ET 61
Candidate Code No.
For Board Use Only
Result Result
Date Date
Int Int
ELECTRICIAN’S THEORY EXAMINATION
17 June 2017
QUESTION AND ANSWER BOOKLET
Time Allowed: Three hours
INSTRUCTIONS – READ CAREFULLY
You have 10 minutes to read this paper but do not start writing until you are told to
do so by the supervisor.
Write your Candidate Code Number in the box provided above. Your name must NOT
appear anywhere on this paper.
Answer all questions.
The pass mark for this examination is 60 marks.
Use a pen for written answers. Do not use a pencil or a red pen.
Drawing instruments and pencils may be used when diagrams are required. Marks
are allocated based on correctness.
Do not use correcting fluid or correcting tape.
For calculation questions all workings, including formulae, must be shown to gain full
marks.
Non-programmable calculators may be used. A formulae sheet may be used.
Warning – You could get 0 marks for any question, or part of a question, if you show
anything hazardous or dangerous in your answer.
Candidates are not permitted to use any Act, Regulation, Standard, Code of
Practice, Handbook or other reference text in this examination.
PLEASE HAND THIS PAPER TO THE SUPERVISOR BEFORE LEAVING THE ROOM.
(turn over)
Question 1
(a) Explain why it is necessary to have a protective earthing conductor in a
final subcircuit protected by an RCD.
(2 marks)
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(b) A voltage test for isolation is being carried out on a three-phase, 400V oven
that may or may not be live.
State the main safety issue that would occur if the only testing done was
between each phase.
(2 marks)
____________________________________________________________
____________________________________________________________
____________________________________________________________
(c) A three-phase PVC flexible cable supplying a 400V, three-phase motor has
been replaced with a three-core neutral-screened flexible cable.
State ONE advantage that a neutral-screen flexible cable has when
compared to a PVC flexible cable.
(2 marks)
____________________________________________________________
____________________________________________________________
____________________________________________________________
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 2
Question 1 continued
(d) Each electrical installation has a prospective short-circuit current rating.
State ONE reason why it is important to consider the prospective-short-
circuit current rating when selecting fittings and accessories (such as the
main switch and earth and neutral busbars) for the main switchboard of the
electrical installation.
(2 marks)
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(e) Each electrical installation has a prospective short-circuit current rating.
State ONE reason why it is important to consider the prospective-short-
circuit current rating when selecting protective devices for the main
switchboard of the electrical installation.
(2 marks)
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 3
Question 1 continued
(f) State ONE reason why a phase-reversal relay would be installed in the final
subcircuit supplying a three-phase crane hoist.
(2 marks)
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(g) A 230V, plug-in heater has an output of 2kW. The supply voltage to the
heater fluctuates between 230V to 240V.
Describe the effect this voltage fluctuation has on the power output of the
heater elements.
(2 marks)
____________________________________________________________
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(h) State how a thermistor protects a three-phase induction motor when a
mechanical overload occurs.
(2 marks)
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 4
Question 1 continued
(i) (i) What do the initials BJT mean when applied to semiconductors?
(1 mark)
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(ii) What is a transducer?
(1 mark)
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(j) State TWO main functions of a ballast (choke) in a fluorescent light fitting.
(2 marks)
(1) ________________________________________________________
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(2) ________________________________________________________
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 5
Question 2
(a) (i) Explain why rewireable fuses are classed as coarse excess current
protection.
(2 marks)
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(ii) Explain why HRC fuses and MCBs are classed as close current
protection.
(2 marks)
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 6
Question 2 continued
(b) This figure shows the operating characteristics of an HRC fuse under fault
conditions.
Refer to the figure and state what each of the numbered parts show.
(4 marks)
A ________________________________________________________
B ________________________________________________________
C ________________________________________________________
D ________________________________________________________
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 7
Question 2 continued
(c) Six 230V final subcircuits are supplied from a distribution switchboard. All
final subcircuits are protected by 20A HRC fuses.
The submains to the distribution switchboard is protected by a 63A HRC
fuse.
Use the terms pre-arcing time and arcing time to explain how correct
discrimination is achieved between the 20A HRC fuse and the 63A HRC
fuse.
