“150”, Aptitude Test Questions and Answers for Artisan II (Electrical) – TRC.
ABSTRACT
This 150-question aptitude test
preparation bank for Artisan II (Electrical) – Tanzania Railways Corporation
(TRC) is designed to help candidates strengthen their technical knowledge and
prepare for employment aptitude tests. It covers electrical installation,
maintenance, fault diagnosis, railway electrification, electrical machines,
control systems, safety procedures and practical calculations. The questions
are scenario-based, with carefully structured answer options, correct answers
and explanations to support deeper understanding of the subject.
Prepared by Artisan II (Electrical) – TRC.
Compiled by Artisan II (Electrical) – TRC.
0628729934.
Date: September 30, 2026
Dear applicants,
This collection of questions and answers
has been prepared to help all of you to understand the key areas tested during
the interview. The goal is to provide a useful, and practical study guide so
you can all perform confidently and fairly in the selection process. I wish you
the best of luck, and may this resource support you in achieving success!
Warm regards,
Artisan II (Electrical) – TRC.
For Personal Use by Applicants Preparing for Artisan II (Electrical) – TRC - at Public Service Recruitment Service.
Artisan II (Electrical)
– TRC.
150 Aptitude
Test Questions and Answers
1. During the
installation of a new electrical circuit in a railway workshop, an electrician
measures the expected supply voltage at the distribution board. However, the
voltage drops significantly when a connected motor starts, while the protective
device does not trip. Which condition should be investigated first?
A. Excessive
circuit impedance causing voltage drop under load B. Incorrect motor rotation
caused by phase displacement C. A developing insulation fault between the motor
winding and earth D. Incorrect protective-device rating causing delayed
disconnection
Answer: A.
Excessive circuit impedance causing voltage drop under load
Rationale:
A significant voltage drop occurring when a motor starts indicates that the
circuit may have excessive impedance, resulting from undersized conductors,
excessively long cable runs, loose terminals, corroded connections, or other
sources of abnormal resistance in the supply path. Since motor starting current
is substantially higher than normal running current, excessive circuit
impedance becomes more evident during starting conditions. The fact that the
protective device does not trip suggests that the current has not exceeded its
operating threshold, making an overload or short-circuit condition less likely.
The supply circuit, cable sizing, terminations, and voltage measurements at
strategic points should therefore be investigated first.
2. A technician
is installing a three-phase electrical motor in a railway maintenance workshop.
After connecting the supply, the motor rotates in the opposite direction to
that required by the driven equipment. The motor runs smoothly and draws
approximately normal current. Which corrective action is most appropriate?
A. Increase the
overload relay setting to accommodate the rotation difference B. Isolate the
supply and interchange any two phase conductors C. Replace the motor windings
because their polarity is reversed D. Increase the supply voltage to restore
the intended rotation
Answer: B.
Interchange any two phase conductors
Rationale: In a
three-phase induction motor, the direction of the rotating magnetic field
determines the direction of rotation. Interchanging any two phase conductors
reverses the phase sequence and consequently reverses the rotating magnetic
field, changing the motor's direction. Since the motor operates smoothly and
draws approximately normal current, the symptoms do not initially indicate
winding failure or insufficient voltage. The supply must first be isolated and
secured before changing connections, followed by appropriate verification of
the rotation before reconnecting the driven equipment.
3. During
inspection of an overhead railway electrification installation, a maintenance
team observes that the contact wire has developed excessive sag between two
supporting structures. Which engineering consideration is most directly
affected by this condition?
A. The resistance
of the railway signalling cable insulation B. The mechanical strength of the
station lighting conductors C. The operating frequency of the traction power
supply D. The consistency of pantograph contact with the contact wire
Answer: D. The
consistency of pantograph contact with the contact wire
Rationale: Excessive
contact-wire sag changes the geometry of the overhead contact system and can
interfere with the intended interaction between the pantograph and the contact
wire. This may result in inconsistent contact pressure, increased arcing,
accelerated wear and possible damage to the current collection equipment. The
condition may be associated with incorrect tensioning, temperature-related
expansion, support defects or mechanical deterioration. A proper inspection
should therefore consider the contact system's mechanical condition and
approved installation tolerances rather than treating sag as merely a visual
defect.
