WorksheetsS6-1 Competition Practice
Total questions: 60
Worksheet time: 4hrs 29mins
How can the direction of a capacitor start single phase motor be reversed?
Reverse the connection to the capacitor
Reverse the connection to the start winding
Reverse the polarity of the supply
Reverse the connection of the start and run windings
When are single phase series universal motors used?
When high starting torque is required
When starting on no-load is required
When constant speeds are required
When low speed drives are required
When is the stator winding of a three phase squirrel-cage induction motor connected in star?
when in full load
when running at rated speed
when on light load
when starting the motor
What is the percentage slip of a motor at full load speed where the nameplate details are: f= 50Hz, poles = 4, full load speed = 1440 rpm?
s = 3%
s = 4%
s = 5%
s = 6%
What is the shape of the laminations used in the magnetic circuit of a single phase shell-type transformer?
LL
TT
LE
EI
What type of transformer is used for interconnecting high voltage transmission systems operating at different voltage levels?
Current transformer
wound transformer
Voltage transformer
Autotransformer
What conditions must be satisfied before connecting two transformers in parallel to share a common load?
Same inductance, same kVA rating
Same polarity, same phase sequence
Same polarity resistance, same current ratio
Same number of polarity turns, same voltage regulation
What is the relationship between the VA rating of a transformer and the internal percentage impedance Z?
VA ∝1/ Z
VA ∝Z
VA ∝2Z
VA ∝ Z/2
What two power losses occur in a transformer when operating on no-load?
Mechanical losses, core losses
Frictional losses, heat losses
Rotational losses, copper losses
Copper losses, core losses
What is the ideal electrical angle of separation of the start and run windings to create a rotating magnetic field in a split phase induction motor?
30
60
90
120
At what speed three-phase cylindrical two-pole synchronous alternator rotates when connected to a 50 Hz infinite busbar system?
1000 rpm
1500 rpm
3000 rpm
6000 rpm
What is the first action to be taken when a thermal overload trips an induction motor?
Short circuit the thermal trip.
Disconnect the thermal trip.
Replace the thermal trip.
Reset the thermal trip.
What is the purpose of the Buchholz relay used with large distribution transformers?
To record operating times.
To check voltage levels
To monitor gas levels
To check current flow.
What is the secondary current for a single-phase transformer which has a primary current of 50 A and a turns ratio of 2:1?
25 A
50 A
100 A
200 a
What is the essential property of the oil used in transformer tanks to facilitate cooling?
High viscosity.
Low hydrogen content
High carbon content
Low viscosity
____________ is a static electrical device that changes AC voltage level from one circuit to another without changing the frequency
transformer
motor
generator
resistor
Losses due to reversal of magnetization in the transformer core are:
mechanical losses
hysteresis losses
eddy current losses
copper losses
Losses due to ohmic resistance of the transformer windings are:
Eddy current losses
Mechanical losses
hysteresis losses
copper losses
Losses due to ohmic resistance of the transformer windings are:
Eddy current losses
Mechanical losses
hysteresis losses
copper losses
At what speed will a three-phase cylindrical six-pole synchronous alternator rotate when connected to a 60 Hz infinite busbar system?
1000 rpm
1200 rpm
1500 rpm
2000 rpm
The purpose that the transformer core is made up of laminations is to _____________ the flux leakage and _____________ the iron losses
reduce ; increase
increase ; reduce
increase ; increase
reduce ; reduce
Universal Motors are so named because:
they can run AC
they can run DC
they can run both AC and DC
they can be used universally
Universal Motor is basically:
A combined shunt and series DC motor
A combined shunt and series DC motor
A parallel DC motor
A series DC motor
Star-delta starter is used in the motors, which are designed to run on:
delta or star
star
delta
singe-phase induction motor
In a star-delta starter, When the stator winding is star connected, voltage over each phase in motor will be reduced by a factor of _______________ of that would be for delta connected winding.
Note:
sqrt = square root
1/3
2/3
1/ sqrt (3)
2/ sqrt (3)
In a star-delta starter, The starting torque as the connection is star will be ___________ times that it will be for delta connected winding.
2/3
1/ sqrt (3)
2 / sqrt (3)
1/3
In Buchholz relay, in case of minor faults:
The float at the bottom of the chamber moves upwards.
The float at the bottom of the chamber remains unaffected because the gases produced will not be sufficient to operate it
The float at the bottom of the chamber moves downwards.
The float at the bottom of the chamber moves upwards and downwards alternately.
Which of the following transformer ratios is true?
Np/Ns = Vs/Vp = Is/Ip
Np/Ns = Vp/Vs = Ip/Is
Np/Ns = Vp/Vs = Is/Ip
Np/Ns = Vs/Vp = Ip/Is
What is the secondary current for a single-phase transformer which has a primary current of 90 A and a turns ratio of 5:3?