(2 marks)
____________________________________________________________
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____________________________________________________________
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 8
Question 3
Introduction
This is a diagram of a 230V bakery oven with three elements.
Final Bake
element
Pre-heat bake First bake
element element
● Final Bake element rated at 12 kW when operating at 230V.
● First Bake element rated at 8 kW when operating at 230V.
● Pre-bake element rated at 4 kW when operating at 230V
Use the information in the introduction to this question to answer parts
3(a), 3(b) and 3(c).
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 9
Question 3 continued
(a) Calculate the total resistance of the oven elements.
(6 marks)
(b) Calculate the total current drawn by the oven when all elements are
operating.
(2 marks)
(c) Calculate the total power dissipated by the oven when all elements are
operating.
(2 marks)
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 10
Question 4
Most electrical installations are supplied from an MEN system of supply.
(a) The earth fault loop is an integral part of an MEN system of supply to an
electrical installation.
(i) State ONE reason why the earth fault loop through the main neutral
conductor to the star point of the distribution transformer is more
important than the earth fault loop through the main earth and mass
of earth to the star point of the distribution transformer.
(2 marks)
________________________________________________________
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(ii) State ONE reason why the main neutral conductor and the main
earthing conductor in an electrical installation could be carrying similar
currents.
(2 marks)
________________________________________________________
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(b) State TWO factors that determine the level of the prospective short-circuit
current in a low voltage electrical installation.
(2 marks)
(1) ________________________________________________________
________________________________________________________
(2) ________________________________________________________
________________________________________________________
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 11
Question 4 continued
(c) One function of the main earthing conductor and the earth electrode in a
low voltage MEN electrical installation is to connect protective earthing
conductors and equipotential bonding conductors to the mass of earth.
State TWO other functions of the main earthing conductor and earth
electrode.
(2 marks)
(1) ________________________________________________________
________________________________________________________
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(2) ________________________________________________________
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(d) State FOUR components of an MEN system that are found in a MEN low
voltage electrical installation.
(2 marks)
(1) ________________________________________________________
(2) ________________________________________________________
(3) ________________________________________________________
(4) ________________________________________________________
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 12
Question 5
(a) Tests have been carried out on a three-phase 400V, Class I, plug-in
electrical appliance after it has been repaired. The test results are as
follows:
Test Result
Earth continuity test Out of range
Insulation resistance test Infinity
(i) What type of fault is indicated by the test results?
(1 mark)
________________________________________________________
(ii) State THREE actions that need to be taken to ensure the electrical
appliance is safe to return to service. Include acceptable test results in
the answer.
(3 marks)
(1) ____________________________________________________
____________________________________________________
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(2) ____________________________________________________
____________________________________________________
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(3) ____________________________________________________
____________________________________________________
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 13
Question 5 continued
(b) AS/NZS 3000 requires that a test voltage of 500V d.c. is used for an
insulation resistance test of a low voltage electrical installation.
State ONE reason why the voltage is a d.c. voltage.
(2 marks)
____________________________________________________________
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(c) State ONE reason why a polarity test is carried out on the mains of a
single-phase, low voltage electrical installation.
(1 mark)
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(d) An earth loop impedance test at the main switchboard of a single-phase low
voltage electrical installation shows a result of 50Ω
There is an MEN link between the neutral busbar and earth bus bar.
State ONE reason why such a high test result would be obtained
(1 mark)
____________________________________________________________
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____________________________________________________________
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 14
Question 5 continued
(e) A single-phase low voltage, MEN electrical installation incorporates only one
MEN switchboard.
In the MEN switchboard, the MEN link between the main neutral bar and
main earth bar is missing.
State ONE hazard that could occur if there was an earth fault in the
electrical installation.
(2 marks)
____________________________________________________________
____________________________________________________________
____________________________________________________________
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 15
Question 6
Introduction
The figure below represents three 230V single fluorescent lights in one section of
a factory. This circuit is supplied from a three-phase switchboard
Switchboard
RCD 16A MCB
Active
(Phase)
Light switch
1A 1A 1A
Earth
Neutral
Light A Light B Light C
The fuses in the light fittings are HRC fuses.
Use the information in the introduction to this question to answer parts
6(a), 6(b) and 6(c)
(a) Fluorescent light A s is above a rotating machine.