4. An electrical
artisan is troubleshooting a single-phase lighting circuit in a railway
administrative building. The circuit breaker remains closed, but the lights do
not operate. A voltage measurement between the phase and neutral terminals at
the distribution board indicates the expected supply voltage. What should be
investigated next?
A. Whether the
circuit breaker has an unnecessarily high breaking capacity B. Whether the
lighting circuit has excessive earth electrode resistance C. Whether an open
circuit exists along the phase or neutral conductor D. Whether the
distribution board has an incorrect three-phase sequence
Answer: C.
Whether an open circuit exists along the phase or neutral conductor
Rationale: The
presence of the expected voltage at the distribution board does not establish
that the complete lighting circuit is electrically continuous. An open phase
conductor, disconnected terminal, failed switch or interrupted neutral can
prevent current from flowing through the lamps even when the incoming supply is
available. The artisan should follow an appropriate isolation and testing
procedure to identify the interruption, checking the circuit's continuity and
connections using suitable instruments. Changing the circuit breaker or
investigating phase sequence would not address the most directly indicated
fault.
5. 230 V
single-phase electrical heater rated at 2,300 W is connected to a supply under
normal operating conditions. Assuming a resistive load and negligible power
losses, what current should the circuit normally carry?
A. 5 A B. 15 A C.
20 A D. 10 A
Answer: D. 10
A
Rationale: For a single-phase resistive load,
electrical power is calculated using (P = VI), where power is measured in
watts, voltage in volts and current in amperes. Therefore, the expected current
is (I = P/V = 2300/230 = 10 A). The assumption of a resistive load means the
power factor is approximately unity, allowing the simple relationship to be
applied. This value represents the expected operating current under the stated
conditions; conductor sizing and protective-device selection must additionally
consider installation method, permissible temperature rise, loading conditions
and applicable electrical regulations.
6. A railway
workshop motor repeatedly trips its overload relay after operating for several
minutes. Measurements indicate that all three phase currents are above the
motor's rated full-load current, although the supply voltage is balanced. Which
condition is the most appropriate initial suspect?
A. Mechanical
overloading of the driven equipment B. Reversed polarity of the motor terminal
connections C. Excessive resistance of the earth electrode D. Incorrect
frequency of the lighting supply
Answer: A.
Mechanical overloading of the driven equipment
Rationale: When
all three phase currents exceed the motor's rated full-load current under
balanced voltage conditions, excessive mechanical loading is an important
initial suspect. A driven machine operating under abnormal friction, excessive
load, restricted movement or mechanical binding may demand greater torque,
causing the motor to draw excessive current and eventually operate its overload
protection. The artisan should investigate the driven equipment, bearing
condition, alignment and load requirements while also verifying the motor's
electrical characteristics. Simply increasing the overload setting would
conceal the symptom without correcting the underlying condition and could
expose the motor to thermal damage.
7. During
preventive maintenance of an electrical distribution board, an artisan
discovers that a cable termination is discoloured and its insulation has become
brittle. The circuit is still operating normally. What is the most appropriate
interpretation of this observation?
A. The cable has
developed excessive capacitive reactance B. The termination may have
experienced abnormal heating due to a poor connection C. The conductor has
acquired an improved electrical conductivity D. The protective device has
necessarily developed an internal short circuit
Answer: B. The
termination may have experienced abnormal heating due to a poor connection
Rationale: Discolouration
and brittle insulation around a cable termination are warning signs of possible
overheating. A loose, corroded or poorly prepared connection can introduce
additional contact resistance, producing heat when current passes through the
joint. The resulting temperature rise may progressively damage insulation and
increase the risk of electrical failure or fire. The absence of an immediate
operational fault does not establish that the connection is safe. The circuit
should be handled under the appropriate isolation procedure, and the
termination should be inspected, tested and repaired in accordance with the
equipment manufacturer's requirements and applicable standards.
