ELECTRICAL TECHNOLOGY: POWER SYSTEMS EXAMINATION GUIDELINES GRADE 12 2021
TABLE OF CONTENTS
Page
1. INTRODUCTION
3
2. ASSESSMENT IN GRADE 12
4
3. ELABORATION OF CONTENT FOR GRADE 12 (CAPS)
6
4. PREPARING LEARNERS FOR THE NSC: ELECTRICAL TECHNOLOGY
17
5. FORMULA SHEET: POWER SYSTEMS
22
6. CONCLUSION
23
1. INTRODUCTION The Curriculum and Assessment Policy Statement (CAPS) for Electrical Technology: Digital outlines the nature and purpose of the subject Electrical Technology. This guides the philosophy underlying the teaching and assessment of the subject in Grade 12. The purpose of these Examination Guidelines is to:
Provide clarity on the depth and scope of the content to be assessed in the Grade 12 National Senior Certificate (NSC) Examination in Electrical Technology.
Assist teachers to adequately prepare learners for the NSC examinations.
This document deals with the final Grade 12 external examinations. It does not deal in any depth with the School-based Assessment (SBA), Performance Assessment Tasks (PATs) or final external practical examinations as these are clarified in a separate PAT document which is updated annually. These Examination Guidelines should be read in conjunction with:
The National Curriculum Statement (NCS) Curriculum and Assessment Policy Statement (CAPS): Electrical Technology
The National Protocol of Assessment: An addendum to the policy document, the National Senior Certificate: A qualification at Level 4 on the National Qualifications Framework (NQF), regarding the National Protocol for Assessment (Grades R–12)
The national policy pertaining to the programme and promotion requirements of the National Curriculum Statement, Grades R–12
2. ASSESSMENT IN GRADE 12
2. ASSESSMENT IN GRADE 12 2.1 Structure/Format of the question paper:
QUESTION
TOPIC
MARKS
TIME
GENERIC – ALL
1
Multiple-choice Questions
15
14 min.
2
Occupational Health and Safety
10
9 min.
GENERIC – ELECTRONICS AND POWER SYSTEMS
3
RLC Circuits
35
32 min.
SPECIFIC
4
Three-phase AC Generation
35
31min
5
Three-phase AC Generation
30
27 min.
6
Three-phase AC Generation
35
31min
7
Three-phase AC Generation
40
36min
TOTAL
200
180min
2.2 Cognitive levels
Bloom's Taxonomy consists of six levels, as shown below.
DESCRIPTION OF COGNITIVE LEVEL
LEVEL
EXPLANATION
SKILLS DEMONSTRATED
ACTION VERBS
CREATING
4
The learner creates new ideas and information using the knowledge previously learned or at hand. At the extended abstract level, the learner makes connections, not only within the given subject area, but also beyond it and generalises and transfers the principles and ideas underlying the specific instance. The learner works with relationships and abstract ideas.
The learner appreciates the significance of the parts in relation to the whole. Various aspects of the knowledge become integrated, the learner shows a deeper understanding and the ability to break down a whole into its component parts. Elements embedded in a whole are identified and the relations among the elements are recognised.
The learner grasps the meaning of information by interpreting and translating what has been learned.
Exemplifying
Comparing
Explaining
Inferring
Classifying
summarise, describe, interpret, calculate, contrast, associate, distinguish, estimate, differentiate, discuss, extend, comprehend, convert, explain, give example, rewrite, infer, review, observe, give main idea
REMEMBERING
1
The learner is able to recall, remember and restate facts and other learned information.
