110 TOP Switchgear Protections Multiple choice Questions answers freshers experienced pdf

Real Time Switchgear Protections Online Quiz Questions and 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

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

20. 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 con¬tacts
(c) high speed opening of contacts
(d) all of the above
Ans: d

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

51. The line insulation is the insulation level of the station equipment.
(a) less than
(b) same as
(c) more than
(d) proportional to
(e) not directly related with
Ans: e

52. The interaction between a transmission line and communication line js minimized by
(a)     transposing transmission as well as communication lines
(b)     increasing the height of the trans-mission line tower
(c) increasing the distance between the two lines
(d) all of the above
Ans: d

53. When a wave propagates on a transmission line, it suffers reflection several times at
(a)     tapping
(b)     load end
(c)     sending end
(d) sending and other end
(e) all of the above
Ans: d

54. Which of the following statements is incorrect?
(a)     Station batteries are used to operate relay only
(b)     The lightning arresters are basical¬ly surge diverters
(c) An impedance relay has maximum fault current when fault occurs near the relay
(d) A high speed relay has an operation of 1 to 2 cycles
Ans: a

55. Discrimination between main and back up protection is provided by the use of relays which are
(a) fact
(b) sensitive
(c) slow
(d) none of the above
Ans: c

56. Induction cup relay is operated due to changes in
(a) current
(b) voltage
(c) impedance
(d) all of the above
Ans: d

57. A.C. network analyser is used to solve problems of
(a) load flow
(b) load flow and short-circuit
(c) load flow and stability
(d) load flow, short-circuit and stability
(e) none of the above
Ans: d

58. Which of the following statements is incorrect ?
(a) Lightning arrestors are used before the switchgear
(b) Shunt reactors are used as compensation reactors
(c) The peak short current is (1.8 xV2) times the A.C. component
(d) The MVA at fault is equal to base MVA divided by per unit equivalent fault reactance
Ans: a

59. Short-circuit currents are due to
(a)     single phase to ground faults
(b)     phase to phase faults
(c) two phase to ground faults
(d) three phase faults
(e) any of these
Ans: e

60. To reduce short circuit fault currents are used.
(a)     reactors
(b)     resistors
(c) capacitors
(d) none of the above
Ans: a

61. Bus coupler is very essential in arrangement
(a) single bus
(b) double bus, double breaker
(c) main and transfer bus
(d) all of the above
Ans: c

62. For cost and safety, the outdoor substa tions are installed for voltages above
(a) 11 kV
(b) 33 kV
(c) 60kV              
(d) 110kV
Ans: b

63. The short circuit in any winding of the transformer is the result of
(a) mechanical vibration
(b) insulation failure
(c) loose connection
(d) impulse voltage
Ans: d

64 relays are used for phase faults on long line.
(a) Impedance
(b) Reactance
(c) Either of the above
(d) None of the above
Ans: a

65.   For which of the following protection from  negative  sequence  currents  is provided ?
(a) Generators      
(b) Motors
(c) Transmission line
(d) Transformers
Ans: a

66 relay is preferred for phase fault on short transmission line.
(a) Induction type
(b) Reactance
(c) Impedance
(d) None of the above
Ans: b

67. Distance relays are generally
(a) split-phase relays
(b) reactance relays
(c) impedance relays
(d) none of the above
Ans: d

68. For which of the following ratings of the transformer differential protection is recommended ?
(a)     above 30 kVA.
(b)     equal to and above 5 MVA
(c) equal to and above 25 MVA
(d) none of the above
Ans: b

69. A _______ is used to measure the stator % winding temperature of the generator.
(a)     thermocouple
(b)     pyrometer
(c) resistance thermometer
(d) thermometer
Ans: c

70. The under voltage relay can be used for
(a) generators
(b) busbars
(c) transformers    
(d) motors
(e) all of the above
Ans: e

71. The relay with inverse time characteristic will operate within
(a) 1.5 sec
(b) 5 to 10 sec
(c) 5 to 20 sec
(d) 20 to 30 sec
(e) none of the above
Ans: b