8. A railway
maintenance team is inspecting a catenary system after a report of intermittent
electrical arcing between the pantograph and the contact wire. The overhead
conductor appears mechanically intact. Which additional condition should
receive particular attention?
A. The resistance
of the station's lighting neutral conductor B. The insulation rating of nearby
office socket outlets C. The frequency of the workshop ventilation motor D.
The condition of the pantograph contact surface and overhead contact geometry
Answer: D. The
condition of the pantograph contact surface and overhead contact geometry
Rationale: Intermittent
arcing at the pantograph-contact-wire interface can arise from inadequate or
unstable electrical contact. Even when the overhead conductor appears intact,
worn or damaged pantograph contact strips, incorrect contact pressure, uneven
contact-wire height, misalignment or other geometric irregularities may
interrupt current collection. These conditions can produce repeated separation
and reconnection, resulting in arcing and accelerated wear. The investigation
should consider both sides of the contact interface rather than assuming that
the overhead conductor alone is responsible. Inspection and testing must follow
the railway operator's approved procedures because overhead traction systems
can present hazardous voltages even when trains are not moving.
9. An electrician
is selecting a protective device for a circuit supplying several workshop
socket outlets. The calculated design current is 18 A, while the selected cable
has a current-carrying capacity of 24 A after applying the relevant
installation correction factors. Which protective-device rating satisfies the
basic overload coordination relationship, assuming the device's conventional
operating current is also suitable for the cable?
A. 25 A B. 32 A C.
20 A D. 16 A
Answer: C. 20 A
Rationale: The
basic overload coordination relationship requires the design current of the
circuit not to exceed the rated current of the protective device, and the
protective-device rating not to exceed the corrected current-carrying capacity
of the cable. Here,
10. A technician
is testing the insulation of an isolated electrical motor using an insulation
resistance tester. Which preparation is most important before conducting the
test?
A. Connect the
motor winding directly to the protective earth terminal B. Ensure the motor is
isolated and sensitive connected components are appropriately disconnected C.
Increase the supply voltage until the motor begins rotating D. Connect the
insulation tester across the motor's operating overload relay
Answer: B. Ensure
the motor is isolated and sensitive connected components are appropriately
disconnected
Rationale: An
insulation resistance test applies a DC test voltage to assess the condition of
electrical insulation. The motor must be safely isolated from all supply
sources, and sensitive electronic components, control equipment and other
devices that could be damaged by the test voltage must be disconnected as
appropriate. The artisan must verify the absence of supply voltage before
testing and follow the instrument manufacturer's instructions. After testing,
the winding may retain an electrical charge and should be discharged using an
appropriate safe procedure. Connecting the tester without considering the
complete circuit arrangement can damage equipment and expose personnel to
electrical hazards.
11. A transformer
supplying a railway building produces an unusual humming sound, and its
secondary voltage decreases noticeably when additional loads are connected. The
transformer is not exceeding its rated current. Which explanation is most
technically relevant?
A. The
transformer may have excessive internal impedance or deteriorated connections B.
The transformer secondary must always produce a higher voltage under increased
loading C. The transformer core has necessarily lost all magnetic properties D.
The protective earth conductor is reversing the transformer's phase sequence
Answer: A. The
transformer may have excessive internal impedance or deteriorated connections
Rationale: A
transformer experiences voltage regulation effects when load current increases
because current flowing through its internal winding resistance and leakage
reactance produces internal voltage drops. Deteriorated terminals, poor
connections or abnormal internal conditions may further increase these effects.
