a GARCIA COLLEGE OF TECHNOLOGY
Kalibo, Aklan
Electrical Engineering Department
NOTES IN ENGINEERING COMPETENCE II (ELECTRICAL ENGINEERING SUBJECTS)
FAULTS
SYMMETRICAL 3-PHASE FAULT CALCULATIONS
The symmetrical fault occurs when all the three conductors of a 3-phase
lne are brought together simultaneously into a short circuit conditions.
This type of fault gives rise to symmetrical currents (i.e. equal currents
with 120° displacement). The symmetrical fault rarely occurs in practice
as majority of the faults are of unsymmetrical in nature. It is the most
severe fault and exposes more heavy duty on the circuit breaker.
STEPS IN SOLVING 3-PHASE FAULT CALCULATIONS
STEP 1: Draw a one-line diagram of the network indicating the rating
and and per unit impedance of each element of the network.
STEP 2: Choose a numerically convenient value of base power and
convert all per unit impedances to this base value.
𝒁𝒑𝒖 =
𝑺𝒃𝒂𝒔𝒆 𝒁𝒐𝒉𝒎𝒊𝒄
𝑬𝟐𝒃𝒂𝒔𝒆
𝟐
𝑺 𝒏𝒆𝒘−𝒃𝒂𝒔𝒆) 𝑬𝒐𝒍𝒅−𝒃𝒂𝒔𝒆)
𝒁𝒑𝒖(𝒏𝒆𝒘) = 𝒁𝒑𝒖(𝒐𝒍𝒅) (
)(
)
𝑺 𝒐𝒍𝒅−𝒃𝒂𝒔𝒆) 𝑬𝒏𝒆𝒘−𝒃𝒂𝒔𝒆)
STEP 3: In reference to the one-line diagram do the network, draw the
equivalent impedance diagram showing one phase of the system and
the neutral. Indicate the per unit impedance on the base power in the
impedance diagram. Transformers are represented by a series per unit
impedance in the idagram.
STEP 4: Find the total per unit impedance Zt of the network up to the
point of fault using circuit analysis.
Zt = Thevenin’s equivalent impedance (3-phase fault is calculate using
Thevenin’s theorem)
STEP 5: Find the base current Ibase corresponding to the selected base
power and the normal voltage at the fault point.
𝑰𝒃𝒂𝒔𝒆 =
𝑺𝒃𝒂𝒔𝒆
√𝟑𝑬𝒇𝒂𝒖𝒍𝒕
STEP 6: The various fault currents are then calculated
Problem 3. A 30-MVA alternator with 15% reactance is connected to a
bus bar. A second alternator rated 25-MVA with 10% reactance is also
connected through a 10% bus bar reactor to the same bus bar. Both of
these reactances are based on 25 MVA. If a feeder taken out from the
bus bars through a circuit breaker develops a 3-phase fault, what should
be the appropriate MVA rating of the circuit breaker?
(325 MVA)
Problem 4. The estimated short circuit MVA at the bus bars of a
generating station A is 1500 MVA and of another station B is 1200 MVA.
The generated voltage at each stations is 33 kV. If these stations are
interconnected through a line having a reactance of 1  and negligible
resistance, calculate the possible short circuit MVA at station B.
(1831 MVA)
Problem 5. At a certain point in an electrical network, the Thevenin’s
equivalent impedance is known to be 2%. Solve for the 3-phase fault
current if the base power is 10 MVA and the base voltage is 13.8 kV?
(20,918 A)
Problem 6. Four identical alternators rated 10 MVA each has a per unit
reactance of 0.20. These alternators are supplying power to a common
bus bar at 13.8 kV. Solve the 3-phase short circuit MVA at the bus bar.
(200 MVA)
Problem 7. At a certain point in an electrical system, the 3-phase fault
current is 10 pu. Determine the series reactance in pu to be inserted in
the fault point in order to limit the short circuit current to 8 pu.
(0.025 pu)
Problem 8. The section bus bars A and B are linked together by a bus bar
reactor rated 5 MVA with 10% reactance. On bus-bar A, there are two
generators connected each rated 10 MVA with 10% reactance and on B,
two generators are connected each rated 8 MVA with 12% reactance.
Find the short circuit MVA fed into a 3-phase fault on bus bar B.
