## electrical

### Switchgear and Protection MCQ Objective Questions and Answers Page - 1

1.
 Short-circuit currents are due to A Single-phase to earth fault B Phase-to-phase fault C All the three phases-to earth fault D Any of the above Answer : D Explanation :
2.
 The most serious consequence of a major uncleared short-circuit fault could be A Blowing of fuse B Fire C Heavy voltage drop D None of these Answer : B Explanation :
3.
 Which of the following results in a symmetrical fault  ? A Single-phase-to earth B Phase-to-phase C All the three phases-to earth D Two phase-to earth Answer : C Explanation :
4.
 Which portion of the transmission system is more prone to faults  ? A Alternator B Transformer C Overhead lines D Underground cable Answer : C Explanation :
5.
 Which portion of the power system is least prone to faults  ? A Alternator B Switchgear C Transformers D Overhead lines Answer : A Explanation :
6.
 The magnitude of fault current depends upon A Total impedance up to fault B Voltage at the fault point C Load current being supplied before occurrence of fault D Both (a) and (b) Answer : D Explanation :
7.
 The most common type of fault is A Single-phase-to ground B Phase-to-phase C Two- phase-to ground D Three - phase-to ground Answer : A Explanation :
8.
 The maximum short-circuit current occurs in the case of  : A Three-phase bolted fault B Double-line-to-ground fault C Line-to-line fault D Single- line-to-ground fault Answer : A Explanation :
9.
 For a fault at the terminals of synchronous generator, the fault current is maximum for a A 3-phase fault B 3-phase to ground fault C Line-to-ground fault D Line-to-line fault Answer : C Explanation :
10.
 A 25 MVA, 33 kV transformer has a pu impedance of 0.9 . the per unit impedance at a new base  50 MVA at 11 kV would be A 10.4 B 12.2 C 14.4 D 16.2 Answer : D Explanation :
11.
 The per unit value of a  4 Ω  resistor at  100 MVA  base and  10 kV base voltage is A 2 pu B 4 pu C 0.4 pu D 40 pu Answer : B Explanation :
12.
 A  10 kVA, 400  V/200 V  single-phase transformer with  10% impedance, draws a steady short-circuit current of A 50 A B 150 A C 250 A D 350 A Answer : C Explanation :
13.
 A transformer rated for  500 kVA.  1 =1kV/0.4 kV has an impedance of  10%  and is connected to an infinite bus. The fault level of the transformer is A 500 kVA B 5000 kVA C 500 MVA D None of these Answer : C Explanation :
14.
 In the above system, the three-phase fault MVA at the bus 3 is A 82.55 MVA B 85.11 MVA C 170.91 MVA D 181.82 MVA Answer : D Explanation :
15.
 The per-unit impedance of an alternator corresponding to base values  13.2 kV  and  30 MVA  is  0.2 p.u. value of the impedance for base values of  13.8 kV  and  50 MVA  in pu will be A 0.131 B 0.226 C 0.305 D 0.364 Answer : C Explanation :
16.
 Four alternators, each rated at  5 MVA, 11 kV, with 20% reactance are working in parallel. The short-circuit level at bus-bars is A 6.25 MVA B 20 MVA C 25 MVA D 100 MVA Answer : D Explanation :
17.
 The post fault voltages at buses  1 and 3  in per unit respectively are A 0.24, 0.63 B 0.31, 0.76 C 0.33, 0.67 D 0.67, 0.33 Answer : D Explanation :
18.
 The instant (t0) of the fault will be A 4.682 ms B 9.667 ms C 14.667 ms D 19.667 ms Answer : A Explanation :
19.
 The  rms  value of the ac component of fault current (Ix) will be A 3.59 kA B 5.07 kA C 7.18 kA D 10.15 kA Answer : A Explanation :
20.
 Instead of the three phase fault, if a single line-to ground fault occurs on phase  ‘a’  point  ‘F’  with zero fault impedance, then the rms of the ac component of fault current (Ix) for phase  ‘$\alpha$’  will be A 4.97 pu B 7.0 pu C 14.93 pu D 29.85 pu Answer : C Explanation :
21.
 Series reactors are used to A Improve the transmission efficiency B Improve the power factor of the power system C Improve the voltage regulation D Bring down the fault level within the capacity of the switchgear Answer : D Explanation :
22.
 Current limiting reactors may be A Air-cored air-cooled B Oil immersed magnetically shielded C Oil immersed non-magnetically shielded D Any of the above Answer : D Explanation :
23.
 Symmetrical  components are used in power system for the analysis of A Balanced  3-phae fault B Unbalanced  3-phae fault C Normal power system under steady conditions D Stability of system under disturbance Answer : B Explanation :
24.