A noticeable secondary voltage reduction accompanied by unusual humming
warrants investigation even when the measured current remains within the
nameplate rating. The humming may be associated with normal magnetostriction,
excessive magnetic excitation, mechanical looseness or other defects, so it
should not independently be treated as proof of a particular failure. The
technician should compare operating measurements with the manufacturer's
specifications and investigate the transformer under controlled conditions.
12. During
maintenance of a railway building's electrical installation, an artisan
discovers that the metallic enclosure of a distribution board is not connected
to the protective earthing system. The installation is operating normally. What
is the principal safety consequence?
A. The
distribution board will necessarily consume more electrical energy B. The
supply frequency may increase during periods of high demand C. The neutral
conductor will automatically carry zero current D. A fault could leave the
enclosure at a dangerous voltage without an effective fault-current path
Answer: D. A
fault could leave the enclosure at a dangerous voltage without an effective
fault-current path
Rationale: Protective
earthing provides a path intended to carry fault current and facilitate the
operation of the relevant protective device when an exposed conductive part
becomes live due to insulation failure. Without an effective protective
connection, a metallic enclosure may remain at a dangerous potential, exposing
anyone touching it to electric shock. Normal operation does not demonstrate
that the installation is safe because the danger may only become apparent when
a fault occurs. The defect requires appropriate isolation, correction and
verification of protective continuity before the equipment is returned to
service. Earthing should not be confused with ordinary load-current carrying
through the neutral conductor.
13. A three-phase
induction motor is connected to a supply through a contactor. When the start
command is released, the motor immediately stops even though the supply remains
available. Which control-system component should be investigated first?
A. The motor's
mechanical coupling alignment B. The contactor's auxiliary holding contact C.
The motor's protective earth electrode D. The distribution transformer's
secondary winding ratio
Answer: B. The
contactor's auxiliary holding contact
Rationale: In a
conventional three-wire motor control circuit, an auxiliary holding contact
connected in parallel with the start push-button maintains the contactor coil's
energisation after the start button is released. If this contact is defective,
incorrectly wired or failing to close, the contactor may remain energised only
while the start button is pressed. Releasing the button then interrupts the
coil circuit, causing the motor to stop even though the main supply is
available. The artisan should examine the control circuit, auxiliary contact
operation and associated wiring using an appropriate safe troubleshooting
procedure. This symptom is more directly associated with the control circuit
than with the motor's mechanical coupling or earthing arrangement.
14. An artisan
measures the resistance of a healthy metallic conductor and obtains a higher
value than expected. The conductor is longer than the reference conductor, but
both have the same material and cross-sectional area and are measured at the
same temperature. Which relationship best explains the result?
A. Resistance
decreases proportionally with conductor length B. Resistance is determined
exclusively by the conductor's insulation thickness C. Resistance increases
proportionally with conductor length D. Resistance remains constant whenever
conductor material is unchanged
Answer: C.
Resistance increases proportionally with conductor length
Rationale: The
resistance of a uniform conductor is expressed by (R=\rho L/A), where (\rho)
represents the material's resistivity, (L) is its length and (A) is its
cross-sectional area. For conductors made from the same material, having equal
cross-sectional areas and operating at the same temperature, increasing the
length increases the resistance proportionally. Therefore, the longer conductor
is expected to have a greater resistance even when both conductors are in
healthy condition. This relationship is important when selecting cables for
electrical installations because excessive conductor length can contribute to
voltage drop and power losses.
15. During an
inspection of a railway workshop's electrical installation, a technician finds
that an RCD trips when a particular machine is connected, but does not trip
when the machine is disconnected. The machine's phase conductor carries its
normal operating current. Which fault is most consistent with this observation?
A. Leakage
current from the machine's live circuit to earth B. Excessive current flowing
equally through all three phases C. An increase in the machine's normal
operating frequency D. A reduction in the resistance of the machine's
protective conductor alone
Answer: A.