BASIC SKILLS LINKED TO THE SUBJECT: The following skills are measured in the question paper. Visibility of these skills gives an indication of the overall skills required in the subject:
Extraction and/or manipulation and/or evaluation of data
Explaining functional operation of circuits and/or components
NOTE:
CALCULATIONS
WAVEFORMS/FLOWCHARTS/CIRCUITS
Generally the criteria used for calculations are as follows:
Correct formula
Substitution of values
Simplifying of values
Answer and correct units
Waveforms will be assessed according to the following criteria:
Type of waveform (input/output)
Correct labelling
Correct plotting of values (correct values, proportional plotting)
Labelling and units on Y-axis
Labelling and units on X-axis
Phase relationship
3. ELABORATION OF CONTENT FOR GRADE 12 (CAPS)
TOPIC
PRESCRIBED CONTENT
MARKS
Multiple-choice
Covers all content
15
Occupational health and safety
OHS Act, 1993 (Act 85 of 1993)
Definitions
Purpose of the Act
General duties of employers to their employees
General duties of employers and self-employed persons to persons other than their employees
General duties of manufacturers and others regarding articles and substances for use at work
Duty to inform
General duties of employees at work
Duty not to interfere with, damage or misuse devices/items
Functions of health and safety representatives
Report to inspector regarding certain incidents
Victimisation forbidden
Offences, penalties and special orders of court
Safety Revision
Unsafe actions
Unsafe conditions
Dangerous practices
Risk analysis
Human rights in the workplace
Work ethics
Revision of emergency procedures (Grade 10)
10
RLC circuits (generic)
Effect of alternating current on R, L and C components in series and parallel circuits.
Describe/Define/Explain the following concepts and terminologies of RLC series and parallel circuits in words (not formulae):
Impedance
Inductive reactance
Capacitive reactance
Power factor
Phase angle
True power,
Resonant frequency
Q-factor
Bandwidth
Calculating the following in RLC series and parallel circuits containing ONE resistor, ONE capacitor and ONE inductor:
Frequency
Inductance
Inductive reactance
Capacitance
Capacitive reactance
Impedance
True power
Power factor
Phase angle
Voltage drop
Total current
Capacitive current
Inductive current
Q-factor
Bandwidth
Resonant frequency
Drawing of the phasor and wave representation of RLC circuits
Applications of RLC circuits
Conditions of series and parallel resonant circuits
Drawing of phasor diagrams of series, parallel and resonant circuits
Drawing of the characteristics curve and phasor diagram of resonant circuits
Illustrate the effect of changing frequency in an RLC circuit toward resonance
Calculations
Series RLC circuits containing ONE resistor, ONE capacitor and ONE inductor
Parallel RLC circuits containing ONE resistor, ONE capacitor and ONE inductor
(35)
Three phase AC generation (specific)
Principle of three-phase AC generation
Distribution networks – (outline generation network to distribution network)
Generation
Transmission
Distribution
Advantages of three phase over single phase
Disadvantages of single phase over three phase
Waveform of single- and three-phase systems
Know how to draw a fully labelled waveform of a three-phase system
The labelling of a waveform should be according to universally adopted sequence which is R.Y.B
Phasor diagram of single- and three-phase systems
Phasor diagram of a single- and three-phase systems
Know how to draw a fully labelled phasor diagram for a threephase systems
Show phase angles.
Show the direction of rotation
Three Phase Systems (3ϕ)
Star connection and symbol
Delta connection and symbol
Delta vs. Star (Know difference between the two)
Schematic (sketch without indication of components)
Diagrammatic (sketch with components) representations of threephase systems.(overview of distribution network, power station to end users)
Only balanced loads
Power in Three-phase (3ɸ) Systems and Calculations
Describe/Define/Explain the following:
Active Power
Reactive Power
Apparent Power
Calculations
Active power (True power) P = √3 ×V× LIL × Cos θ
Reactive Power Q ( PR )= √3 × VL ×IL ×Sin θ
Apparent Power s (Papp) = √3 x VL x LL
NOTE: Know power formula as the formula sheet is the guide not all formulae are there. Manipulation of formulae is key to the calculations in this section
Introduction to Star and Delta Calculations Star
Line voltage and current VL = √3VPH and IL = IPH
Delta
Line voltage and current VL = VPH and IL = √3IPH
Losses
Efficiency η = output power x 100% or η = input - losses x 100% input power input power
Importance of power factor correction for consumers and suppliers: No calculation
Only Application of Meters in Three Phase (3ϕ)
Wattmeter
kWh meter (Energy meter)
Power factor meter
Two- and three-wattmeter connection/method
Calculation of two- and three-wattmeter
Methods of connections
(35)
Three-phase transformer (specific)
Introduction of three-phase transformers
Principle of operation and connections of three-phase transformers
Describe/Explain the principle of operation of three-phase transformer in the correct order of processes.