72. The single phasing relays are used for the protection of
(a) single phase motors only
(b) two phase motors only
(c) two single phase motors running in parallel
(d) three phase motors
Ans: d

73. Which of the following devices will receive voltage surge first travelling on the transmission line ?
(a) Lightning arresters
(b) Relays
(c) Step-down transformer
(d) Switchgear
Ans: a

74. Which of the following parameter can be neglected for a short line ?
(a) Inductance
(b) Capacitance
(c) Resistance
(d) Reactance
Ans: b

75. Series reactors should have
(a) low resistance
(b) high resistance
(c) low impedance
(d) high impedance
Ans: a

76. Which of the following circuit breakers has high reliability and minimum maintenance ?
(a)     Air blast circuit breakers
(b)     Circuit breaker with SF6 gas
(c) Vacuum circuit breakers
(d) Oil circuit breakers
Ans: b

77. Arc in a circuit breaker is interrupted at
(a)     zero current
(b) maximum current
(c) minimum voltage
(d) maximum voltage
Ans: a

78 transmission line has reflection coefficient as one.
(a) Open circuit    
(b) Short-circuit
(e) Long
(d) None of the above
Ans: a

79. What will be the reflection co-efficient of the wave of load connected to transmission line if surge impedance of the line is equal to load ?
(a)     Zero
(b)     Unity
(c) Infinity
(d) None of the above
Ans: a

80. The inverse definite mean time relays are used for over current and earth fault protection of transformer against
(a)     heavy loads
(b)     internal short-circuits
(c) external short-circuits
(d) all of the above
Ans: b

81. Over voltage protection is recommended for
(a)     hydro-electric generators
(b)     steam turbine generators
(c) gas turbine generators
(d) all of the above
(e) none of the above
Ans: d

82. Air blast circuit breakers for 400 kV power system are designed to operate in
(a) 100 micro-second
(b) 50 milli-second
(c) 0.5 sec
(d)     0.1 sec
Ans: b

83. Overfluxing protection is recommended for
(a) distribution transformer
(b) generator transformer of the power plant
(c) auto-transformer of the power plant
(d) station transformer of the power plant
Ans: b

84. Series capacitors are used to
(a)     compensate for line inductive reactance
(b)     compensate for line capacitive reactance
(c) improve line voltage
(d) none of the above
Ans: a

85. Admittance relay is _______ relay.
(a)     impedance
(b)     directional
(c) non-directional
(d) none of the above
Ans: b

86. The material used for fuse must have
(a) low melting point and high specific resistance
(b) low melting point and -low specific resistance
(c) high melting point and low specific resistance
(d) low melting point and any specific resistance
Ans: a

87. If the fault occurs near the impedance relay, the VII ratio will be
(a) constant for all distances
(b) lower than that of if fault occurs away from the relay
(c) higher than that of if fault occurs away from the relay
(d) none of the above
Ans: b

88. The torque produced in induction type relay (shaded pole structure) is
(a)     inversely proportional to the current
(b)     inversely proportional to the square of the current
(c) proportional to the current
(d) proportional to square of the current
Ans: b

89. The steady state stability of the power system can be increased by
(a)     connecting lines in parallel
(b)     connecting lines in series
(e) using machines of high impedance
(d) reducing the excitation of machines
(e) none of the above
Ans: a

90. The inductive interference between power and communication line can be minimized by
(a) transposition of the power line
(b) transposition of the communication line
(c) both (a) and (b)
(d) increasing the distance between the conductors
Ans: c

91. The power loss is an important factor for the design of
(a) transmission line
(b) motor
(c) generator
(d) feeder
Ans: a

92. A fuse is connected
(a) in series with circuit
(b) in parallel with circuit
(c) either in series or in parallel with circuit
(d) none of the above
Ans: a

93. H.R.C. fuse, as compared to a rewirable fuse, has
(a) no ageing effect
(b) high speed of operation
(c) high rupturing capacity
(d) all of the above
Ans: d