Leakage current from the machine's live circuit to earth
Rationale: An RCD
detects an imbalance between the currents flowing through the live conductors
passing through its sensing mechanism. If some current escapes from the
intended circuit through an alternative path, such as earth, the outgoing and
returning currents are no longer equal and the device may operate. Since the
tripping is associated with connecting a particular machine, insulation
deterioration, moisture ingress, damaged wiring or leakage within connected
components should be investigated. A protective earth conductor alone does not
necessarily cause RCD operation; its purpose is to provide a protective
fault-current path. The machine must be safely isolated and tested before being
returned to service.
16. A maintenance
team is replacing a damaged section of electrical cable supplying equipment in
a railway workshop. The replacement cable has the same conductor material and
cross-sectional area as the original, but a lower insulation temperature rating.
Which consideration is most important before approving its installation?
A. Whether the
replacement cable has a more attractive external appearance B. Whether its
conductor resistance is lower at all operating temperatures C. Whether the
cable can be installed without using protective conduit D. Whether its
insulation rating is suitable for the expected operating and environmental
temperatures
Answer: D.
Whether its insulation rating is suitable for the expected operating and
environmental temperatures
Rationale: Cable
insulation must withstand the temperatures expected during normal operation,
overload conditions where applicable, and the environmental conditions of the
installation. A cable with an unsuitable insulation temperature rating may
deteriorate prematurely even if its conductor material and cross-sectional area
match those of the original. Its suitability must therefore be assessed against
the expected loading, installation method, ambient temperature, grouping,
exposure conditions and applicable electrical standards. Matching the conductor
size alone does not establish that the replacement cable is technically
equivalent or safe for the intended application.
17. A technician
is investigating a railway station's three-phase motor circuit. Measurements
show that one phase current is significantly lower than the other two, while
the motor produces reduced torque and abnormal heating. Which condition should
be suspected?
A. Excessive
resistance in the protective earth electrode B. An open or high-resistance
connection in one phase circuit C. An increase in the motor's rated mechanical
output D. A normal consequence of using a three-phase supply
Answer: B. An
open or high-resistance connection in one phase circuit
Rationale: Unequal
phase currents accompanied by reduced torque and abnormal heating may indicate
a supply imbalance or a defect such as a loose terminal, damaged conductor or
high-resistance connection in one phase. In a three-phase motor, loss or
substantial impairment of one phase can result in abnormal current
distribution, reduced electromagnetic performance and excessive heating in the
remaining phases. The artisan should investigate the supply, contactor
contacts, terminals, protective devices and motor windings before concluding
that the motor itself is defective. Continuing operation under such conditions
may accelerate insulation deterioration and lead to winding failure.
18. A railway
electrical installation uses a transformer with a primary voltage of 11,000 V
and a secondary voltage of 415 V. Assuming an ideal transformer, what is the
approximate primary-to-secondary turns ratio?
A. 1:26.5 B.
26.5:1 C. 11:415 D. 415:11
Answer: B. 26.5:1
Rationale: For an
ideal transformer, the turns ratio is proportional to the voltage ratio.
Therefore, the primary-to-secondary turns ratio is (N_p/N_s = V_p/V_s =
11000/415 \approx 26.5). This means the primary winding has approximately 26.5
times as many turns as the secondary winding. Since the transformer reduces
voltage from 11 kV to 415 V, it is a step-down transformer. The relationship
assumes an ideal transformer and does not account for voltage regulation,
winding losses or other practical effects. Understanding this ratio helps
technicians interpret transformer nameplate information and distinguish
primary-side and secondary-side characteristics.
19. An artisan is
inspecting a railway workshop's electric motor and notices that its cooling fan
is damaged. The motor continues to operate at its normal load, and the measured
current is within the rated value. What is the most appropriate technical
response?
A. Continue
operating the motor because rated current guarantees safe temperature B.
Increase the motor's overload protection setting to prevent unnecessary trips C.
Investigate the cooling defect and arrange corrective maintenance before
thermal damage develops D. Reduce the supply voltage to compensate for the
damaged cooling fan
Answer: C.
