Electrical Trade Theory N3 — Document
April 2021 — N3 — Electrical Trade Theory
N3
2021
Nated Engineering
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Q1— Section header —
1.1.1Explain how the temperature of a laundry iron can be controlled. (2\
1.1.2Give TWO possible fault conditions that could lead to an iron tripping the main earth leakage relay
1.1.3Which type of thermostat is used in this appliance (1\
1.2Draw a neat labelled circuit diagram to show how the two elements of a stove plate are connected to a three
Q2— Section header —
2.1Indicate whether each of the following must be earthed, must not be earthed or must be bonded together and earthed
2.1.1Portable drilling machine
2.1.2PVC conduits
2.1.3Metallic working surfaces in kitchens and laundries
2.1.4Hot water and cold water pipe systems
2.1.5
2.2Give the components of a magnetic circuit breaker/overload relay (not thermal magnetic type responsible for each of the following:
2.2.1Quick tripping in case of a short circuit (2\
2.2.2Time delay before tripping (3\ [10] 1104126310
Q3— Section header —
3.1FIGURE shows two discharge lamps connected to an electrical supply. FIGURE
3.1.1Name the discharge lamps labelled A and B. (2 x 1\ (2\
3.1.2Give TWO uses of each lamp. (2 2\ (4\
3.1.3Give ONE advantage of lamp B. (1\
3.2Redraw lamp B and indicate each of the following on the sketch: Electrodes Neon and argon gas Metallic sodium (3\ [10]10
Q4— Section header —
4.1The maximum value of the emf (1\
4.2The frequency of the supply (3\
4.3The effective value of the voltage (2\
4.4The period of the waveform (1\
4.5The instantaneous value 0,008 seconds from the start of the cycle (3\ [10] 1104126310
Q5— Section header —
5.1Calculate the following for a resonan condition:
5.1.1The capacitance of the capacitor (2\
5.1.2The current through the circuit (2\
5.1.3The voltage drop across the inductor (2\
5.1.4The voltage drop across the capacitor (1\
5.1.5The voltage drop across the resistor (1\
5.2Prove that the vector sum of the three voltage drops is 150 V. (2\ [10]10
Q6— Section header —
6.1Indicate whether the following statements are TRUE or FALSE by writing only 'True' or 'Fals e' next to the question number (6.1.1
6.1.5\in the ANSWER BOOK.
6.1.1The colours represent ing the three phases of a phase system are red, white and black.
6.1.2
6.1.3Three phase alternator windings are usually connected in star.
6.1.4The line voltage of a star connected system is 1,73 times the phase voltage.
6.1.5If the phase voltage of a three ph ase star connected system is 240 V the line voltage is 415 ,692
6.2are connected in star to a 415 three phase supply system. Calculate:
6.2.1The system phase voltage (2\
6.2.2The phase current (2\
6.2The line current (1\ [10] 1104126310
Q7— Section header —
7.1Electrical transformers are machines that transfer electricity from one circuit to another
7.1.1State how electrical energy is transferred from the primary of the transformer to its secondary winding.
7.1.2
7.2A three phase transformer has a rating of 30 kVA and is connected in delta star. The primary line voltage is 11 k V and the secondary line voltage is
7.2.1The transformer ratio (3\
7.2.2The secondary phase current (3\
7.2.3The primary line current (2\ [10]10
Q8— Section header —
8.1Name TWO protective devices found in a face plate starter used to start large DC motors. (2\
8.2Briefly explain armature reaction with reference to DC motors. (2\
8.3Draw a neat, fully labelled circuit diagram of face plate starter used to start a shunt motor.
Q9— Section header —
9.1Name the type of three phase motor starter used in each of the following starting methods:
9.1.1Resistance starting
9.1.2Reduced voltage starting (2 1\ (2\
9.2Explain how the direction of rotation of a three phase induction motor may be reversed. (2\
9.3Draw a neat, labelled circuit diagram of a capacitor start split phase motor. (4\
9.4What is the purpose of a centrifugal switch in single phase motors? (2\ [10] 1104126310
Q10— Section header —
10.1Draw a basic circuit diagram of the connections required for a wattmeter to be connected to a high current and high voltage AC supply. (5\
10.2Name ONE semi conductor material used to manufacture diodes and transistors. (1\
10.3FIGURE shows a configuration of a transistor. FIGURE
10.3.1Identify the configuration (1\
10.3.210
Mark Allocation
Total100 marks
Sections10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 + 10 = 100 marks
Paper information
TitleDocument
SubjectElectrical Trade Theory
LevelN3
Document typeQuestion paper
Year2021
MonthApril
Pages8
Downloads3
Examination context
Decade2020s
Years ago5y
SeasonAutumn (SH)
Start dayThursday
Exam periodApril 2021
Days since exam2,018
About Electrical Trade Theory examination preparation
This TVET examination resource can be used as part of a structured revision programme. Past papers are most useful when learners attempt questions independently, work within a realistic time limit, and review their answers afterwards.
How to use this resource for revision
A TVET past examination paper is most useful when it is treated as an active revision exercise. Try the questions yourself before checking the corresponding memorandum. Where you make an error, identify the reason — was it a knowledge gap, a calculation mistake, an interpretation issue, or an examination technique problem?
For stronger examination practice, complete the paper without unnecessary interruptions and keep track of the questions that take the most time. Afterwards, review the memorandum and separate knowledge gaps from mistakes caused by interpretation, calculation, structure, or examination technique.
understanding question requirements
applying subject knowledge
working under time constraints
checking answers
identifying revision priorities
A practical revision approach
Start by completing the resource without looking at the memorandum. Review incorrect and incomplete answers afterwards and make a short revision list based on the areas that caused difficulty.
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