 In a star-connected system without neutral grounding, zero sequence currents are A Zero B Phasor sum of phase currents C Same as rms value of phase currents D Same as peak value of phase currents Answer : A Explanation :
25.
 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 Answer : B Explanation :
26.
 The Positive sequence current of a transmission line is  : A Always zero B 1/3 of Negative sequence current C Equal to Negative sequence current D 3 times Negative sequence current Answer : C Explanation :
27.
 For a fully transposed transmission line A Positive, Negative and zero sequence impedances are equal. B Positive and Negative sequence impedances are equal. C Zero and Positive sequence impedances are equal. D Negative and zero sequence impedances are equal. Answer : B Explanation :
28.
 In case of an unbalanced star-connected load supplied from an unbalanced 3-phase, 3-wire system, load currents will consist of A Positive- sequence components. B Negative- sequence components. C Zero- sequence components. D Only (a) and (b) Answer : D Explanation :
29.
 For an unbalanced fault, with paths for zero-sequence currents, at the point of fault A The negative-and zero-sequence voltages are minimum. B The negative-and zero-sequence voltages are maximum. C The negative-sequence voltages is minimum and zero-sequence voltages is maximum. D The negative-sequence voltages is maximum and zero-sequence voltages is minimum. Answer : B Explanation :
30.
 In case of single line to ground fault A All sequence networks are connected in parallel. B All sequence networks are connected in series. C Positive and negative sequence networks are connected in parallel. D Zero and negative sequence networks are connected in series. Answer : B Explanation :
31.
 When a line-to-ground fault occurs, the current, the current in the faulted phase is  100 A. the zero-sequence current in this case will be  : A Zero B 33.3 A C 66.6 A D 100 A Answer : B Explanation :
32.
 Zero sequence fault current is absent when fault is A Single-line-to ground fault. B Line-to-line ground fault. C Line-to-line D Double-line-to ground fault. Answer : D Explanation :
33.
 The current of a single-phase load drawn from a 3-phase system has  : A Zero- sequence component of current as zero. B Negative- sequence component of current more than positive sequence component. C Positive, negative-and zero- sequence components equal. D Negative- sequence component of current less than positive sequence component. Answer : C Explanation :
34.
 If the positive, negative – and zero-sequence reactances of an element of a power system are 0.3, 0.3 and 0.8 pu respectively, then the element would be a A Synchronous generator B Synchronous motor C Static load D Transmission line Answer : D Explanation :
35.
 If all the sequence voltages at the fault point in a power system are equal, then the fault is a A Three-phase fault B Line-to ground fault C Line-to-line fault D Double-line-to ground fault Answer : D Explanation :
36.
 The three sequence voltages at the point of fault in a power system are found to be equal. The nature of the fault is A L – G B L – L – L C L – L D L – L – G Answer : D Explanation :
37.
 The zero-sequence current of a generator for line-to ground fault is j2.4 pu. Then the current through the neutral during the fault is A J2.4 pu B J0.8 pu C J7.2 pu D J0.24 pu Answer : C Explanation :
38.
 One current transformer (CT) is mounted over a 3-phase, 3-core cable with its sheath and armour removed from the portion covered by the CT. an ammeter placed in the CT secondary would measure A The positive- sequence current B The negative- sequence current C The zero- sequence current D Three times the zero- sequence current Answer : D Explanation :
39.
 The positive sequence driving point reactance at the bus is A 2.5 Ω B 4.033 Ω C 5.5 Ω D 12.1 Ω Answer : D Explanation :
40.
 The zero sequence driving point reactance at the bus is A 2.2 Ω B 4.84 Ω C 18.18 Ω D 22.72 Ω Answer : B Explanation :
41.
 The main function of a fuse is to A Protect the line B Open the circuit C Protect the appliance D Prevent excessive currents Answer : D Explanation :
42.
 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 Answer : D Explanation :
43.
 The arcing contacts in a circuit breaker are made of A Copper tungsten alloy B Porcelain C Electrolytic copper D Aluminium alloy Answer : A Explanation :
44.
 Which of the following medium is employed for extinction of arc in air circuit breaker? A Water B Oil C Air D SF6 Answer : C Explanation :
45.
 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 All of the above Answer : C Explanation :
46.
 Fault diverters are basically A Fuses B Relays C Fast switches D Circuit breakers Answer : C Explanation :
47.
 A thermal protection switch can protect against A Short circuit B Temperature C Overload D Over voltage Answer : C Explanation :
48.
 Arc in a circuit breaker behaves as A A capacitive reactance B an inductive reactance C A resistance increasing with voltage rise across the are D A resistance decreasing with voltage rise across the are Answer : D Explanation :
49.
 Thermal circuit breaker has A Delayed trip action B Instantaneous trip action C Both of the above D None of the above Answer : A Explanation :