Switchgear Protections -- MCQs with Answers

Switchgear Protections -- MCQs with Answers
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1.    The main function of a fuse is to
(a)     protect the line
(b)     open the circuit
(c)    protect the appliance
(d)    prevent excessive currents
(e)    none of the above
Ans: d

2.    On which of the following routine tests are conducted ?
(a)    Oil circuit breakers
(b)    Air blast circuit breakers
(c)    Minimum oil circuit breakers
(d)    All of the above
Ans: d
3.    SF6 gas
(a)    is yellow in colour
(b)    is lighter than air
(c)    is nontoxic
(d)    has pungent small
(e)    none of the above
Ans: c

4. The arcing contacts in a circuit breaker are made of
(a)     copper tungsten alloy
(b)     porcelain
(c)    electrolytic copper
(d)    aluminium alloy
Ans: a

5. Which  of the following  medium is employed for extinction of arc in air circuit breaker ?
(a) Water   
(b) Oil
(c) Air   
(d) SF6
Ans: c

6. With which of the following, a circuit breaker must be equipped for remote operation ?
(a) Inverse time trip
(b) Time-delay trip
(c) Shunt trip   
(d) None of the above
(e) All of the above
Ans: c

7.    Fault diverters are basically
(a) fuses   
(b) relays
(c) fast switches    
(d) circuit breakers
Ans: c

8.    A thermal protection switch can protect against
(a) short-circuit     
(b) temperature
(c) overload   
(d) over voltage
Ans: c

9.    Arc in a circuit behaves as
(a)     a capackive reactance
(b)     an inductive reactance
(c)    a resistance increasing with voltage rise across the arc
(d)    a resistance decreasing with voltage rise across the arc
Ans: d

10.    Thermal circuit breaker has
(a)    delayed trip action
(b)    instantaneous trip action
(c)    both of the above
(d)    none of the above
Ans: a

11.    Relays can be designed to respond to changes in
(a)     resistance, reactance or impedance
(b)     voltage and current
(c)    light intensity
(d)    temperature
(e)    all above
Ans: e

12.    Overload relays are of...... type.
(a) induction   
(b) solid state
(c) thermal   
(d) electromagnetic
(e) all above
Ans: e

13.    Thermal overload relays are used to protect the motor against over current due to
(a) short-circuits   
(b) heavy loads
(c) grounds   
(d) all of the above
Ans: b

14.    Magnetic circuit breaker has ______ trip action.
(a)     delayed   
(b)     instantaneous
(c)    both of the above
(d)    none of the above
Ans: b

15.    D.C. shunt relays are made of
(a)    few turns of thin wire
(b)    few turns of thick wire
(c)    many turns of thin wire
(d)    many turns of thick wire
Ans: c

16.    The relay operating speed depends upon
(a)     the spring tension
(b)     the rate of flux built up
(c)    armature core air gap
(d)    all of the above
Ans: d
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17.    In order that current should flow without causing excessive heating or voltage drop, the relay contacts should
(a) have low contact resistance
(b) be clean and smooth
(c) be of sufficient size and proper shape
(d) have all above properties
Ans: d

18.    Circuit breakers usually operate under
(a)     transient state of short-circuit current
(b)     sub-transient state of short-circuit current
(c)    steady state of short-circuit current
(d)    after D.C. component has ceased
Ans: a

19.    Circuit breakers are essentially
(a)     current carrying contacts called electrodes
(b)     arc extinguishers
(c)    circuits to break the system
(d)    transformers to isolate the two systems
(e)    any of the above
Ans: a

    The current zero interruption, in oil and air blast circuit breakers, is achieved by
(a)    lengthening of the gap
(b)    cooling and blast effect
(c)    both (a) and (b)
(d)    deionizing the oil with forced air
(e)     none of the above
Ans: c

21.    Air blast circuit breaker is used for
(a)     over currents   
(b)     short duty
(c)    intermittant duty
(d)    repeated duty
Ans: d

22.    An efficient and a well designed protective relaying should have
(a)     good selectivity and reliability
(b)     economy and simplicity
(c)    high speed and selectivity
(d)    all of the above
Ans: d

23.    Burden of a protective relay is the power
(a)     required to operate the circuit breaker
(b)     absorbed by the circuit of relay
(c)    developed by the relay circuit
(d)    none of the above
Ans: b

24.    Directional relays are based on flow of
(a) power   
(b) current
(c) voltage wave    
(d) all of the above
Ans: a

25.    A differential relay measures the vector difference between
(a)     two currents    
(b)     two voltages
(c)    two or more similar electrical quan¬tities
(d)    none of the above
Ans: c

26.    A transmission line is protected by
(a)     inrush protection
(b)     distance protection
(c)    time graded and current graded over current protection
(d)    both (b) and (c)
(e)    none of the above
Ans: d