(173.3 MVA)
SYMMETRICAL COMPONENTS
According to Fortescue’s Theorem, three unbalance phasors of a 3phase system can be resolved into three balanced systems of phasors
namely,
𝟏
𝑰𝒇(𝒑𝒖) =
𝒁𝒕
Positive sequence components consisting of three phasors of equal
magnitude, displaced from each other by 120 and having the same
phase sequence as the original phasors.
𝑰𝒇 = 𝑰𝒇(𝒑𝒖)  𝑰𝒃𝒂𝒔𝒆
𝑰𝒂𝟏 = 𝑰𝒂𝟏 ∠𝟎 = 𝑰𝒂𝟏
𝑺𝒇 =
𝑺𝒃𝒂𝒔𝒆
𝒁𝒕
If = fault current
Sf = fault power
Problem 1. Two alternators rated 15 MVA and 20 MVA have per unit
reactance of 0.30 and 0.50 respectively are connected to a common 12kV bus bar. Solve the fault current if a symmetrical 3-phase fault occurs
at the bus bar.
(4330 A)
Problem 2. A 5-MVA, 3-phase, 6.6 kV, alternator has an internal
reactance of 6% based on its rating. Find the reactance per phase of the
limiting reactor if the steady apparent power (MVA) on short circuit is
not to exceed 5 times the full load value.
(1.219  )
𝑰𝒃𝟏 = 𝑰𝒂𝟏 ∠𝟐𝟒𝟎° = 𝒂𝟐 𝑰𝒂𝟏
𝑰𝒄𝟏 = 𝑰𝒂𝟏 ∠𝟏𝟐𝟎° = 𝒂𝑰𝒂𝟏
𝒂 = 𝟏∠𝟏𝟐𝟎°
𝒂𝟐 = 𝟏∠𝟐𝟒𝟎°
𝟏 + 𝒂 + 𝒂𝟐 = 𝟎
Negative sequence components consisting of three phasors of equal
magnitude, displaced from each other by 120 and having phase
sequence opposite to that of the original phasors.
𝑰𝒂𝟎 = 𝑰𝒃𝟎 = 𝑰𝒄𝟎
𝑰𝒂 = 𝑰𝒂𝟎 + 𝑰𝒂𝟏 + 𝑰𝒂𝟐
Page 1 of 11
𝑰𝒃 = 𝑰𝒃𝟎 + 𝑰𝒃𝟏 + 𝑰𝒃𝟐 → 𝑰𝒂𝟎 + 𝒂𝟐 𝑰𝒂𝟏 + 𝒂𝑰𝒂𝟐
b.
c.
Negative sequence impedance, Z2
Zero sequence impedance, Z0
Zero sequence components consisting of three phasors of equal
magnitude, and with zero displacement from each other.
SEQUENCE IMPEDANCES OF SOME POWER SYSTEM ELEMENTS
INVOLVED IN SHORT CIRCUIT CALCULATIONS
𝑰𝒄 = 𝑰𝒄𝟎 + 𝑰𝒄𝟏 + 𝑰𝒄𝟐 → 𝑰𝒂𝟎 + 𝒂𝑰𝒂𝟏 + 𝒂𝟐 𝑰𝒂𝟐
Synchronous Generators. The positive, negative, and zero sequence
impedances of rotating machines are generally different. The positive
sequence of a synchronous generator is equal to the synchronous
impedance of the machine. The negative sequence is much less than the
positive sequence impedance. The zero sequence impedance is a
variable item and if its value is not given, it may be assumed to be equal
to the positive sequence impedance.
𝑰𝒂𝟎 =
𝟏
(𝑰 + 𝑰𝒃 + 𝑰𝒄 )
𝟑 𝒂
𝑰𝒂𝟏 =
𝟏
(𝑰 + 𝒂𝑰𝒃 + 𝒂𝟐 𝑰𝒄 )
𝟑 𝒂
𝑰𝒂𝟐 =
𝟏
(𝑰 + 𝒂𝟐 𝑰𝒃 + 𝒂𝑰𝒄 )
𝟑 𝒂
Problem 9. In symmetrical components, simplify the expression,
𝟏+𝒂
𝟏 + 𝒂𝟐
(-0.5 + j0.866)
Problem 10. The symmetrical components of a set of unbalanced 3phase currents are: 𝐼𝑎0 = 3∠ − 30°; 𝐼𝑎1 = 5∠90°; 𝐼𝑎2 = 4∠30°.
Determine the original unbalanced phasors.