94. The fuse rating is expressed in terms of
(a) current
(b) voltage
(c) VAR
(d) kVA
Ans: a

95. The fuse blows off by
(a) burning
(b) arcing
(c) melting
(d) none of the above
Ans: c

96. On which of the following effects of electric current a fuse operates ?
(a) Photoelectric effect
(b) Electrostatic effect
(c) Heating effect
(d) Magnetic effect
Ans: c

97. An isolator is installed
(a) to operate the relay of circuit breaker
(b) as a substitute for circuit breaker
(c) always independent of the position of circuit breaker
(d)     generally on both sides of a circuit breaker
Ans: d

98. A fuse in a motor circuit provides protection against
(a) overload
(b) short-circuit and overload
(c) open circuit, short-circuit and overload
(d) none of the above
Ans: b

99. Protection by fuses is generally not used beyond
(a) 20 A
(b) 50 A
(c) 100 A
(d) 200 A
Ans: c

100.   A fuse is never inserted in
(a)     neutral wire
(b) negative of D.C. circuit
(c) positive of D.C. circuit
(d) phase dine
Ans: a

101. Oil switches are employed for
(a) low currents circuits
(b) low voltages circuits
(c) high voltages and large currents circuits
(d)     all circuits
Ans: c

102. A switchgear is device used for
(a)     interrupting an electrical circuit
(b)     switching an electrical circuit 111.
(c) switching and controlling an electri¬cal circuit
(d) switching, controlling and protect¬ing the electrical circuit and equipment
Ans: d

103. The fuse wire, in D.C. circuits, is inserted in
(a)     negative circuit only
(b)     positive circuit only
(c) both (a) and (b)
(d) either (a) or (b)
Ans: c

104. By which of the following methods major portion of the heat generated in a H.R.C. fuse is dissipated ?
(a) Radiation
(b) Convection
(c) Conduction      
(d) All of the above
Ans: c

105. A short-circuit is identified by
(a) no current flow
(b) heavy current flow
(c) voltage drop    
(d) voltage rise
Ans: b

106. The information to the circuit breaker under fault conditions is provided by
(a) relay
(b) rewirable fuse
(c) H.R.C. only      
(d) all of the above
Ans: a

107. To limit short-circuit current in a power system are used.
(a) earth wires      
(b) isolators
(c) H.R.C. fuses    
(d) reactors
Ans: d

108. A balanced 3-phase system consists of
(a)     zero sequence currents only
(b)     positive sequence currents only
(c) negative and zero sequence currents
(d) zero, negative and positive sequence currents
Ans: b

109. In a single bus-bar system there will be complete shut down when
(a) fault occurs on the bus itself
(b) fault occurs on neutral line
(c) two or more faults occur simultaneously
(d) fault occurs with respect to earthing
Ans: a

Latest Switchgear Protections Objective type Questions for freshers

30 TOP Electrical Switchgear and Protection Interview Questions answers pdf download

Real Time Electrical Switchgear and Protection Interview Questions with Answers PDF

=> Also Read Switchgear and Protection MCQs

1. What are the main functions of control gear?
1. To provide a means of starting and stopping the motor and, at the same time, of limiting the starting current if required.
2. To give adequate protection to the motor under all conditions.
3. To allow speed changing when required.
4. To provide means of braking the motor when required.
5. To reverse the direction of rotation when required.
Protection of the motor must be automatic, but the other operations may be arranged to be under the control of an operator, or may be partly or fully-automatic.

2. What provision must be made for short-circuit conditions in motor circuits?
Since overload relays are not designed to operate and clear the circuit in the event of a short-circuit. Circuit-breaker or fuse protection of suficient breaking capacity to deal with any possible short-circuit that may occur must be provided.

3. What are the usual forms of overload relay in motor-control gear?
In small contactor starters, generally thermal relays, either of the ‘solder pot’ or bimetal type. With large contactors or oil switches, magnetic relays of the solenoid type with dashpots. Either type of overload relay may be used within intermediate sizes.

4. How do thermal relays work?
The bimetallic thermal relay consists of a small bimetallic strip that is heated by an element connected in series with the supply.
When the current rises above a preset value, the movement of the strip releases a catch which opens the trip contacts.
In recent years more modern electronic relays are used which simulate the thermal overload. Many of these relays also incorporate a memory, i.e. simulates the temperature rise / cooling curve of the winding.


