Electrical Trade Theory N2 — Document
April 2020 — N2 — Electrical Trade Theory
N2
2020
Nated Engineering
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Q1— Section header —
1.1Cable can carry fault currents that are much higher than their rated current. Name THREE factors that affect the permissible fault current that a cable can carry. (3)3
1.2Determine the full-load line current of a 380 V, 60 kW, three-phase motor that has a full-load power factor of 0, 8. (3)3
1.3Name FOUR types of inductive loads used in an AC circuit. (4)4
1.4In which unit is apparent power measured? (1) [11]11
Q2— Section header —
2.1Indicate whether the following statements are TRUE or FALSE by writing only 'True' or 'False' next to the question number (2.1.1–2.1.3) in the ANSWER BOOK.
2.1.1The casing of a circuit breaker is moulded glass fibre.
2.1.2A circuit breaker may be used as a local switch disconnector provided that it complies with the standards of a switch disconnector.
2.1.3The contacts of a circuit breaker are generally made of silver tungsten. (3 × 1) (3)3
2.2Use the table below to compare relays and contactors with regard to factors as indicated by the headings by filling in the missing word(s) next to the question number (2.2.1 – 2.2.6) in the ANSWER BOOK.
2.3List THREE types of joints used to join conductors in high-voltage applications. (3) [12] Copyright reserved Please turn over (11041872) -4-12
Q3— Section header —
3.1Complete the following sentence by writing only the missing word or words next to the question number (3.1.1 – 3.1.2) in the ANSWER BOOK. If the direction of a current through the field windings of a compound motor is
3.2Give ONE disadvantage of a series motor. (1)1
3.3Refer to FIGURE 1 below and answer the following questions: B A D E FIGURE 1
3.3.1Identify FIGURE 1 above. (1)1
3.3.2I dentify the parts labeled A to E . Write only th e answer next to the letter (A – E ) in the ANSWER BOOK. (5 × 1) (5)5
3.4Draw a labelled circuit diagram of a series motor. (3) [12] Copyright reserved Please turn over (11041872) -5-12
Q4— Section header —
4.1Name TWO types of windings that can be found on a capacitor-start / capacitor-run motor. (2)2
4.2Explain the purpose of a centrifugal switch as used in a single-phase induction motor. (2)2
4.3With reference to AC motors, explain what is meant by the following:
4.3.1Stator
4.3.2Rotor (2 × 1) (2)2
4.4Explain why the rotor of an induction motor cannot rotate at synchronous speed. (2)2
4.5Compare the following types of rotors with reference to their construction:
4.5.1Squirrel-cage rotors
4.5.2Wound rotors (2 × 1) (2)2
4.6Name FIVE advantages of three-phase induction motors. (5) [15]15
Q5— Section header —
5.1A distribution system consists of overhead lines and an outdoor substation. The outdoor substation contains switch disconnectors, surge arrestors, overload protection and star-delta transformers Explain how each of the following is earthed in this system:
5.1.1Transformers (2)2
5.1.2Overhead lines (2)2
5.1.3Entire substation (3)3
5.2Briefly explain the term floating earth . (2)2
5.3Give THREE reasons why the supplier's neutral conductor in a low-voltage system is earthed. (3) [12] Copyright reserved Please turn over (11041872) -6-12
Q6— Section header —
6.1Explain how time delay is obtained with bi-metal type overload relay. (2)2
6.2Briefly explain the effect of ambient temperature on the accuracy of thermal overload relays. (4)4
6.3The earth-leakage device has a test button that requires the user to test regularly for correct operation, thus ensuring that earth protection is optimal. Explain the condition that exists and the resulting action when the test button
Q7— Section header —
7.1Explain the function of an energy meter. (2)2
7.2Name TWO coils that a wattmeter in a single-phase system consists of. (2)2
7.3Give TWO types of wattmeters. (2) [ 6 ]
Q8— Section header —
8.1The turn’s ratio of a single-phase transformer is 25:1. Calculate the following:
8.1.1The secondary voltage when the transformer is connected to a 220 V AC supply
8.1.2The input current if the transformer delivers 10 A (2 × 2) (4)4
8.2A 220 / 11 V single-phase step-down transformer has 2000 primary turns. Determine the following:
8.2.1The transformer ratio
8.2.2The number of secondary turns (2 × 2) (4) Copyright reserved Please turn over (11041872) -7-
8.3A three-phase delta-delta transformer is connected to a 2,2 kV supply. The secondary phase voltage is measured and found to be 220 V. Determine the following:
8.3.1The primary phase voltage (1)1
8.3.2The secondary line voltage (1)1
8.3.3The primary phase current when it draws its full load current of 120 A from the supply (2) [12]12
Q9— Section header —
9.1Explain ho w the Z ener diode operates in reverse - bias mode. (3)3
9.2Explain the difference in current flow in NPN- and PNP-transistors. (2)2
9.3Answer the following questions on P-N junction diodes:
9.3.1State what happens when the reverse voltage of a diode is exceeded. (1)1
9.3.2Explain the function of a rectifier. (2)2
9.3.3When is the diode said to be forward-biased? (2) [10] TOTAL: 100 Copyright reserved Please turn over (11041872) FORMULA SHEET Any applicable formula may also be used. Star V L = 3 V PH I = I L PH Delta V = V L PH10
Mark Allocation
Total100 marks
Sections11 + 12 + 12 + 15 + 12 + 10 + 12 + 10 = 94 marks
Per-question sum100 marks
Paper information
TitleDocument
SubjectElectrical Trade Theory
LevelN2
Document typeQuestion paper
Year2020
MonthApril
Pages8
Downloads3
Examination context
Decade2020s
Years ago6y
SeasonAutumn (SH)
Start dayWednesday
Exam periodApril 2020
Days since exam2,385
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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