## electrical

### Switchgear and Protection MCQ Objective Questions and Answers Page - 3

1.
 In circuit breakers, oil, as quenching medium, has the disadvantage(s) of A Need of periodical reconditioning or replacement. B Risk of formation of explosive mixture with air. C Possibility of causing fire hazards. D All of the above. Answer : D Explanation :
2.
 Main purpose of oil in OCB is to A Provide insulation B Provide cooling of contacts C Quenching arc D None of above Answer : C Explanation :
3.
 Air used in air blast circuit breaker must A Be ionized B Be free from moisture. C Have least CO2. D Have oil mist. Answer : B Explanation :
4.
 While using air-blast circuit breaker, current chopping is a phenomenon often observed when A A long overhead line is switched off. B A bank of capacitor is switched off. C A transformer on no load is switched off. D A heavy load is switched off. Answer : C Explanation :
5.
 In air-blast circuit breaker resistance switching is used to A Reduce the magnitude of fault current. B Control the CB operating time. C Damp out the fast transient. D Charge the fault current power factor. Answer : C Explanation :
6.
 SF6  gas has excellent heat transfer properties because of its A Low gaseous viscosity. B High dielectric strength. C Higher molecular weight. D Both (a) and (c) Answer : D Explanation :
7.
 SF6  circuit breakers have the advantages of A Very much reduced electrical clearance, reduced moisture problem and minimum current chopping problem. B Noiseless operation, less arcing time resulting in less contact erosion. C No reduction in dielectric strength of SF6 D All of the above Answer : D Explanation :
8.
 SF6  circuit breakers have the drawback(s) of A Sealing problem of gas. B Ingress of moisture in the gas system-dangerous. C Deterioration of  SF6 with time. D Both (a) and (b) Answer : D Explanation :
9.
 Which of the following statements is not correct  ? A SF6  is non-toxic and non-inflammable gas. B SF6  has very high dielectric strength roughly  24 times of that of air. C SF6  is yellow in colour. D SF6  is about  100 times more effective that air in arc extinction. Answer : C Explanation :
10.
 The single most important property that makes  SF6 a very efficient medium for circuit breaking is A It is non-toxic and non-inflammable. B It has high dielectric constant. C It has high breakdown strength. D It is highly electronegative in character. Answer : D Explanation :
11.
 During arc extinction  SF6  gas gets A Decomposed into SF4  and SF2 . B Decomposed into S and F  ions. C Reduced to SF2 . D Oxidised. Answer : A Explanation :
12.
 SF6  gas is transported in A Air cylinders. B Gas cylinders. C Liquid form in cylinders. D Solid form. Answer : C Explanation :
13.
 The current chopping tendency is minimised by using   SF6  gas at relatively A Low pressure and high viscosity. B Low pressure and Low viscosity. C High pressure and Low viscosity. D High pressure and high viscosity. Answer : B Explanation :
14.
 To limit current chopping in vacuum circuit breakers, the contact material employed should have the properties of A Low conductivity  and  High vapour pressure. B Low conductivity  and  Low vapour pressure. C High conductivity  and  High vapour pressure. D High conductivity  and  Low vapour pressure. Answer : A Explanation :
15.
 A thermal protection switch provides protection against A Overhead B Temperature C Short circuit D Overvoltage Answer : A Explanation :
16.
 For remote operation, the circuit breaker must be equipped with A Inverse time trip B Shunt trip C Time delay trip D All of these Answer : B Explanation :
17.
 Which of the following circuit breakers is generally used in railway electrification  ? A Air-blast circuit breaker B Minimum oil circuit breaker C Bulk oil circuit breaker D SF6  circuit breaker Answer : A Explanation :
18.
 What is the preferred type of  CB  to be installed in extra high voltage ac system  ? A Bulk oil type  CB B Air-blast CB C Vacuum CB D Sulphur hexafluoride  (SF6 ) CB Answer : D Explanation :
19.
 Keeping in view the cost and overall effectiveness, the following circuit breaker is beat suited for capacitor bank switching A Vacuum B Air-blast C SF6 D Oil Answer : A Explanation :
20.
 Which of the following circuit breakers has the lowest voltage range  ? A SF6  circuit breaker B Air-blast circuit breaker C Tank type oil circuit breaker D Air-break circuit breaker Answer : D Explanation :
21.
 Which of the following circuit breakers produce the least arc energy  ? A Plain oil B Minimum oil C Air-blast D Air break Answer : C Explanation :
22.
 Which of the following circuit breakers has high reliability and negligible maintenance  ? A Air-blast B SF6 C Oil D Vacuum Answer : B Explanation :
23.