5. How does the magnetic overload relay operate?
A solenoid connected in series with the supply contains a plunger whose movement is damped by a dashpot. When the safe current is exceeded, the solenoid pulls the plunger up – disconnecting the supply. The damping provided by the dashpot prevents unwarranted tripping on short-time overloads.

6. How many overload relays are required in the control gear?
On three-phase supplies where the neutral point of the system is connected to earth, as is usually the case, three overload relays (one in each line) are necessary for complete protection.
For 2-phase 3-wire and 4-wire supplies, two overload relays are required, one in each phase line, none being connected in any neutral or earth conductor.
With single-phase motors one overload relay in any conductor except an earthed conductor or neutral.

7. What happens when one of the three lines supplying a three-phase induction motor becomes open-circuited?
The motor, if already running, will continue to run as a single-phase motor on the remaining single-phase supply. The condition is called single-phasing. If the motor is loaded to more than about 30 per cent of full load, the currents in the motor windings tend to become excessive and overheating occurs.
With one line broken, the motor will not start up and, due to the heavy standstill current, burn-out is likely unless the motor is quickly disconnected.

8. Will normal overload relays trip on single-phasing?
If correctly set, the normal overloads will trip when the motor is fully loaded due to the rise in current passing through the closed supply lines. With a delta-connected motor partially loaded, the rise in line current may not be su??cient to operate the overload trip and one phase may became excessively overheated.


9. What special protection can be provided against single-phasing?
One method is to incorporate a combined overload and single-phase relay in the control gear. A typical relay of this type includes three overload relays with trip contacts so arranged that it will trip if the displacement of one overload element differs from that of the others.
This type of relay will operate if single-phasing occurs at or near full load with the same time delay as on overload, but at light loads, the time delay for single-phase protection is longer. Another device is a phase-failure relay in the control gear. Its principle is based on the fact that the currents in the supply lines or the voltages between them at the motor terminals are unbalanced when the motor is single-phasing.
The phase-failure relay may be of the current or voltage-operated type which trips out the line switch when one of the supply lines becomes open circuited.

10. What are the alternatives to the use of overload releases?
Direct protection against overheating or burning-out of motor windings may be built into the motor. Built-in protectors may take the form of thermostats or thermistors embedded in the end windings of the stator while the motor is under construction.

11. How are built in thermal overload protector arranged?
On smaller motors LV mush winding motors, these detectors are embedded in the overhang of the winding. On the medium voltage motors these are placed in between bottom and top coils in the slot portion of the core.

12. How do built-in thermal overload protectors work?
Thermistors are very small semiconductor devices whose resistance changes rapidly with temperature. Three thermistors are inserted in the end-windings of the stator, one in each phase, and are connected in series. The two thermistor terminals at the motor are connected to an electronic-amplifier-control unit in the starter, through which the tripping circuit of the starter is operated. The response of the thermistors to temperature change is extremely rapid, allowing this type of protection to be effective under all motor overload conditions.

13. Resistance temperature devices (RTD)
This is a resistance which increases linearly with temperature rise. The most commonly used in motors is embedded in an epoxy glass type wedge which can be inserted between the upper and lower coils. The resistance is measured with an electronic amplifier control unit which is converted to temperature.
This unit has adjustable settings to allow for alarm and trip with contacts which are then used in the motor starter circuit.
Thermocouples
A thermocouple is two dissimilar metals which are joined together and with a change in temperature, creates a voltaic action. This gives out a milli-volt signal which is then measured with an electronic amplifier control unit converting the measurement to temperature.

14. When is direct-on-line starting used for three-phase squirrel-cage motors?
It is usual for small LV machines; for larger motors it is often necessary to use other methods of starting in order to avoid excessive starting currents. HV motors are usually DOL started. (since amps are low)

15. What methods are employed to reduce the starting current of squirrel-cage motors?
Where the starting conditions are light, the starting current can be lessened by some method of reducing the stator voltage when switching on. There are four ways of starting on reduced voltage:
1. Primary-resistance starting – introducing resistance between the supply and the stator windings.
2. Primary-reactor starting – introducing a reactor in series with the stator windings, usually connected in the star point.
3. Star-delta starting – connecting the stator windings in star for starting and in delta for running.
4. Auto-transformer starting – supplying the stator windings through tappings on an auto-transformer.