Connection of three-phase transformer
Delta/star
Star/star
Star/delta
Delta/delta
Concept, explanation and understanding of losses
Copper losses (IR2): Losses due to the resistance of the wire.
Iron losses (core losses): Heat losses occurring due to the hysteresis of the core and eddy current
Eddy current
Hysteresis losses
Conceptual meaning of losses
Sources of heat generation in a transformer
Factors contributing to heat
Three-phase transformers compared to single-phase transformers
Comparison of single-phase transformers and three-phase transformers
Application of delta/star connection transformers
Construction of transformers
Know the types of the transformer construction
The comparison between the types of transformers
Application of transformers
Know and understand the :
Application of a transformer
The main function of a transformer
Cooling
Understand the cooling methods
Safety
Know the importance of safety in the workshop when working with a transformer
Protection
Identify/Describe/Explain the use of the protection devices in transformers
Calculations (Balanced Loads only)
Know, understand and apply the formula to :
Calculate the:
Transformation ratio
Number of turns
Line and Phase current, voltage and power
Know how to manipulate the formula
Power factor
Define/Explain power factor
Power
Know the calculations of power
Load including losses and efficiency
Know how to describe/Explain losses in transformers
Calculate efficiency
30
Three-phase motors and starters (specific)
Introduction to Three-phase (3ϕ) Motors
Three-phase squirrel-cage induction motor
Know and Understand :
How to read and interpret the details on the name plate of the motor
The purpose of three phase squirrel-cage induction motor
Explain the function/use of starter to start the induction motor
Principle of operation
Explain and know the function of each parts of squirrel-cage motor
Describe/Explain the operation of three-phase squirrel-cage induction motor
Know how the rotating magnetic field is produced or created
Construction
Describe the construction of a squirrel-cage motor
Explain and know the function of each parts of squirrel-cage motor
Advantages
Know and understand the:
Advantages of three phase squirrel-cage induction motor compared to single-phase motor.
Applications
State where to use the induction motor
Calculations on slip, power and efficiency
Describe/Explain the conceptual meaning of the following:
Slip
Efficiency
Power
Know the calculation of :
Slip – revs per minute: S = ns - nr
Per unit slip: S = ns - nr ns
Percentage slip: S = ns - nr x 100% ns
NOTE: Know and understand all the formulae as the formula sheet is the guideline and not all formulae are there
Understand the:
Relationship of speed vs torque
How the maximum torque is determined
Characteristic curve of torque vs speed
Synchronous Speed
What is synchronous speed? Knowledge and understanding the:
Conceptual meaning of synchronous speed
The equation of calculating synchronous speed
Relationship between synchronous speed and generated power
Electrical and Mechanical Aspects of Three-phase (3ϕ) Motors
Fault-finding/Troubleshooting
Motor testing. Know and understand the
Insulation resistance test
Continuity test
Loose connection test
Bearing test
Commissioning. The process involved in preparing the motor and starter to be used by the operator
Understand the checklist for commissioning a new installed motor.