Investigate the cooling defect and arrange corrective maintenance before
thermal damage develops
Rationale: A
motor's cooling system is essential for removing heat generated by electrical
and mechanical losses. A damaged fan may reduce airflow and cause the winding
and other components to operate at temperatures above their permitted limits,
even when the motor current remains within its rated value. Current
measurements alone cannot confirm that the motor is operating within its
thermal limits. The defect should therefore be assessed and corrected according
to the maintenance procedure, with the motor's operational status determined by
the responsible personnel. Increasing overload protection settings or reducing
supply voltage would not restore the required cooling performance.
20. A catenary
maintenance team is preparing to inspect an overhead electrical system
following a suspected conductor defect. A supervisor proposes relying only on
the opening of a nearby circuit breaker before permitting work. Which response
reflects the correct electrical safety principle?
A. Opening the
circuit breaker is sufficient because it guarantees complete isolation of every
conductor B. The conductor can be handled after a visual inspection confirms
that no current is flowing C. Earthing is unnecessary if the overhead line has
remained disconnected for several minutes D. Isolation must be established and
verified, with appropriate earthing and other approved safety precautions
applied
Answer: D.
Isolation must be established and verified, with appropriate earthing and other
approved safety precautions applied
Rationale: Opening
a circuit breaker does not, by itself, establish that an overhead electrical
conductor is safe to touch. Other supply sources, backfeed, induced voltages,
incorrect switching, stored electrical energy or accidental re-energisation may
present hazards. The authorised procedure must establish the correct isolation
boundaries, secure against unintended reconnection, verify the absence of
voltage using approved equipment and apply protective earthing where required.
Additional railway-specific precautions and permits must also be observed. Work
on catenary systems must only be undertaken by appropriately authorised
personnel under the operator's established electrical safety rules.
21. A technician
measures a 24 V DC control circuit and finds that the voltage at the power
supply terminals is correct. However, the voltage across a relay coil is only
15 V when the relay is energised. The circuit has no apparent short circuit.
Which explanation is most consistent with these measurements?
A. Excessive
voltage drop across resistance in the control circuit wiring or connections B.
An increase in the power supply's rated output voltage C. A reversal of the
magnetic polarity of the relay enclosure D. A reduction in the earth
resistance of the distribution system
Answer: A.
Excessive voltage drop across resistance in the control circuit wiring or
connections
Rationale: When a
relay coil receives a voltage significantly below its intended operating value
despite a correct source voltage, voltage drop along the circuit should be
investigated. Loose terminals, corroded contacts, damaged conductors,
undersized wiring or excessive contact resistance can introduce additional
voltage drop when current flows. The artisan should measure voltage at
appropriate points under load to identify where the reduction occurs, following
the correct isolation and testing requirements. A no-load voltage measurement
alone may fail to reveal a high-resistance connection because the voltage drop
becomes more significant when current is drawn.
22. An electrical
artisan is maintaining a motor-driven pump used in a railway facility. The pump
starts normally but delivers insufficient flow. The motor current is lower than
its previous recorded operating value, and there is no abnormal electrical noise.
Which investigation is most relevant before replacing the motor?
A. Whether the
motor's protective earth conductor has excessive cross-sectional area B.
Whether the electrical supply frequency has increased above its rated value C.
Whether the pump has a hydraulic restriction, air ingress or an abnormal
operating condition D. Whether the distribution board's incoming voltage is
necessarily above its rated value
Answer: C.
Whether the pump has a hydraulic restriction, air ingress or an abnormal
operating condition
Rationale: A
motor-driven pump may deliver insufficient flow because of hydraulic or
mechanical conditions that do not originate in the motor windings. Blocked
strainers, restricted pipelines, air entering the suction line, incorrect valve
positions, damaged impellers or other abnormal operating conditions may reduce
pump performance. A lower motor current than previously recorded may indicate
that the pump is operating under reduced load, although the precise
interpretation depends on the pump design and operating point. The artisan
should therefore assess the complete motor-pump system, compare readings with
historical operating data and avoid replacing an electrically functional motor
without evidence of an electrical defect.