𝑰𝒂 = 𝟖. 𝟏𝟖𝟓∠𝟒𝟐. 𝟐𝟏𝟔° 𝑨; 𝑰𝒃 = 𝟒∠ − 𝟑𝟎° 𝑨; 𝑰𝒄 = 𝟖. 𝟏𝟖𝟓∠ − 𝟏𝟎𝟐. 𝟐𝟏𝟔° 𝑨
Problem 11. Solve the symmetrical components for the following set of
unbalanced voltages: 𝑉𝑎 = 300∠ − 120°; 𝑉𝑏 = 200∠90°; 𝑉𝑐 = 100∠ − 30°.
Determine the original unbalanced phasors.
Transformers. Since transformers have the same impedance with
reversed phase rotation, their positive and negative sequence
impedance impedance are equal, this value being equal to the
impedance of the transformer. The zero sequence impedance depends
upon earth connection. If there is a circuit for earth current, zero
sequence impedance will be equal to positive sequence impedance,
otherwise it will be infinite.
Transmission lines. The positive sequence impedance and negative
sequence impedance of a line are the same, this value being equal to the
normal impedance of the line. This is expected because the phase
rotation of the currents does not make any difference in the constants
of the line. The zero sequence impedance is usually much greater than
the positive or negative sequence impedance. It may be taken as equal
to three times the positive sequence impedance, if not given.
𝑽𝒂𝟎 = 𝟒𝟐. 𝟐𝟔𝟓∠ − 𝟏𝟐𝟎° 𝑽; 𝑽𝒂𝟏 = 𝟏𝟗𝟑. 𝟏𝟖𝟓∠ − 𝟏𝟑𝟓° 𝑽; 𝑽𝒂𝟐 = 𝟖𝟔. 𝟗𝟒𝟕∠ − 𝟖𝟒. 𝟖𝟗𝟔° 𝑽
ANALYSIS OF UNSYMMETRICAL FAULTS
NEUTRAL CURRENT IN A 3-PHASE, 4-WIRE SYSTEM
In the analysis of unsymmetrical faults, the following conditions are
assumed:
a. The generated emf of the system is of positive sequence only.
b. No current flows in the network other than due to the fault,
i.e. load currents are neglected.
c. Phase a shall be taken as the reference phase.
Ia
a
Ib
SINGLE-LINE TO GROUND (SLG) FAULT
b
Any one of the three lines is accidentally connected to the ground.
Ic
c
𝑰𝒇(𝑺𝑳𝑮) =
In
n
𝑰𝒏 = 𝟑𝑰𝒂𝒐
𝑰𝒂𝒐 = 𝑰𝒃𝒐 = 𝑰𝒄𝒐
Problem 12. In a 3-phase system, phase a is on no-load and phase b and
c are short-circuited to ground. The currents are given:
𝐼𝑏 =
91.65∠160.9° 𝐴; 𝐼𝑛 = 60∠90° 𝐴. Find the symmetrical components of
currents Ia0, Ia1, and Ia2.
𝟑𝑬𝒂
𝒁𝟏 + 𝒁𝟐 + 𝒁𝟎 + 𝟑𝒁𝒏
Z1 = positive sequence per unit impedance
Z2 = negative sequence per unit impedance
Z0 = zero sequence per unit impedance
Zn = per unit impedance of reactor connected to ground
Ea = per unit positive sequence voltage of phase A (reference)
Problem 16. A 3-phase, 11-kV, 25 MVA generator with X0 = 0.05 pu, X1 =
0.2 pu, X2 = 0.2 pu is grounded through a reactance of 0.3 Ω. Solve the
fault current for a single line to ground fault.
(6189.45 A)
LINE TO LINE (LL) FAULT
𝑰𝒂𝟎 = 𝟐𝟎∠𝟗𝟎° 𝑨; 𝑰𝒂𝟏 = 𝟔𝟎∠ − 𝟗𝟎° 𝑨; 𝑰𝒂𝟐 = 𝟒𝟎∠𝟗𝟎° 𝑨
Problem 13. In symmetrical components, “a – a2” is equal to ____.
(j1.732)
Any two line terminals are accidentally connected or shorted together.
𝑰𝒇(𝑳𝑳) =
Problem 14. The phase currents in a Y-connected unbalanced load are Ia
= 44 – j33, Ib = -32 – j24, and Ic = -40 + j25. Determine the negative
sequence component current of phase a.
