## electrical

### Switchgear and Protection MCQ Objective Questions and Answers Page - 2

1.
 Zero- sequence currents can flow from a line to transformer bank if the windings are in A Grounded star/delta B Delta/star C Star/grounded star D Delta/delta Answer : A Explanation :
2.
 Fuses have got advantages of A Cheapest type of protection B Inverse time-current characteristic C No maintenance D All of the above Answer : D Explanation :
3.
 The primary function of a fuse is to A Open the circuit B Protect the appliance C Protect the line D Prevent excessive currents from flow through the circuit Answer : D Explanation :
4.
 The material used in liquid fuses is A $SF_{6}$ B Distilled water C Carbon tetra chloride D Transformer oil Answer : C Explanation :
5.
 In comparison to rewirable fuses HRC fuses have the advantage(s) of A High speed operation B High rupturing capacity C No ageing effect D All of the above Answer : D Explanation :
6.
 In HRC fuse the time between cut-off and final current zero is called the A Pre-arcing time B Arcing time C Total operating time D None of these Answer : B Explanation :
7.
 If a combination of HRC fuse and a circuit breaker is employed, the circuit breaker operates for A Short- circuit current B Low overload currents C Under all abnormal currents D The combination is newer employed in practice Answer : B Explanation :
8.
 The least expensive protection for overcurrent in low-voltage system is A Rewirable fuse B Isolator C Circuit breaker D Air-break switch Answer : A Explanation :
9.
 A Circuit breaker normally operates A When the power is to be supplied B When the line is to be tested C When the switch is to be put on D Whenever fault occurs in the line Answer : D Explanation :
10.
 When a fault occurs in a high voltage transmission line, first the A Circuit breaker operates then the relay B Relay operates then the Circuit breaker C Relay operates then successively the isolator and the Circuit breaker D Isolator operates, then successively the Relay and the Circuit breaker Answer : B Explanation :
11.
 The initiation of electric arc at the instant of contact separation is caused by A Thermionic emission of electrons B Field emission of electrons C (a) and (b) both D None of the above Answer : C Explanation :
12.
 Arcing voltage will be the least in case of A Carbon B Copper C Silver D Tungsten Answer : C Explanation :
13.
 The heat produced at the contact point owing to flow of electric current is least affected by A Temperature of the surrounding medium B Contact resistance C Magnitude of electric current flowing D Duration of flow of current Answer : A Explanation :
14.
 Which of the following should have low value for the contacts and their material  ? A Thermal capacity B Contact resistance C Thermal conductivity D None of these Answer : B Explanation :
15.
 In a CB Contact wipe is necessary A To decrease the speed of opening of the CB B To increase the speed of opening and reduce the arcing of contacts C To increase the dielectric strength across the CB contacts D None of the above Answer : C Explanation :
16.
 Interrupting medium in a contactor may be A Air at atmospheric pressure B Oil C Compressed air D Any of these Answer : D Explanation :
17.
 The fluids used in circuit breakers should be of A High dielectric strength and thermal stability B Noninflammability C Arc extinguishing ability D All of the above Answer : D Explanation :
18.
 Sparking between contacts can be reduced by inserting A A capacitor in parallel with the contacts B A capacitor in series with the contacts C A resistor in the line D A reactor in the line Answer : A Explanation :
19.
 For magnetic blow out of arc the magnetic field is produced A In the load circuit B At right angles to the axis of the arc C In line with the axis of the arc D Any of the above Answer : B Explanation :
20.
 In circuit breakers the contact space is ionized by A Field emission from the contact surface B Thermal emission from the contact surface C Thermal ionization of gas D Any of the above Answer : B Explanation :
21.
 In a circuit breaker, ionization is facilitated by A Increase in field strength B Increase of mean free length C High temperature of surrounding medium D All of the above Answer : D Explanation :
22.
 The stability of arc in vacuum depends on A The contact material only B The contact material and its vapour pressure C The circuit parameters only D The combination of (b) and (c) Answer : D Explanation :
23.
 The interrupting time of a circuit breaker is the period between the instant of A Initiation of short circuit and the arc extinction on an opening operation. B Energizing of the trip circuit and the arc extinction on an opening operation. C Initiation of short circuit and the parting of primary arc contacts. D Energizing of the trip circuit and the parting of primary arc contacts. Answer : B Explanation :
24.