16. When is primary-resistance starting employed?
Generally only for small motors on light-starting duty. The method is easily adjustable to suit the load and gives a smooth breakaway against low torque. If the resistance is adjustable, as in a faceplate starter, starting can be very smooth and this is useful for motors that must be started without any shock that might cause injury to the material being handled by the driven machine.

17. When is the primary reactor method of starting employed?
Mainly for high-tension motors on very light-starting load where a fairly heavy starting current can be permitted e.g. boiler-feed pumps in a large power station.

18. When is an auto-transformer starter used?
When more flexibility is required for starting a squirrel-cage motor than is provided by the star-delta method, which is limited as far as starting torque is concerned. Auto-transformer starting permits the stator to be wound for running in star. The starting torque can be adjusted to suit the load by changing the voltage tapping on the auto-transformer. Both starting torque and current are reduced in the same proportion.
It is used for motors of medium and large size on light starting loads (e.g. centrifugal pumps, fans, compressors and mills). Up to about 75kW the simple auto-transformer starter is employed; above this, the Korndorfer connection is recommended.

19. What are the initial-starting line current and motor torque when star-delta starting?
Both line current and torque are approximately one-third of the motor standstill values on full volts.

20. How do the various methods of starting on reduced voltage compare as regards torque per ampere?
Star-delta and auto-transformer methods have the advantage over primary resistance and primary reactor methods.

21. What mechanical methods of reducing starting current can be adopted?
The starting duty can be reduced by fitting a centrifugal or other type of clutch which only picks up the load when the motor is well up to speed.

22. What is sequence starting?
A system of starting by which several motors of similar rating are started in sequence off one starter in conjunction with interlocked switching.

23. How are slip-ring motors started?
By first switching the supply on the stator winding with all the external rotor resistance in circuit across the slip rings and then cutting out the rotor resistance progressively as the motor speeds up until finally the rotor winding is short-circuited.

24. What is the usual arrangement of connections for a hand-operated slip-ring starter?
Small slip-ring starters usually consist of a contactor for the stator circuit and a face plate-type starting resistance for the rotor circuit. The basic essentials are shown, the three wires from the stator going to slip-ring terminals R, S and T on the motor. An actual wiring diagram is also shown. The starter must be fitted with interlocks to ensure that the resistance is all-in when starting.
With a contactor controlling the stator supply, interlocking is simply effected, as shown, through electrical contacts on the arm of the rotor starter, no current reaching the contactor coil ‘I’ unless the arm is in the starting position. The start button must be kept depressed until all resistance has been taken out; this ensures that the motor is not accidentally left running with some of the rotor resistance still in circuit. When the operating arm of the face- plate is in the ‘run’ position the start button is short-circuited.
If the motor is fitted with a device designed to lift the brushes and short-circuit the slip rings when the motor is up to speed, an interlock must be arranged in the control circuit to ensure that the brush-gear is in the starting position before the stator contactor can close.

25. How is speed control of a slip-ring motor effected?
By introducing resistance into the rotor circuit similar to a starting resistance except that the heat losses in the resistance must be dissipated continuously. Unless the duty is intermittent, all except small sizes require some means of cooling the resistors.
Grid resistances with a motor-driven fan may be used in conjunction with a drum controller. Alternative methods are oil-immersed resistances or a liquid resistance cooled by circulating water through cooling tubes.

26. What is liquid resistance?
Insulated pots filled with a resistance solution of electrolyte, e.g. caustic soda or washing soda. Plates connected to the slip rings dip into the pots and are shorted out in the full-speed position.
Liquid starters and controllers are used for large motors.

27. What is the advantage of a liquid resistance for starting purposes?
Resistance may be reduced continuously so that, with close control over the current as indicated on an ammetre, a very smooth start can be obtained.