Understand the starting of an electrical induction motor
3Φ Direct-on-line Starter with Overload
Function of components on diagrams
Identifying the components from the given circuit diagram
Know the description of each components
Describe/Explain the function of the starter
Principle of operation
Describe/Explain the operation of direct-on-line motor starter
Diagram
Identifying the control circuit
Know how to draw the circuit diagram
Wiring on a panel
Understand the wiring and working of the panel
Calculation of the overload value and setting of the overload
Calculate the power at full load.
3Φ Forward and Reverse Starter with Overload
Function of components on diagrams
Know all the components and their description.
Know the function of each component in the starter circuit.
Principle of operation
Know and understand:
In chronological order describe/explain the 3Φ forward and reverse starter with overload
How the direction of rotation of three-phase motors can be reversed
Diagram
Identifying the control circuit
Know how to draw the circuit diagram
Wiring on a panel
Understand the wiring and working of the panel
Explain the direction of rotation of a three-phase motor
Calculation of the overload value and setting of the overload
3Φ Sequence Motor Control Starter with Overload (Without Timer)
Function of components on diagrams
Know all the components and their description
Know the function of each component
Principle of operation:
Describe/Explain the operation of 3Φ sequence motor control starter with overload
Diagram
Identifying the type of the control circuit
Know how to draw the circuit diagram
Wiring on a panel
3Φ Sequence Motor Control Starter with Overload (With Timer)
Function of components on diagrams
Know all the components and their description
Know the function of each component
Principle of operation
Describe/Explain the sequential operation of 3Φ sequence motor control starter with overload (with timer)
Diagram
Identifying the type of the control circuit
Know how to draw the circuit diagram
Wiring on a panel
3Φ Automatic Star Delta Starter with Overload
Function of components on diagrams
Know all the components and their description
Know the function of each component.
Principle of operation
In chronological order the operation of 3Φ automatic star delta starter with overload
Diagram
Identifying the control circuit
Know how to draw the circuit diagram
Wiring on a panel
Understand the wiring and working of the panel
Calculation of the overload value and setting of the overload
Programmable logic controllers (PLCs) (specific)
Introduction to the Programmable Logic Control Device
History of the PLC
Method used for automated control for industrial production and processes at the time before PLC where developed.
Disadvantages of using relays for automated control as compared to PLCs
Know and understand why the PLC's where developed
Hard wiring vs. Soft wiring, know and understand the:
Conceptual meaning of hard wiring and soft wiring
Advantage of soft wiring vs hard wiring
Disadvantage of hard wiring vs soft wiring
Conceptual meaning of each advantage and disadvantage of hard wiring and soft wiring
The programmed scan cycle of a PLC
Steps that a PLC undergo to complete one programmed scan cycle (input scan, process scan and output scan)
Sequential operation of the PLC
Safety measures to be considered during programming of the PLC
PLC Software and Devices Difference between analogue and digital/discrete inputs
Know and understand the conceptual meaning of:
Analogue input
Digital input
Know the importance of converting:
Analogue input to digital input
Digital output into analogue output
Logic gates and truth tables of AND, OR, NAND, NOT, NOR inputs to a PLC (Digital) (Can this be removed)
Relate either the given logic gate symbol, the truth table and the ladder diagram to:
Truth table
Ladder logic diagram
Symbol
Input devices to PLC
Examples of:
Switches as input devices
Push button (N/O) and (N/C)
N/O and N/C contacts/switch
Sensors as input devices
Proximity sensor
Temperature sensor
Light sensor
Level sensor
Overload sensor
Know the application/use of each sensor as input devices to PLC:
Proximity
Temperature
Light
Level
Overload
Outputs on a PLC
Transistor/Relay
Operation of transistor/relays
Contactors/relays:
Operation of contactors/relays (revision)
Correct symbols as used in:
Hard wiring and
Soft wiring circuit
Timers:
The purpose of timers as used in the operation of a PLC's logic circuit
Explaining/describing the following:
On delay timer contact
Off delay timer contact
Know when each timer contact is applied to the PLC logic circuit
Know and use the symbols of on delay and off delay timer contact in the PLC's logic circuit
