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WorksheetsEE OBJECTIVE QUESTION (CIRCUIT BREAKERS)
Total questions: 85
Worksheet time: 43mins
The function of protective relay in a circuit breaker is
to earth any stray voltages
to close the contacts when the actuating quantity reaches a certain predetermined value
to limit arcing current during the operation of circuit breaker
to provide additional safety in the operation of circuit breaker
Low voltage circuit breakers have rated voltages of less than
220 V
400 V
1000 V
10,000 V
A relay in which the measurement is performed by a stationary circuit and which has no moving parts is known as
Static relay
Differential relay
Instantaneous relay
Time relay
When a H.V. ac circuit breaker is tested for endurance, it is tested for at least
1000 opening-closing operations
100 opening-closing operations
10 opening-closing operations
5 opening-closing operations
For high voltage ac circuit breakers, the rated short circuit current is passed for
0.01 sec
0.1 sec
3 sec
D. 30 sec
Which of the following is not a type of contactor for circuit breakers?
electromagnetic
electro-pneumatic
pneumatic
vacuum
Interrupting medium in a contactor may be
air
oil
SF6 gas
any of the above
In air blast circuit breakers, the pressure of air is of the order of
100 mg Hg
1 kg/cm2
20 to 30 kg/cm2
200 to 300 kg/cm2
SF6 gas
is lighter than hydrogen
is lighter than air
has density 2 times as compared to that of air
has density 5 times as compared to that of air
The pressure of SF6 gas in circuit breakers is of the order of
100 mg Hg
1 kg/cm2
3 to 5 kg/cm2
30 to 50 kg/cm2
While selecting a gas for circuit breaker, the property of gas that should be considered is
high dielectric strength
non-inflammability
non-toxicity
none of the above
Out of the following circuit breakers, which one has the lowest voltage range?
air-break circuit breaker
tank type oil circuit breaker
Air-blast circuit breaker
SF6 circuit breaker
Which of the following circuit breaker can be installed on 400 kV line
Tank type coil circuit breaker
Miniature circuit breaker
Vacuum circuit breaker
Air blast circuit breaker
In a HRC fuse the time between cut-off and final current zero, is known as
total operating time
arcing time
pre-arcing time
pre-arcing time
ing factor for HRC fuse is
Minimum fusing current
/
current rating
minimum fusing current
/
minimum rupturing time
maximum fusing current
/
minimum rupturing time
minimum fusing current
/
prospective current of a circuit
The insulation resistance of high voltage electric circuit breaker is
1 kΩ
10 kΩ
20 Mega ohms
2000 Mega ohms
There is definite objection to use of which of the following medium for extinguishing the arc in case of a circuit breaker?
Air
SF6 gas
Vacuum
Water
In a circuit breaker if the insulation resistance between phase terminal earthed frame is less than the specified limit, the probable cause could be
moisture
dirty insulation surface
carbon or copper particle sticking to the internal surface
any of the above
The transient voltage that appears across the contacts at the instant of arc extinction is called
recovery voltage
restriking voltage
supply voltage
peak voltage
In a circuit breaker the active recovery voltage depends upon
power factor
armature reaction
circuit conditions
all of the above
protection is provided by HRC fuse in case of
Open circuits
Short circuits
Over loads
None of the above
A fuse wire possesses
inverse time characteristics
direct time characteristics
neither of the above
A relay used for protection of motors against overload is
impedance relay
Electromagnetic attraction type
Thermal relay
Buchholz’s relay
Fuse protection is used for current ratings up to
10 A
20 A
50 A
100 A
The fuse current in ampere is related with fuse wire diameter D as
1α^1/D
1α D
1α^3/2
1αD^2
A fuse wire should have
low specific resistance and high melting point
low specific resistance and low melting point
high specific resistance and high melting point
high specific resistance and low melting point
Fuse wire protection system is usually used beyond
10 A
25 A
50 A
100 A
For extra high voltage lines which circuit breaker is preferred
Bulk oil circuit breaker
Vacuum circuit breaker
SF6 gas circuit breaker
Minimum oil circuit breaker
The number of cycles in which a high speed circuit breaker can complete its operation is
3 to 8
10 to 18
20 to 30
40 to 50
A material best suited for manufacturing of fuse wire is
Aluminum
Silver
Lead
Copper
In a circuit breaker, the current which exists at the instant of contact separation is known as
restriking current
surge current
breaking current
recovery current
A Merz-Price protection is suitable for
transformers
alternators
feeders
transmission lines
‘Kick fuse’ has
square law characteristics
linear characteristics
inverse characteristics
direct characteristics
Air-blast breakers for 400 kV power systems are designed to operate in
50 microseconds
50 milliseconds
500 milliseconds
50 seconds
Breaking capacity of a circuit breaker is usually expressed in terms of
amperes
volts
MW
MVA
Which of the following equations correctly represents the relation between contact resistance r and the applied force F?
F = kr
F = k/r
r = k/F^n
r = kF^n
As the force on contact is increased, the contact resistance will
increase linearly
increase exponentially
remain altered
decrease
The heat produced at the contact point, due to passage of current, will least depend on
contact resistance
time during which the current flows
current flowing
temperature of the surrounding medium
Minimum arcing voltage for platinum is 16 V. It can be therefore concluded that when the voltage is below 16V
it will not possible to interrupt the circuit
it will possible to pass the current
it will be possible to interrupt any value of current without arcing
it will be possible to interrupt any value of current without bringing contact closer to another
Which of the following is not the method of arc dispersion?
oil immersion contacts
magnetic blow out of arc
use of rectifiers
deionization of arc path
Which of the following contact point metals has the highest melting point?