Hint:
Np/Ns = Is/Ip
60 A
80 A
120 A
150 A
With increase in temperature the resistivity of oil:
increases rapidly
increases slowly
decreases rapidly
does not get affected
Split phase motors have _________ starting torque and ________ power factor.
high ; poor
high ; high
poor ; poor
poor ; high
In the coordination of conductor and overcurrent protection device, which of the following is true?
Ib ≤ In ≤ Iz
In ≤ Iz ≤ In
I2 ≤ Ib ≤ Iz
Iz ≤ Ib ≥ In
According to SBC 401, the PEN conductor shall have the color of _____________.
Blue
Green and Yellow
Black
Brown
According to SBC 401, the conductor for Phase 2 of three-phase a.c. circuit shall have the color of _____________.
Blue
Red
Black
Brown
Inside the bathroom, which area or object is Zone 0?
The door
The bathtub
The mirror
The ceiling
What degree of protection is necessary for the installation of electrical equipment in Zone 0 of a swimming pool?
IPX2
IPX4
IPX6
IPX8
What is the maximum residual operating current of an RCD protecting socket-outlets installed in Zone 3 of bathtub and shower basins?
300 mA
30 mA
30 A
300 A
What is the maximum operating temperature of a PVC insulation according to SBC401?
60°C
70°C
80°C
90°C
What does RCD mean
Resistive and Capacitive Device
Residual Capacitance Device
Resistive Current Device
Residual Current Device
The earthing system applicable for Caravans electrical installation is ______________
TT
TN-C
IT
TN-S
What is minimum size of PE conductor to be used in caravan electrical installation?
1.5 mm square
4.0 mm square
1.0 mm square
10 mm square
Which protective measure provides protection against indirect contact? (unit 18)
obstacles
placing out of reach
insulation of live parts
automatic disconnection of supply
Which installation condition does not affect the current carrying capacity of a cable? (unit 18)
length of the cable
Type of material of the conductor
number of cables run together
air temperature around the cable
What type of material is Cu-OF?
Conventional copper
Conventional aluminum
High efficiency copper
High conductivity aluminum
Which relationship correctly identifies the requirements for overload protection?
Ib ≤In ≤Iz
Ib ≤Ir ≤Iz
Ib ≤Ix ≤Iz
Ib ≤Ih ≤Iz
What is used to divide electrical supply into subsidiary circuits, providing individual circuit protection?
Contactor.
PV inverter
Distribution Board.
Combined multi-port SPD.
Electric current passing through a human body or through an animal body when it touches one or more accessible parts of an installation or of equipment.
Reference: SBC 401 Definitions
touch current
touch voltage
step current
step voltage
Voltage appearing during an insulation fault, between simultaneously accessible parts.
Reference: SBC 401 Definitions
touch current
touch voltage
step current
step voltage
A nominal voltage not exceeding the relevant voltage limit (50 V a.c. or 75 V d.c.) between conductors or between conductor and earth.
medium voltage
high voltage
extra low current
extra low voltage
A nominal voltage exceeding 50 V a.c. or 75 V d.c., but not exceeding 1000 V a.c. or 1500 V d.c.
medium voltage
low voltage
extra low current
extra low voltage
What protective device provides the function of an residual current device with a circuit breaker, in the same enclosure?
RCD
RCBO
MCB
MCCB
What is the correct switching arrangements for an installation with an alternative supply available if the primary source fails?
the 2 switches should be interlocked so that only 1 switch can be on
the 2 switches should operate independently from each other
both supply switches must be able to open both supplies
both switches must only be operated manually
What is the main effect of annealing copper busbars?
Hint:
Annealing copper makes it softer and less brittle, which allows you to bend it without breaking it.
It improves malleability.
it corrodes more easily.
It improves conductivity
It is resistant to corrosion.
What is a requirement of a switch used for isolation?
The switch must be a semi-conductor device.
It must be designed to prevent unintentional closure.
It must always be positioned next to the equipment it isolates
When the switch is new the leakage current must not exceed 6 mA.
What is the minimum recognized shock current likely to cause muscular contraction?
Approximately 5 mA
Approximately 10 mA
Approximately 20 mA
Approximately 30 mA
What precaution must be taken to prevent the spread of fire when a wiring system passes between floors in multi-storey premises?
Trunking must be made of flame retardant material and provide IP2X protection
Trunking requires an internal fire barrier if the internal csa is less than 710 mm.
Hole around cables must be sealed to prevent any movement of the cables.
Sealing material must provide the same flame retardant properties as the building structure
Which measure provides protection against direct contact?
Electrical separation.
Barriers and enclosures.
Non-conducting location.
Automatic disconnection of supply.
_______________ is the low impedance path to earth of fault current.
Fault current loop
Earth Fault loop impedance
Earthing Loop
Leakage current loop to earth
If the voltage drop across the conductor is 6.7%. Which of the following is true?
The conductor is good to be used
The conductor is not long enough
The conductor size (CSA) must be increased
The conductor size (CSA) must be decreased
In the Earthing System, PEN stands for ________________,
Planet Earth Nations
Proposed earth and neutral
Protective earth neutral
Protect the Earth’s Nature