23. During
installation of a new electrical circuit, an artisan measures continuity
between the protective earth terminal of a socket outlet and the installation's
main earthing terminal. The instrument indicates a very high resistance. What
does this result primarily suggest?
A. The circuit's
phase conductor is carrying excessive load current B. The protective conductor
may be disconnected, damaged or inadequately connected C. The neutral
conductor has developed a lower operating voltage D. The socket outlet has a
higher power factor than the connected equipment
Answer: B. The
protective conductor may be disconnected, damaged or inadequately connected
Rationale: A
protective conductor is intended to provide a reliable, low-impedance path for
fault current between exposed conductive parts and the earthing system. A very
high continuity resistance between a socket outlet's protective earth terminal
and the main earthing terminal suggests that the protective path may be
interrupted, damaged or poorly connected. Such a condition can compromise
automatic disconnection during an earth fault and create a serious electric
shock hazard. The installation should not be accepted as satisfactory on the
basis of normal socket operation. The fault must be located, corrected and
verified through appropriate protective-conductor continuity testing before the
circuit is put into service.
24. A railway
workshop has a three-phase supply with a line voltage of 415 V. A balanced
star-connected resistive load is connected across the supply. What is the
approximate voltage across each individual phase load?
A. 415 V B. 240
V C. 720 V D. 120 V
Answer: B. 240 V
Rationale: In a
balanced star-connected three-phase system, the line voltage is equal to the
phase voltage multiplied by the square root of three. Therefore, the phase
voltage is
25. During
routine maintenance of an electrical installation in a railway building, a
technician discovers that a circuit breaker trips immediately when a particular
circuit is energised, even though no load equipment has been connected. Which
fault should be investigated first?
A. Excessive
mechanical loading of the disconnected equipment B. Incorrect operating speed
of the connected electric motor C. Reduced efficiency of the building's
ventilation system D. A short circuit or wiring fault within the unloaded
circuit
Answer: D. A
short circuit or wiring fault within the unloaded circuit
Rationale: When a
circuit breaker trips immediately upon energisation despite the absence of
connected load equipment, a fault within the fixed wiring or associated
components should be investigated first. Possible causes include
phase-to-neutral contact, phase-to-earth contact, damaged insulation, incorrect
termination or a fault within a permanently connected accessory. The absence of
external load does not establish that the circuit is fault-free because the
fixed installation itself remains energised. The artisan should isolate the
circuit, verify the absence of voltage and perform appropriate inspection,
continuity and insulation resistance tests to locate the defect before
attempting to re-energise the installation. Repeatedly resetting the breaker
without identifying the cause may worsen the fault or expose personnel to
danger.
26. In a
conventional direct-on-line (DOL) motor control circuit used in a railway
workshop, pressing the START push-button energizes the contactor and starts the
motor. Pressing the STOP push-button de-energizes the contactor and stops the
motor. Which arrangement correctly describes the control circuit?
A. The STOP
button is normally open and connected in parallel with the motor overload
contact. B. The START button is normally closed and connected in series with
the contactor coil. C. The STOP button is normally closed and connected in
series with the contactor coil, while the START button is normally open. D. The
auxiliary holding contact is normally open and connected in parallel with the
START button.
Answer: C. The
STOP button is normally closed and connected in series with the contactor coil,
while the START button is normally open.
Rationale: In a
conventional DOL motor control circuit, the STOP push-button is normally closed
(NC) and connected in series with the contactor coil. This keeps the control
circuit complete during normal operation. Pressing the STOP button opens the
circuit, de-energizing the contactor and stopping the motor.