27.    Large internal faults are protected by
(a)     merz price percentage differential protection
(b)     mho and ohm relays
(c)    horn gaps and temperature relays
(d)    earth fault and positive sequence relays
Ans: a

28.    When a transmission line is energized, the wave that propagates on it is
(a)    current wave only
(b)    voltage wave only
(c)    both (a) and (b)
(d)     power factor wave only
Ans: c

29.    Protective relays are devices that detect abnormal conditions in electrical circuits by measuring
(a)     current during abnormal condition
(b)     voltage during abnormal condition
(c)    constantly the electrical quantities which differ during normal and abnormal conditions
(d)    none of the above
Ans: c

30.    The voltage appearing across the contacts after opening of the circuit breaker is called______voltage.
(a) recovery   
(b) surge
(c) operating   
(d) arc
(e) none of the above
Ans: a

31.    Ionization in circuit breaker is facilitated by
(a)     high temperature
(b)     increase of mean free path
(c)    increasing field strength
(d)    all of the above
Ans: d

32.    In a circuit breaker the basic problem is to
(a)     maintain the arc
(b)     extinguish the arc
(c)    transmit large power
(d)    emit the ionizing electrons
Ans: c

33.    Overheating of relay contacts or contact born out is due to
(a)    slow making and breaking of load circuit contacts
(b)    foreign matter on the contact surface
(c)    too low contact pressure
(d)    all of the above
Ans: d

34.    Interruption of large currents by relay requires
(a)    arc suppressing blow out coils
(b)    wide separation of the opened contacts
(c)    high speed opening of contacts
(d)    all of the above
Ans: d
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35.    Shunt capacitance is neglected while considering
(a)     short transmission line
(b)    medium transmission line
(c)    long transmission line
(d)    medium and long transmission lines
Ans: a

36.    The arc voltage produced in A.C. circuit breaker is always
(a)     in phase with the arc current
(b)     lagging the arc current by 90"
(c)    leading the arc current by 90°
(d)    none of the above
Ans: a

37.    The time of closing the cycle, in modern circuit breakers is
(a) 0.003 sec   
(b) 0.001 sec
(c) 0.01 sec   
(d) 0.10 sec
(e) none of the above
Ans: a

38.    Insulation resistance of high voltage circuit breakers is more than
(a) 1 mega ohms    
(b) 10 mega ohms
(c) 100 mega ohms
(d) 500 mega ohms
Ans: c

39.    H.R.C. fuses provide best protection against
(a) overload   
(b) reverse current
(c) open-circuits     
(d) short-circuits
Ans: d

40.   The ground wire should not be smaller than No ______ copper.
(a) 2   
(b) 4
(c) 6   
(d) 10
Ans: d

41.    The delay fuses are used for the protection of ________ .
(a)     motors
(b)     power outlet circuits
(c)    fluorescent lamps
(d)    light circuits
Ans: a

42.    Which of the following is the least expensive protection for overcurrent is low voltage system ?
(a)    Rewireable fuse
(b)    Isolator
(c)    Oil circuit breaker
(d)     Air break circuit breaker
(e)    None of the above
Ans: a

43.    Resistance grounding is used for voltage between
(a)  33kVto66kV
(b)  HkVto33kV
(c)  3.3kVandllkV
(d)  none of the above
Ans: c

44.    The contacts of high voltage switches used in power system are submerged in oil. The main purpose of the oil is to
(a)    lubricate the contacts
(b)    insulate the contacts from switch body
(c)    extinguish the arc
(d)    all of the above
(e)    none of the above
Ans: c

45.    In Railway applications ______ circuit breaker is used.
(a) SFe   
(b) bulk oil
(c) minimum oil     
(d) air break
Ans:

46.    To protect most of the electrical equipment handling low power, the types of relays used are
(a)    thermocouple
(b)    electronic and bimetallic
(c)    both (a) and (b)
(d)    none of the above
Ans: c

47.    Wave trap is used to trap waves of
(a)     power frequencies
(b)     higher frequencies entering genera¬tor or transformer units
(c)    either of the above
(d)    none of the above
Ans: b

48.    Ungrounded neutral transmission system is not recommended because of system
(a)     insulation being overstressed due to pver voltages
(b)    insulation overstress may lead to failure and subsequent phase to phase faults
(c)    being inadequately protected against ground fault
(d)    all of the above
Ans: d

49.    The reflection co-efficient at the open circuited end of a transmission line.
(a)     zero   
(b)     infinity
(c)    unity
(d)    none of the above
Ans: c

50.    For the protection of power station buildings against direct strokes the requirements are
(a)     interception
(b)     interception and conduction
(c)    interception, conduction and dis-sipation
(d)    interception, conduction, dissipation and reflection
(e)    none of the above
Ans: c
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