(12.52 – j13.476 A)
Problem 15. The sequence component of phase b current are: Ib0 = 0.77 +
j1.88, Ib1 = 14.788 – j9.169 and Ib2 = -4.717 + j0.148. What is the phase a
line current?
(3.803 + j23.282 A)
SEQUENCE IMPEDANCES
Each element of a power system will offer impedance to different phase
sequence components of current which may not be same. Thus, in
unsymmetrical fault calculations, each piece of equipment will have
three values of impedance, one in each sequence current namely,
a. Positive sequence impedance, Z1
√𝟑𝑬𝒂
𝒁𝟏 + 𝒁𝟐
Problem 17. The positive, negative, and zero sequence reactances of a 20
MVA, 13.2 kV synchronous generator are 0.3 pu, 0.2 pu, and 0.1 pu
respectively. The generator is not loaded when a line to line fault occurs.
Determine the fault current.
(3030 A)
DOUBLE LINE TO GROUND (DLG) FAULT
Any two line terminals are accidentally connected or shorted together to
the ground.
𝑰𝒇(𝑫𝑳𝑮) =
𝟑𝒁𝟐 𝑬𝒂
𝒁𝟏 𝒁𝟐 + 𝒁𝟐 (𝒁𝟎 + 𝟑𝒁𝒏 ) + (𝒁𝟎 + 𝟑𝒁𝒏 )𝒁𝟏
Page 2 of 11
Problem 18. A generator having a solidly grounded neutral and rated 50MVA, 30 kV has positive, negative, and zero sequence reactances of 25,
15, and 5% respectively. What reactance must be placed in the generator
neutral to limit the fault current for a bolted double line to ground fault
to that for a bolted 3-phase fault?
(0.825 Ω)
Problem 19. A 3-phase, 11 kV, 10 MVA alternator has sequence
reactances of X0 = 0.05 pu, X1 = 0.15 pu, X2 = 0.15 pu. If the generator is
on no-load, find the ratio of fault currents for SLG fault to that when all
3-phases are dead short circuited.
(1.286)
Problem 20. The positive, negative, and zero a 20 MVA, 13.2 kV
synchronous generator are 0.3 pu, 0.2 pu, and 0.1 pu respectively. The
generator is solidly grounded and is not loaded. Neglect all resistances.
Determine the fault current if a double line to ground fault occurs at the
generator terminals b and c.
(4771 A)
Problem 21. A 25 MVA, 11 kV, 3-phase alternator was subjected to the
following faults: 3-phase fault = 1000 A; line to line fault = 1300 A; line to
ground fault = 2100 A. The generator is solidly grounded. Ignoring
resistances, determine the zero sequence reactance of the alternator.
(0.611 Ω)
SEQUENCE CONNECTION DIAGRAMS FOR 3-PHASE TRANSFORMER
BANKS

Y-Y connection with primary neutral grounded

Y-Y connection with both neutrals grounded

Y-∆ connection with grounded neutral

Y-∆ connection with isolated neutral

∆-∆ connection
Problem 22. An ac generator is connected to the primary side of a deltawye grounded transformer bank. An SLG fault on the secondary side of
the bank is equivalent to what type of fault on the generator at the
primary side of the transformer?
(line to line fault)
PRACTICE PROBLEMS
1. A system operates at 220 kVA and 11 kV. Using these quantities as
base values, find the base impedance for the system.
a. 20 
c. 550 
b. 100 
d. 34.64 
2. A 60 Hz turbo generator is rated 100 MVA, 22 kV and solidly grounded
operating at rated voltage at no load. X1 = X2 = 0.20 p.u. and Xg0 = 0.05
pu. Determine the inductance reactance to be inserted in the neutral of
the generator to limit the subtransient line current for a single line to
ground fault to that for a 3-phase fault.
a. 0.333 
c. 0.242 
b. 0.525 
d. 0.375 
3. A generator having a solidly grounded neutral and rated 50-MVA, 30
kV has positive, negative, and zero sequence reactances of 25, 15, and 5
percent respectively. What reactance must be placed in the generator
neutral to limit the fault current for a line to ground fault to that for a 3phase fault?
a. 1.6 
c. 1.4 
b. 1.2 
d. 1.8 
4. Evaluate (1 + a) / (1 + a2).
a. 0.5 + j0.866
c. -0.5 + j0.866
b. -0.5 – j0.866
d. 0.5 – j0.866
5. Four identical alternators each rated for 20 MVA, 11 kV have
subtransient reactance of 16% are working in parallel. Determine the
short circuit level at the bus bar.