(i) Explain how fluorescent light A produces a stroboscopic effect.
(2 marks)
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 16
Question 6
(ii) Describe the danger produced by the stroboscopic effect of
fluorescent light A.
(3 marks)
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(iii) State ONE method of eliminating the stroboscopic effect of
fluorescent light A
(1 mark)
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(b) Explain how discrimination is achieved between the lighting final subcircuit
and fluorescent light B.
(2 marks)
____________________________________________________________
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(c) State ONE reason why a fuse in a fluorescent light must not be shorted out
or increased in current rating.
(2 marks)
____________________________________________________________
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____________________________________________________________
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 17
Question 7
(a) A 250 kVA, three-phase, delta-star-connected step-down transformer has a
phase-turns ratio of 217 to 1. The primary is connected to a 50 kV, three-
phase supply, and the transformer is fully loaded.
(i) Calculate the secondary phase voltage.
(2 marks)
(ii) Calculate the secondary line voltage.
(2 marks)
(iii) Calculate the primary line current.
(2 marks)
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 18
Question 7 continued
(iv) Calculate the secondary line current.
(2 marks)
(b) State ONE reason why a current transformer is dangerous when its
secondary winding is left in the open-circuit condition.
(2 marks)
____________________________________________________________
____________________________________________________________
____________________________________________________________
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 19
Question 8
Introduction
In an office, a 40A RCCB and 20A MCB protect a 230V final subcircuit
supplying two permanently-connected electrical appliances.
The MCB has tripped. When the MCB is reset, it trips again when the supply is
restored to the final subcircuit.
The fault is either in the fixed-wiring or in a permanently-connected electrical
appliance.
The final subcircuit has been isolated at the switchboard.
Use the information in the introduction to answer parts 8(a) and 8(b).
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 20
Question 8 continued
(a) If the fault is in the fixed wiring.
The test instrument to be used is a multi-function test instrument with
insulation resistance (with 500V and 250V ranges), resistance, voltage and
temperature functions.
(i) State the action taken before the testing.
(1 mark)
________________________________________________________
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(ii) State the function and range selected on the test instrument.
(1 mark)
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(iii) Describe how testing to locate the fault is carried out.
(2 marks)
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(iv) State the test result when the expected fault is found.
(1 mark)
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(turn over)
ET61 QUESTIONS – 17 JUNE 2017 21
Question 8 continued
(b) If the fault is in a permanently-connected electrical appliance.
The test instrument to be used is a multi-function test instrument with
insulation resistance (with 500V and 250V ranges), resistance, voltage and
temperature functions.
(i) State the action taken before the testing.
(1 mark)
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
(ii) State the function and range selected on the test instrument.
(1 mark)
________________________________________________________
________________________________________________________
(iii) Describe how testing to locate the fault is carried out.
(2 marks)
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
________________________________________________________
(iv) State the test result when the expected fault is found.
(1 mark)
________________________________________________________
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 22
Question 9
Introduction
A three-phase, 400 V, 50 Hz, 4 pole squirrel-cage induction motor when running
fully loaded has:
● Full load speed (FLN) 1440 rpm.
● Efficiency (η) 90 %.
● Power factor (pf) 0.86
The load requires a full-load torque of 66.315 Nm. at 1440 rpm.
Use the information in the introduction to answer parts 9(a), 9(b), 9(c),
9(d) and 9(e).
(a) Calculate the output power of the motor in kW.
(2 marks)
(b) Calculate the input power of the motor in kW.
(2 marks)
(turn over)
ET61 QUESTIONS – 17 JUNE 2017 23
Question 9 continued
(c) Calculate full-load current the motor will draw.
(2 marks)
(d) Calculate the synchronous speed of the motor.
(2 marks)
(e) Calculate the percentage (%) slip of the motor.
(2 marks)
ET61 QUESTIONS – 17 JUNE 2017 24
For Candidate’s Use For Examiner’s Use Only
Questions Marks
Answered
In the box, write the
number of EXTRA 2
sheets you have
used. Write NIL if
you have not used 3
any
TOTAL
ET61 QUESTIONS – 17 JUNE 2017 25