28. What is a slip resistance?
A fixed step of rotor resistance used to limit the current taken from the supply at the instant when peak load is applied to the motor. It is often desirable to do this on press drives, guillotines, etc. As the resistance value is small, it is usual to have a conventional starter so arranged that the last step of resistance is not cut out when the starting handle is right home. This last step of resistance is continuously rated.

29. What is meant by motors in synchronous tie?
When the two slip-ring motors are required to run at the same speed, it is possible to do this by connecting their rotors together through the slip rings in conjunction with a single slip resistance. The starter for such a scheme includes a single rotor resistance, In order to limit the circulating current in the event of the motors being out of phase when started, a reactance is usually inserted in the interconnecting tie. The reactor is wound in two sections, and connected so that it is non-inductive to currents flowing through each half into the slip resistance but inductive to circulating currents between rotors. This reactance also assists load sharing when the two motors are driving a common load, as for example travel motors at opposite ends of an overhead crane.
The controller gives speed control by varying resistance in series with the rotor windings and also breaks the three-stator phase in the ‘off’ position. The moving-copper-contact rings are shown as thick horizontal lines, while the forward and reverse steps are indicated by the numbered vertical lines.

Latest Electrical Switchgear and Protection Interview Questions for freshers and Experienced pdf

50 TOP Switchgear Protection Interview Questions and answers freshers experienced pdf

1. What are the functions of protective relays.
To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system,  thereby protecting the system from damages consequent to the fault.

2. Give the consequences of short circuit.
Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts.The trip circuit is then closed, the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.

3. Define protected zone.
Are those which are directly protected by a protective system such as relays, fuses or switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.

4. What are unit system and non unit system?
A unit protective system is one in which only faults occurring within its protected zone are isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A non unit system is a protective system which is activated even when the faults are external to its protected zone.


5. What is primary protection?
Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker.It serves as the first line of defence.

6. What is back up protection?
Is the second line of defence , which operates if the primary protection fails to activate within a definite time delay.

7. Name the different kinds of over current relays.
Induction type non-directional over current relay,Induction type directional over current relay & current differential relay.

8. Define energizing quantity.
It refers to the current or voltage which is used to activate the relay into operation.


9. Define operating time of a relay.
It is defined as the time period extended from the occurrence of the fault through the relay detecting the fault to the operation of the relay.

10. Define resetting time of a relay.
It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.

11. What are over and under current relays?
Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg: Induction type nondirectional/ directional overcurrent relay, differential overcurrent relay)whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(eg: differential over-voltage relay)

12. Mention any two applications of differential relay.
Protection of generator & generator transformer unit; protection of large motors and busbars .

13. What is biased differential bus zone reduction?
The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.
The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be suficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system.

14. Mention the short comings of Merz Price scheme of protection applied to a power transformer.
In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition. Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay under normal condition.

15. What is an under frequency relay?
An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value.

16. Define the term pilot with reference to power line protection.
Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.

17. Mention any two disadvantage of carrier current scheme for transmission line only.
The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response time of band pass filter capacitance phase-shift of the transmission line .

18. What are the features of directional relay?
High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive.

19. What are the causes of over speed and how alternators are protected from it? Sudden loss of all or major part of the load causes over-speeding in alternators.
Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.

20. What are the main types of stator winding faults?
Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same phase winding.

21. What are the types of graded used in line of radial relay feeder? Definite time relay and inverse-definite time relay.
What are the various faults that would affect an alternator?
Stator faults
Phase to phase faults 2, Phase to earth faults 3, Inter turn faults
(b)
1, Earth faults
2, Fault between turns
3, Loss of excitation due to fuel failure
1, Over speed
2, Loss of drive
3, Vacuum failure resulting in condenser pressure rise, resulting in shattering of the turbine low pressure casing
1, Fault on lines
2, Fault on busbars

22. Why neutral resistor is added between neutral and earth of an alternator?
In order to limit the flow of current through neutral and earth a resistor is introduced between them.

23. What are faults associated with an alternator?
External fault or through fault
Internal fault
1, Short circuit in transformer winding and connection 2, Incipient or slow developing faults

24. What are the problems arising in differential protection in power transformer and how are they overcome?
Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.
Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.
Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.
Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary. CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.