 For a high speed circuit breaker the total clearing time is around A Few minutes B Few seconds C 1 to 2 cycles D 5 to 20 cycles Answer : C Explanation :
25.
 The resistance of an electric arc can be increased by A Increasing the concentration of ionized particles. B Reducing the arc length. C Splitting the arc. D Increasing the arc x-section. Answer : C Explanation :
26.
 The arc voltage in a circuit breaker is A In the phase with the arc current. B Lagging the arc current by 900. C Leading the arc current by 900. D Lagging the arc current by 1800. Answer : A Explanation :
27.
 In a circuit breaker, the active recovery voltage depends upon A Circuit conditions B Power factor C Armature reaction D All of these Answer : D Explanation :
28.
 In a circuit breaker the current that exists at the instant of contact separation is called the  _____________   current. A Re-striking B Breaking C Arc D Recovery Answer : B Explanation :
29.
 The rate of rise of re-striking voltage (RRRV) depends upon A System voltage B Circuit pf only C Switching condition only D Both (B) and (c) Answer : D Explanation :
30.
 The rate of rise of re-striking voltage (RRRV) is dependent upon A Resistance of the system only B Inductance of the system only C Capacitance of the system only D Inductance and capacitance of system Answer : D Explanation :
31.
 It is difficult to interrupt a capacitive circuit because A The current has a leading  power factor. B The re-striking voltage can be high. C Current magnitude is very small. D Stored energy in the capacitor is very high. Answer : B Explanation :
32.
 In circuit breakers, oil, as quenching medium, has the advantage(s) of A Good cooling properties B Absorption of arc energy C High dielectric strength D All of the above Answer : D Explanation :
33.
 The chances of arc interruption in subsequent current zeros A Increase in case of ABCB but decrease in OCB. B Decrease in case of ABCB but increase in OCB. C Decrease in both cases. D Increase in both cases. Answer : B Explanation :
34.
 In a minimum oil circuit breaker the oil is used A To act as circuit breaking medium only B For circuit breaking and providing insulation C For providing insulation only D For none of the above purposes Answer : A Explanation :
35.
 The air-blast circuit breakers for  400 kV  system are designed to operate in A 0.1 s B 0.5 s C 50 ms D 100 μs Answer : C Explanation :
36.
 The minimum oil circuit breaker has less volume of oil because A The oil between the breaker contacts has greater strength B There is insulation between the breaker contacts. C Solid insulation is provided for insulating the contacts from the earth. D None of the above. Answer : C Explanation :
37.
 Where voltages are high and current to be interrupted is low, the circuit breaker preferred is  _________  one. A Air-break B Vacuum C Oil D Air-blast Answer : B Explanation :
38.
 For rural electrification in a country like India with complex network, the circuit breaker preferred is  _________  One. A Air-break B Oil C Vacuum D Minimum oil Answer : C Explanation :
39.
 The most suitable circuit breaker for short line fault without switching resistor is  ________  One. A Minimum oil B Air-blast C $SF_{6}$ D Air-break Answer : C Explanation :
40.
 An isolator is meant for A Breaking abnormal current. B Making under fault condition. C Breaking the circuit under no-load condition. D None of the above. Answer : C Explanation :
41.
 Relays can be designed to respond to changes in A Resistance, reactance or impedance B Voltage and current C Temperature D All above Answer : D Explanation :
42.
 Overload relays are of ……. Type. A Induction B Electromagnetic C Thermal D All above Answer : D Explanation :
43.
 Thermal overload relays are used to protect the mother against overcurrent due to A short-circuits B heavy loads C grounds D all of the above Answer : B Explanation :
44.
 Magnetic circuit breaker has   ___________   trip action A Delayed B Instantaneous C Both of the above D None of the above Answer : B Explanation :
45.
 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 Answer : C Explanation :
46.
 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 Answer : D Explanation :
47.
 On 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 Answer : D Explanation :
48.
 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 DC component has ceased Answer : A Explanation :
49.
 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 Answer : A Explanation :
50.
 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 Answer : C Explanation :