformers
TRANSFORMER EQUATION
Because the efficiency of a transformer is so high, losses are often neglected
in simple calculations. If losses are neglected, the transformer is referred to
as an ideal transformer. If a transformer has no losses then the input will be
equal to the output:
𝑉1 𝐼1 𝑐𝑜𝑠𝜙1 = 𝑉2 𝐼2 𝑐𝑜𝑠𝜙2
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Module 8: Transformers (continued)
THREE-PHASE TRANSFORMERS
Three-phase transformers may be regarded as three single-phase
transformers with their primary and secondary windings connected in either
star or delta.
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Module 8: Transformers (continued)
TRANSFORMER TAPPINGS
In order to compensate for volt drops which might happen owing to extra
loads, tappings are made on the secondary side of supply transformers. Tap
changing switches are fitted to increase the voltage during peak periods and
to reduce it when the load reduces. Tap changing switches can be of the
manual or automatic types.
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Module 9: Electronics
DIODES
Semiconductor components are generally made in such a way that a p-type
material is in close contact with an n-type material. The interface between
the two is termed a p-n junction. Diodes are p-n junction devices.
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Module 9: Electronics (continued)
RATING OF DIODES
Following are four different ratings which are applied to diodes:
• Continuous current rating;
• Intermittent current rating;
• Power rating; and
• Reverse voltage rating.
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Module 9: Electronics (continued)
BIASING
Biasing refers to the application of a specific voltage polarity and value to
achieve desired working conditions.
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Module 9: Electronics (continued)
TESTING
A diode is tested to ensure that it will, within specified limits, conduct with the
polarity in one direction and be non-conductive when the polarity is reversed.
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Module 9: Electronics (continued)
DC OPERATION
A diode can be used to prevent a feedback signal or reverse voltage from
being applied to a sensitive circuit thus protecting the circuit from damage.
This is made possible by the very high impedance in reverse polarity.
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Module 9: Electronics (continued)
AC OPERATION
Owing to the reverse-blocking characteristics of a diode, it may be used as a
rectifier to convert ac to dc by allowing conduction during one half cycle and
blocking conduction during the following half cycle.
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Module 9: Electronics (continued)
DIODES AS RECTIFIERS
One of the first and still most popular uses of a diode, is to rectify alternating
current by converting it to direct current for a wide range of purposes.
These come in the forms of:
• Half-wave rectifier;
• Full-wave rectification with a centre-tap transformer; and
• Full-wave bridge rectifier.
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Module 9: Electronics (continued)
EFFICIENCY OF RECTIFIER CIRCUITS
A half-wave rectifier only makes use of half of the ac input cycle and its
utilisation efficiency cannot exceed 50%. In practice, it is always less than
50% owing to losses such as resistance losses (𝐼2 𝑅 losses).
A full-wave rectifier makes use of the full ac input cycle and its efficiency is
only dependent on circuit losses such as resistance losses (𝐼2 𝑅 losses).
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Module 9: Electronics (continued)
ZENER DIODES
Where constant voltage power supplies are required, use is often made of
Zener diodes in order to stabilise the voltage.
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Module 9: Electronics (continued)
NPN AND PNP TRANSISTORS
The circuit symbols of an NPN and a PNP transistor with their relative current
flows are seen:
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Module 9: Electronics (continued)
TRANSISTOR IN A COMMON EMITTER CIRCUIT
These can be formed as:
• Common emitter amplifiers;
• Common emitter switches (inverting mode); and
• Common emitter switches (non-inverting mode).
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Module 9: Electronics (continued)
OPERATION OF A THYRISTOR
Thyristors are used mainly as converters and for controlling electrical power.
Some examples of the applications of thyristors are speed control for motors,
self regulating battery chargers and static inverters.
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