25. What is REF relay?
It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be suffcient to activate the relay, unless the relay is set for a very low current.
Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

26. What is over fluxing protection in transformer?
If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing.

27. Why busbar protection is needed?
Fault level at busbar is high
The stability of the system is affected by the faults in the bus zone.
A fault in the bus bar causes interruption of supply to a large portion of the system network.

28. What is field suppression?
When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself. Hence the field circuit breaker is opened and stored energy in the field winding is discharged through another resistor. This method is known as field suppression.

29. What are the causes of bus zone faults?
Failure of support insulator resulting in earth fault
Flashover across support insulator during over voltage Heavily polluted insulator causing flashover Earthquake, mechanical damage etc.

30. What are the problems in bus zone differential protection?
Large number of circuits, different current levels for different circuits for external faults.
Saturation of CT cores due to dc component and ac component in short circuit currents. The saturation introduces ratio error.
Sectionalizing of the bus makes circuit complicated.
Setting of relays need a change with large load changes.

31. What is static relay?
It is a relay in which measurement or comparison of electrical quantities is made in a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device.

32. What is power swing?
During switching of lines or wrong synchronization surges of real and reactive power flowing in transmission line causes severe oscillations in the voltage and current vectors. It is represented by curves originating in load regions and traveling towards relay characteristics.

33. What is a programmable relay?
A static relay may have one or more programmable units such as microprocessors or microcomputers in its circuit.

34. What is resistance switching?
It is the method of connecting a resistance in parallel with the contact space(arc). The resistance reduces the restriking voltage frequency and it diverts part of the arc current. It assists the circuit breaker in interrupting the magnetizing current and capacity current.

35. What do you mean by current chopping?
When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping.

36. Give the two methods of arc interruption?
High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished Current zero method:-The arc is interrupted at current zero position that occurs100 times a second in case of 50Hz power system frequency in ac.

37. What is restriking voltage?
It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished.

38. What is meant by recovery voltage?
The power frequency rms voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage.

39. Write the classification of circuit breakers based on the medium used for arc extinction?
Air break circuit breaker Oil circuit breaker
Minimum oil circuit breaker Air blast circuit breaker
SF6 circuit breaker
Vacuum circuit breaker

40. What is the main problem of the circuit breaker?
When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which may cause damage to the system or to the breaker itself. This is the main problem.

41. What are demerits of MOCB?
Short contact life
Frequent maintenance Possibility of explosion
Larger arcing time for small currents Prone to restricts

42. What are the advantages of oil as arc quenching medium?
It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties
It acts as an insulator and permits smaller clearance between line conductors and earthed components

43. What are the advantages of MOCB over a bulk oil circuit breaker?
It requires lesser quantity of oil
It requires smaller space
There is a reduced risk of fire
Maintenance problem are reduced

44. What are the advantages of synthetic testing methods?
The breaker can be tested for desired transient recovery voltage and RRRV.
Both test current and test voltage can be independently varied. This gives flexibility to the test
The method is simple With this method a breaker capacity (MVA) of five time of that of the capacity of the test plant can be tested.

45. How does the over voltage surge affect the power system?
The over voltage of the power system leads to insulation breakdown of the equipments. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line.

46. Define composite testing of a circuit breaker.
In this method the breaker is first tested for its rated breaking capacity at a reduced voltage and afterwards for rated voltage at a low current.This method does not give a proper estimate of the breaker performance.

47. How direct tests are conducted in circuit breakers?
Using a short circuit generator as the source.
Using the power utility system or network as the source.

48. What is dielectric test of a circuit breaker?
It consists of overvoltage withstand test of power frequency lightning and impulse voltages.Testa are done for both internal and external insulation with switch in both open and closed conditions.

49. Define negative sequence component.
It has 3 vectors equal in magnitude and displaced from each other by an angle 120 degrees and has the phase sequence in opposite to its original phasors.

50. Differentiate between a fuse and a circuit breaker.
Fuse is a low current interrupting device. It is a copper or an aluminium wire.Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions.
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