Explain/Describe the sequential operation of the ladder logic diagram with timer function
Latching concepts (retaining circuits):
Identification of the 'latching/retaining' circuit in a control circuit
Understanding the use/function of the latching circuit
Interlocking:
Conceptual meaning of interlocking as used in the control circuit
Correct symbols used for interlocking
Markers/Flags (Memory elements): Purpose of markers as used in the ladder logic diagram
Conversion of hard-wired schematics (Control circuits) to ladder logic and labelling of symbols (motor starters only)
Identify the type of the control circuit to be converted
Know how to draw:
The power rails of the ladder logic diagram
Rungs with input and outputs between the power rails of the ladder logic diagram
Know the function of each components as in the control circuits
Know the sequence in which ladder logic operate
NOTE: Use the Engineering Graphics and Design (EGD) approach and not crude freehand drawings and that ladder logic circuit is drawn from left to right and operate from top to bottom. Applications of PLCs: The PLC as a motor starter
The variable speed drive (VSD) as a programmable motor controller (concepts only)
Know and understand the function of VSD as used in motors
Explain/describe the positive effect of applying VSD in motors
Basic principle of operation
Input to VSD,
Output of VSD to the motor
Relationship between the speed of an AC motor and the frequency supplied to the motor
Introduction to VSD, know and understand the following:
Part (sections) of the VSD, namely:
Rectifier circuit
Filter circuit
Inverting circuit
Understand how the switches functions
Understand the effect the switching time has on the output waveform and the frequency
Function of each part
Sequential operation of the VSD
Methods of speed control
The V/Hz
Vector drives
Conception understanding of V/Hz and vector drive
Basic block diagram
Types of motors used with a VSD
Know the types of motors used with a VSD and where they are used.
Regenerative braking
Conceptual understanding of the term regenerative breaking
Understand the conversion from one form of energy to another during the braking process
Conceptual understanding of the term regenerative energy
Purpose of the braking resistor as used in the motor control circuit
Examples of regenerative braking
Know and understand the diagram showing how regenerative braking is applied to a motor control circuit
Basic applications of VSD
Application of VSD
Safety aspects to consider in the application of VSD
Start-up and run profiles (with applications) (programming – optional)
Characteristics curve of speed vs torque
Understand the relationship between speed and torque
Know and understand the speed range of (e.g. breakdown speed)
Characteristics curve of current vs torque
Understand the relationship between current and torque
(40)
TOTAL
200
4. PREPARING LEARNERS FOR THE NSC: ELECTRICAL TECHNOLOGY Learners do not intuitively know how to answer a question paper successfully. Teachers need to prepare learners to have the skills needed to negotiate a question paper successfully. This preparation process starts in Grade 10 and culminates in Grade 12. Learners need to be coached in some of the following skills, which will help them in answering the question paper effectively: Manipulation of formulae: The learners must learn how to use the standard formula, manipulate the formula correctly, correctly substitute values and remember to always add a value/unit with an answer. Prefixes and units: Learners must have a clear understanding of the conversion and uses of units and abbreviations, such as kilo, milli, micro, nano, etc. Teachers should drill leaners on this skill. Learners must be taught on how to approach a question paper and ANSWER BOOK. Planning the answers: Learners must know how to answer in a chronological order of sequence and know how much space should be taken up by a typical answer. Do not break questions up and answer it haphazardly out of order. Ensure the numbering convention in the question paper is followed in the ANSWER BOOK. Open spaces in the ANSWER BOOK: Teachers should encourage learners to answer ALL questions, including subquestions, and not leaving open spaces. Even when learners are unsure of the expected response, they are urged to answer to the best of their ability. This may lead their train of thoughts in the correct direction leading the learner to a correct or partially correct answer. Where learners leave an open space to proceed with the question paper, they should be taught to return to that space when the rest of the question