The START push-button
is normally open (NO). When pressed, it momentarily energizes the contactor
coil. An auxiliary normally open holding contact, connected in parallel with
the START button, then closes and maintains coil energization after the START
button is released. If the control circuit is interrupted, the contactor will
normally de-energize, preventing the motor from continuing to run through a
maintained start command.
27. A technician
is investigating a DC battery-powered control circuit in a railway signalling
support installation. The battery voltage is correct when measured without a
connected load, but falls significantly when the control equipment is
energised. Which electrical property of the battery is most directly
responsible for this behaviour?
A. Internal
resistance causing voltage drop when current flows B. External resistance
increasing the battery's rated voltage C. Capacitance preventing current from
entering the control equipment D. Inductance reversing the battery's polarity
under load
Answer: A.
Internal resistance causing voltage drop when current flows
Rationale: A
practical battery has internal resistance, which produces an internal voltage
drop whenever current is drawn. The terminal voltage under load is therefore
lower than its open-circuit voltage, with the reduction approximately equal to
the product of the load current and internal resistance. A battery with
deteriorated cells, poor internal connections or ageing may develop increased
effective internal resistance, resulting in an excessive voltage reduction when
equipment is energised. The technician should evaluate the battery using an
appropriate load test and inspect external connections rather than relying
exclusively on an open-circuit voltage measurement.
28. A railway
electrical artisan is inspecting the suspension arrangement of an overhead
contact system. One insulator shows visible surface contamination and tracking
marks, although no complete electrical breakdown has occurred. Which risk is
most directly associated with this condition?
A. Increased
mechanical tension in the supporting messenger wire B. Automatic reversal of
the traction supply phase sequence C. Reduced resistance of the railway
running rails D. Leakage current developing across the contaminated insulating
surface
Answer: D.
Leakage current developing across the contaminated insulating surface
Rationale: Insulators
are intended to prevent unwanted current flow between electrically live
components and supporting structures or earth. Moisture, dust, industrial
pollution and other contaminants can reduce the surface insulation resistance,
allowing leakage current to develop along the insulator. Under certain
conditions, this may lead to surface tracking, flashover and eventual
insulation failure. Visible tracking marks indicate that the insulation may
already have experienced electrical stress and requires assessment. The
maintenance team should follow approved inspection and cleaning procedures,
evaluate the condition of the insulator and ensure that the installation
remains electrically and mechanically safe.
29. A three-phase
induction motor has an output power of 15 kW, a line voltage of 415 V, a power
factor of 0.80, and an efficiency of 90%. What is the approximate full-load
line current of the motor?
Answer: B. 29.0 A
Rationale:
The motor output
power is:
Since the
efficiency is 90%:
For a three-phase
motor:
Therefore:
Substituting the
values:
Therefore, the
approximate full-load line current is 29.0 A.
30. During the
maintenance of a railway building's electrical installation, an artisan
discovers that a capacitor in a power-factor correction panel has swollen and
developed signs of leakage. The panel remains connected to the supply. Which
action is most appropriate?
A. Increase the
capacitor bank's operating voltage to restore its capacitance B. Disconnect
the protective earth to prevent further capacitor deterioration C. Isolate the
affected equipment and arrange safe discharge and inspection before handling D.
Connect the damaged capacitor directly across another healthy capacitor
Answer: C.
Isolate the affected equipment and arrange safe discharge and inspection before
handling
Rationale: A
swollen or leaking capacitor indicates possible internal deterioration and
creates a risk of electrical failure, overheating or rupture. Capacitors may
retain a dangerous electrical charge after disconnection, particularly where
discharge resistors have failed or are inadequate. The affected equipment must
therefore be isolated using the approved procedure, verified as de-energised
and discharged using appropriate equipment before inspection or replacement.
The artisan should also investigate the cause of failure, including operating
voltage, harmonic conditions, temperature and ventilation. Connecting the
damaged capacitor to another unit or attempting to restore its operation by
increasing voltage would introduce additional hazards.
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