 Which of the following circuit breakers take minimum time in installation  ? A Air-blast B Minimum oil C Bulk oil D $SF_{6}$ Answer : D Explanation :
24.
 The rating of a circuit breaker is usually determined on the basis of  ___________  fault. A Symmetrical B Line to line C Single line to ground D Double line to ground Answer : A Explanation :
25.
 The transient phenomenon lasts in a power system for a period ranging from A Few ms to 1 s. B 1   to  2 s. C 2 s  to  3 s. D Greater than 3 s. Answer : A Explanation :
26.
 Circuit breakers usually operate under A Steady short-circuit current. B Sub-transient state of short-circuit current. C Transient state of short-circuit current. D None of these. Answer : B Explanation :
27.
 The re-striking voltage is measured in A RMS value B Peak value C Instantaneous value D Average value Answer : B Explanation :
28.
 The making and breaking currents of  3-phase ac circuit breakers in power system are respectively in what form  ? A Rms value, rms value B Instantaneous value, rms value C Rms value D Instantaneous value, Instantaneous value Answer : B Explanation :
29.
 Rated breaking  capacity (MVA)  of a circuit breaker is equal to A The product of rated breaking current (kA) and rated voltage (kV). B The product of rated symmetrical breaking current (kA) and rated voltage (kV). C The product of breaking current (kA) and fault voltage (kV). D Twice the value of rated current (kA) and rated voltage (kV). Answer : B Explanation :
30.
 The making to breaking current ratio  for an  EHV  circuit breaker is A More than 1. B Equal to 1. C Less than 1. D A negative number Answer : A Explanation :
31.
 The making capacity of a circuit breaker is A Less than the symmetrical breaking capacity of the breaker. B Greater than the symmetrical breaking capacity of the breaker. C Equal to the symmetrical breaking capacity of the breaker. D Equal to the symmetrical breaking capacity. Answer : B Explanation :
32.
 Isolators used in transmission lines are capable of breaking  : A Fault current B No current C Charging current D Load current Answer : C Explanation :
33.
 The oil switches may be A Remote or manually controlled. B Used for capacitor switching, street lighting control and automatic disconnect on power failure. C Used for high voltage and large current circuits. D All of the above. Answer : D Explanation :
34.
 Which of the following sequence of operation is correct for operation of circuit breaker, isolator and earthing switch while opening a circuit  ? A Open circuit breaker – open isolator – close earthing switch. B Open isolator – Open circuit breaker – close earthing switch. C Open earthing switch – open isolator – then Open circuit breaker. D Open circuit breaker – close earthing switch – open isolator. Answer : A Explanation :
35.
 Which of the following sequence of operation is correct for operation of circuit breaker, isolator and earthing switch while closing a circuit  ? A Ensure circuit breaker is open – close isolator – open earthing switch – close circuit breaker. B Ensure circuit breaker is open – open earthing switch – close isolator – close circuit breaker. C Close circuit breaker – open earthing switch – close isolator. D Open earthing switch – Close circuit breaker – close isolator. Answer : B Explanation :
36.
 Purpose of backup protection is A To increase the speed B To increase a reach C To leave no blind spot D To guard against failure of primary Answer : D Explanation :
37.
 Protective relays are the devices that detect abnormal conditions in electrical circuit by measuring A Voltage. B Current. C Constantly the electrical quantities which differ during normal and abnormal conditions. D None of the above. Answer : C Explanation :
38.
 The actuating quantity for the relays may be A Magnitude B Frequency C Phase angle D Any of these Answer : D Explanation :
39.
 Protective relays can be designed to respond to A Light intensity B Temperature C Voltage or current D All of the above Answer : D Explanation :
40.
 The operating speed of a relay depends upon the A Rate of flux built up B Armature core air-gap C Spring tension D All of these Answer : D Explanation :
41.
 Air blast Circuit breaker is used for A Over currents B Short duty C Intermittent duty D Repeated duty Answer : D Explanation :
42.
 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 Answer : D Explanation :
43.
 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 Answer : B Explanation :
44.
 Directional relays are based on flow of A Power B Current C Voltage wave D All of the above Answer : A Explanation :
45.
 A differential relay measures the vector difference between A Two currents B Two voltages C Two or more similar electrical quantities D None of the above Answer : C Explanation :
46.
 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) Answer : D Explanation :
47.
 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 Answer : A Explanation :
48.
 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 Answer : C Explanation :
49.
 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 condition D None of the above Answer : C Explanation :
50.
 The voltage appearing across the contacts after opening of the circuit breaker is called   _____________   voltage. A Recovery B Surge C Operating D Arc Answer : A Explanation :