Silver
B. Tungsten
Gold
Copper
The arc voltage produced in the circuit breaker is always
in phase with arc current
leading the arc current by 90 deg
lagging the arc current by 90 deg
leading the arc current by 45 deg
Sparking between contacts can be reduced by
inserting resistance line
inserting a capacitor in series with the contacts
inserting a capacitor in parallel with the contacts
inserting an inductor in series with the contacts
For magnetic blow out of arc the magnetic field is produced
in the load circuit
parallel to the axis of the arc
at right angle to the axis of the arc
at 45 degrees with respect to the axis of the arc
Sparking occurs when a load is switched off because the circuit has
high inductance
high capacitance
high resistance
low resistance
For the same current, which of the following fuse wires will have the least fusing time?
18 SWG TIN – 12.5 A
20 SWG TIN – 10 A
22 SWG TIN – 7.5 A
24 SWG TIN – 5 A
An automatic device that operates at present value is known as
mercury switch
relay
fuse
contactor
The basic function of a circuit breaker is to
produce arc
ionize the surrounding air
transmit voltage by arcing
extinguish the arc
The power factor of arc in the circuit breaker is
always zero
always unity
always lagging
always leading
Flame proof switch gears are usually preferred
on transmission lines of low voltage
substations
in mines
in high MVA capacity circuits
Air is used in air-blast circuit breaker
must have least carbon dioxide
must be ionized
must be free from moisture
none of these
In a circuit breaker the time duration from the instant of fault to the instant of energizing of the trip coil is known as
lag time
lead time
protection time
operation time
In a circuit breaker the time duration from the instant of the fault to the extinction of arc is known as
operation time
total clearing time
lag time
lead time
In a circuit breaker the time duration from the instant of fault to the instant of closing of contact is known as
recycle time
total time
gross time
reclosing time
For high speed circuit breaker the total clearing time is nearly
1 to 2 cycles
5 to 10 cycles
10 to 15 cycles
less than 50 cycles
If the power is zero, the active recovery voltage will be
minimum
0.5
0.707
maximum
A circuit breaker will normally operate
when the switch is put on
when the line is to be checked
when the power is to be supplied
whenever fault in the line occurs
Which of the following circuit breaker will produce the least arc energy?
minimum oil circuit breaker
air blast circuit breaker
plain oil circuit breaker
all will produce the same energy
The breaking capacity of a circuit breaker in MVA ( 3 phase ) is given by
rated service voltage x rated symmetrical current
1.1 x rated service voltage x rated symmetrical current
√2x rated service voltage x rated symmetrical current
√3x rated service voltage x rated symmetrical current
Which relay is used for feeders?
MHO relay
Translay relay
Merz-Price protection
Buchholz relay
Which of the following relays is used in transformers?
Buchholz relay
MHO relay
Merz-Price relay
None of the above
MHO relay is used for
rectifier
circuit breaker
transmission lines
feeders
Merz-Price protection is used on
substations
capacitor banks
induction motors
generators
The value of the fault current depend on
voltage at the faulty point
total impedance up to the fault
both A and B
none of the above
The advantage of the neutral earthing is
simplified design of earth fault protection
over-voltages due to the lightening can be discharges to the earth
freedom from persistent arcing grounds
all of the above
The over voltage surges in power system may be caused by
lightening
switching
resonance
any of the above
The protection against the over-voltage due to the lightning is provided by
use of surge diverters
low tower footing resistance
use of overhead ground wires
any of the above
Which of the following is a conducting medium for electric current?
Low temperature gas
High temperature gas
Dissociated gas
Plasma
In circuit breakers the contact space is ionized by
thermal ionization of gas
thermal emission from surface contacts
field emission from the surface contacts
any of the above
Which of the following are air-break switching devices?
Isolator
Limit switch
Earthing switch
All of the above
Which of the following is the deremit of SF6 circuit breakers?
Sealing problem of gas
In flux of moisture in the gas system is dangerous
Deterioration of quality of circuit breaker affects reliability of circuit breakers
All of the above
A fuse is normally a
current limiting device
voltage limiting device
power limiting device
power factor limiting device
Normally the fuse elements are in parts which are connected in the middle by tin bridge. The melting point of tin bridge is
35 deg C
88 deg C
230 deg C
540 deg C
The material used for bus bars should have
low resistivity
higher softening temperature
low cost
all of the above
Which of the following is not valid in case of aluminum as compared to copper? Aluminum is higher
resistivity
coefficient of linear expansion
tensile strength
joint resistance
Over current protection for motor is provided by
cartridge fuses
kit kat fuses
over current relay
all of the above
Fuse in motor circuits provides
over current protection
short-circuit protection
open-circuit protection
none of the above
In which method of starting a motor, the starting current is the maximum?
Auto-transformer
Star-delta transformer
Stator rotor starter
Direct-on-line
Over voltage transients may occur due to
lightening
switching
arcing grounds
any of the above
Switching surges may be caused by
closing of the unchanged line
load shedding or receiving end of line
switching of magnetizing current
any of the above
The disadvantages offered by underground system is
frequent arcing grounds
difficult earth fault relaying
voltage oscillations
all of the above
Solid grounding is used for voltages
above 220 kV
above 11 kV
below 660 V
below 115 V
Switching over voltages are more hazardous than lightening surges in case of
low voltage systems
11 kV system
unbalanced systems
EHV and UHV systems
Current limiting reactors may be
air cooled, air cored
oil immersed magnetically shielded
oil immersed non-magnetically shielded
any of the above
Series reactors are installed at strategic locations of the power systems to
bring down the fault level within the capacity of the switchgear
directly pass the fault surges to ground
pass the neutralizing surges of opposite nature
discharge the capacitor
