Domain:
Certificate
in
Electrical
(Electrical General)
Subject:
Electrical Trade Theory
Tittle:
Mock assessment
Level:
3
Trainer:
Ms Hilma Shikongo
Moderator:
Mr Faustinus Faustinus
Marks:
100
Due date:
Monday, 27 October 2020
Instructions to candidates
1.
2.
3.
4.
5.
6.
Fill in your name, surname and date in the space provide
Read carefully before you answer.
Answer all the questions
Avoid looking at your neighbour’s paper
Ensure that your writing is neat and legible
This question paper consists of 5 pages including the cover page
NB: You must obtain 50% and above, to be declared competent.
Competent
Not
Competent
Yet
Engineering
QUESTION 1: DOMESTIC APPLIANCES
Marks [10]
1.1 Name TWO types of space heaters. (2)
1.2 Name the alloy that is normally used to manufacture the heating elements mentioned in
QUESTION 1.1 above. (1)
1.3 A stove-plate connected to a three-heat switch has elements each having a resistance of 50
ohms.
Calculate the resistance of the circuit during each of the following switching positions:
1.3.1 Low (2)
1.3.2 Medium (1)
1.3.3 High (3)
1.4 State how the two elements in QUESTION 1.3.1 above are connected in order to produce a low
level of heat. (1)
QUESTION 2: PROTECTION
MARKS (10)
2.1 Circuit breakers and fuses perform similar function in electrical circuits. State TWO functions of
these protective devices. (4)
2.2 FIGURE 2.1 below illustrates one compulsory test to be conducted in an installation. The purpose
of this test is to simulate an earth fault.
Name the components identified in the diagram. Write only the name of the components next to
the question number (2.2.1–2.2.3)
(3)
FIGURE 2.1
2.3 Earth continuity conductors form part of the earthing chain of an installation.
Explain what are earth continuity conductors?
(3)
QUESTION 3: ILLUMINATION
MARKS (10)
3.2 Why is a reversing switch required in a DC fluorescent lamp circuit? (6)
3.3 Indicate whether the following statements are TRUE or FALSE. Choose the answer and write only
'True' or 'False' next to the question number (3.3.1–3.3.2)
3.3.1 Light emitting diodes emit light when a voltage is applied across them.
3.3.2 A glow starter actually starts the lamp by opening the circuit. ( 2× 2) (4)
QUESTION 4: ALTERNATING CURRENT THEORY MARKS (10)
An alternating voltage waveform is represented by e= 200 sin (628,319 t x 57,3) 0 volts. Calculate the
following:
4.1 The effective value of the emf (2)
4.2 The average value of the emf (2)
4.3 The frequency (2)
4.4 The period of the waveform (1)
4.5 The instantaneous value of the emf 2 milliseconds after the commencement of a cycle. (3)
QUESTION 5: SERIES RLC CIRCUITS
MARKS [10]
5.1 The series RLC circuit in FIGURE 5.1 has a leading power factor of 0,89.
FIGURE 5.1
Study the circuit diagram above and calculate the following:
5.1.1 Inductive reactance (2)
5.1.2 Capacitive reactance (2)
5.1.3 The current flowing (4)
5.2 Draw the phasor diagram of the circuit. (2)
QUESTION 6: THREE-PHASE AC SYSTEMS MARKS (10)
Various options are given as possible answers to the following questions. Choose the answer and
write only the letter (A–D) next to the question number (6.1.1–6.1.4)
6.1.1 A three-phase supply has more advantages than a single-phase
supply because …
A)
B)
C)
D)
the line voltage is in phase with the line current.
the phase voltage is greater than the line voltage.
two different voltages are available
None of the above
6.1.2 The line current of a star-connected system is …
A)
B)
C)
D)
1,73 times the phase current.
√3 times the current.
equal to the phase current.
the phase current divided by √3 .
6.1.3 The colours used to represent the three phases are …
A)
B)
C)
D)
red – white – blue.
red – white – black.
red – yellow – black.
red – green – blue.
6.1.4 Three-phase alternator windings are usually connected in …
A)
B)
C)
D)
star.
delta.
star and delta.
None of the above
6.1.5 Single-phase loads can be connected to a three-phase distribution system that is …
A)
B)
C)
D)
three wire delta connected.
four wire star connected.
four wire delta connected
three wire star connected
(5 × 1) (5)
6..2 If the voltage in a three-phase, four wire AC supply is measured between lines, it gives a reading
of 415 volts.
Calculate the voltage between line and neutral. (3)
6..3 If the current drawn from the supply in QUESTION 6.2 above is 25 A, calculate the apparent
power. (2)
QUESTION 7: TRANSFORMERS
MARKS (10)
7.1 State the function of each of the following components of a transformer:
7.1.1 Iron core
7.1.2 Primary and secondary winding insulation
(2 × 1) (2)
7.2 A 300 kVA three-phase transformer is connected in star-delta and the winding ratio is 15:1. The
secondary voltage is 380V. Calculate the following:
7.2.1 The secondary line current (2)
7.2.2 The primary phase current (3)
7.3 Draw the circuit diagram for the transformer connection in QUESTION 7.2 above. (3)
QUESTION 8: DC MACHINES
8.1 FIGURE 8.1 below illustrates the load characteristics of a compound motor.
Name the components identified in the diagram. Write only the name of the
components next to the question number (8.1.1–8.1.5) in the
ANSWER BOOK.
(5)
8.2 State THREE applications where the load characteristics in QUESTION 8.1 can be used. (3)
8.3 Compare the operating characteristics of the shunt motor and series motor with reference to
starting torque. (2)
QUESTION 9: AC MACHINES
MARKS [10]
9.1 Name TWO types of single phase AC motors. (2)
9.2 Name the type of three-phase motor that will use the following starting method:
9.2.1 Resistance starting
9.2.2 Reduced voltage starting (2 × 1) (2)
9.3 State TWO reasons why universal motors are more suitable than single phase induction motors
for use in portable tools. (4)
9.4 State how the start winding is automatically disconnected after starting in a
single-phase split-phase motor. (2)
QUESTION 10: MEASURING INSTRUMENTS AND ELECTRONICS MARKS (10)
10.1 Draw a neat circuit diagram to show how one wattmeter is connected to measure the input
power to a balanced three-phase four wires load. Clearly indicate the voltage and current coil on
your circuit diagram. (4)
10.2 FIGURE 10.1 below illustrate a full-wave rectifier using four diodes. Name the components
identified by arrows in the diagram. Write only the name of the components next to the question
number (10.2.1–10.2.4)
(4)
FIGURE 10.1
10.3 Name TWO semiconductor materials used in the electrical industry. (2)