paper is completed. Teachers are urged to pay attention to Bloom's Taxonomy and should prepare learners to answer basic recall questions as well as more complex and intricate sentence-type questions, e.g. the paragraph- type answers such as the operation of a transformer. Learners should be coached to regularly read questions and answers to homework and tests aloud in the class. Teachers must encourage learners to engage in intelligent debate and discussion around subject content and on how an answer should be constructed. Learners must know how to structure their sentences in order to communicate what they are trying to say. Learners must learn how to list facts. Answers are assessed on the principle of a single mark for a single fact. Teachers must show the learners the difference between a sketch, a symbol and what a block diagram represents. Below is an example of a sketch. It was drawn freehand and is a resemblance of a real-world device. Marks are awarded for drawings WITH LABELS. A drawing cannot be assessed without labels. FIGURE 1: SKETCH Symbols: Symbols are simple representations of electronic devices and relates to the theory of how the device works and not necessarily to the appearance of the device. Without labelling and a title, it cannot be marked effectively. FIGURE 2: SYMBOL AND HOW IT IS MARKED Block diagrams: Block diagrams are used extensively in Electrical Technology. It usually relates to processes and how devices operate. They are representative of the operation of a system/device and may not contain any physical resemblance to the device. Note that block diagrams may be given semi-complete, requiring the learner to fill or complete the other sections. FIGURE 3: BLOCK DIAGRAM All sketches, symbols, diagrams and waveforms must always be labelled and have a caption. Learners must be shown how to interpret and use waveforms to support their answers. Ladder logic: Ladder logic diagrams must be labelled and have each of the operands identified. FIGURE 4: LADDER LOGIC Sketches, diagrams and waveforms should be clear, not too small and easily interpretable. Guard against small and illegible drawings. Circuit diagrams: Circuit diagrams are marked on the following premise:
The circuit or portion of the circuit must be correct.
All components must have labels.
Note that whole circuits or portions of a circuit may be given and interrogated.
FIGURE 5: CIRCUIT DIAGRAM Calculations: Calculations should be done showing ALL steps. Values must be placed correctly. Units allocated to the calculation must be shown. Wrong units will result in the answer being marked wrong. No units will result in the answer being marked wrong. It is good practice to draw a line underneath the final answer ending it in a small arrow. This indicates that the calculation is done. FIGURE 6: A CALCULATION AND HOW TO MARK IT Lines must be drawn between questions. Rough work should be labelled as rough work and have a line drawn through it. Power factor (Cosθ): When using the power factor, learners should note whether they are given the power factor or the phase angle. Learners use the power factor as an angle, resulting in their answers being incorrect. Phasor diagrams should always include an arrow showing its direction of rotation. As a phasor is a rotating vector and always rotates anticlockwise, it is required of learners to show this when doing graphical representations of phasors. FIGURE 7: PHASOR DIAGRAM Input and output waveforms: It is common in Electrical Technology to enquire from the learner what the result of an input waveform in a circuit may have on the output of the circuit. This is because the principle of input, process, output forms the cornerstone of how electric and electronic circuits operate. By placing input and output waveforms directly underneath each other, in a synchronised fashion, the manner in which a circuit will affect a waveform is easily illustrated. The same applies to digital circuits. FIGURE 8: INPUT AND OUTPUT WAVEFORMS
5. FORMULA SHEET: DIGITAL ELECTRONICS NOTE: This formula sheet is only a guide and may not contain ALL the formulae as in the prescribed textbook and/or CAPS policy document.
6. CONCLUSION It is envisaged that these Examination Guidelines will serve as an instrument to strengthen and empower teachers to set valid and reliable assessment items in all their classroom activities. This Examination Guidelines document is meant to articulate the assessment aspirations espoused in the CAPS document. It is therefore not a substitute for the CAPS document which teachers should teach to. Qualitative curriculum coverage as enunciated in the CAPS cannot be over-emphasised.