a. 500 MVA
c. 400 MVA
b. 125 MVA
d. 80 MVA
6. A 3-phase, 13.8 kV, 50 MVA alternator has positive, negative, and zero
sequence of 0.20, 0.20, and 0.05 pu. If the alternator is on no-load,
determine the fault current for a 3-phase fault.
a. 12,200 A
c. 15,200 A
b. 14,500 A
d. 10,460 A
7. The line to ground voltages on the high voltage side of a step-up
transformer are 100 kV, 33 kV, and 38 kV on phases a, b, and c
respectively. The voltage of phase a leads that of phase b by 100 and
lags that of phase c by 176.5. Determine the symmetrical negative
phase sequence component of phase a.
a. 50.65 + j14.32
c. 18.79 – j10.06
b. 45.32 + j25.12
d. 30.55 – j4.26
8. A single line to ground fault occurs on an unbalanced generator in
phase a. For this generator, X1 = X2 = 0.25 p.u. and X0 = 0.15 pu. The
reacatnce connected in neutral is Xn = 0.05 pu. If the initial prefault
voltage is 1 pu, what is the magnitude of the fault current in pu?
a. 1.25 pu
c. 1.54 pu
b. 1.43 pu
d. 3.75 pu
9. At a certain location in an electric system, the available fault MVA is
400 MVA. A 15 MVA, 34.5 kV/6.24 kV, 2.5% impedance transformer is
installed at that location. Determine the short circuit MVA at the
secondary side of the transformer.
a. 200 MVA
c. 240 MVA
b. 150 MVA
d. 280 MVA
10. A 10 MVA, 6.9 kV alternator has a positive, negative, and zero
sequence per unit reactance components of 0.30, 0.25, and 0.05
respectively. If the neutral of the generator is grounded through a
reactor of 0.48 , determine the fault current during a single line to
ground fault.
a. 2789 A
c. 2340 A
b. 2890 A
d. 2560 A
11. A 30 MVA generator with 15% reactance is connected to a bus bar. A
25 MVA transformer with 10% reactance is also connected through a
10% bus bar reactor to the same bus. Both of these reactance is based
on 25 MVA. If a feeder taken out from the bus bars through a circuit
breaker develops a 3-phase fault, what should be the appropriate rating
of the circuit breaker?
a. 923 MVA
c. 675 MVA
b. 325 MVA
d. 500 MVA
12. The sequence component of phase a currents are: Ia0 = 0 A; Ia1 = 10 +
j5 A; Ia2 = 8 - j5 A. Determine current in phase c.
a. 17.74 cis 174.4
c. 15.34 cis 164.5
b. 15.45 cis 134.3
d. 12.2 cis (-45.4)
13. The available fault current of a point in a certain electrical system is
9 pu. Determine the short circuit MVA using a base of 110 MVA.
a. 1222.2 MVA
c. 12.2 MVA
b. 9.9 MVA
d. 990 MVA
14. A generator is rated 10 MVA, 33 kV. It is Y-connected and solidly
grounded at its rated voltage at no load. Its reactances are X1 = 0.15 pu;
X2 = 0.15 pu; X0 = 0.05 pu. Determine the value of inductive reactance in
ohms to be inserted in the neutral to limit the subtransient current for a
single line to ground fault equal to that for a 3-phase fault.
a. 5.75 Ω
c. 3.63 Ω
b. 3.48 Ω
d. 3.12 Ω
15. Determine the zero sequence component of the following
unbalanced system voltage: Va = 100 cis 30; Vb = 150 cis (-30); Vc =
120 cis (-120).
a. 62.257 cis (-38.28)
c. 77.023 cis (-40.48)
b. 67.589 cis (-39.48)
d. 68.589 cis (-29.88)
16. Evaluate: (a – a2)(a2 – a).
a. 3∠0°
c. 3.464∠30°
Page 3 of 11
b. 3∠90°
d. 3.464∠60°
17. A generator is rated 500 MVA, 22 kV. The positive, negative, and
zero sequence components of the generator are 0.20, 0.20, and 0.05 pu,
respectively. What is the ratio of the fault current for a single line to
ground fault to the fault current for a 3-phase fault?
a. 1.50
c. 1.33
b. 1.42
d. 1.25
18. The available fault MVA of a point in a certain electrical system is
600 MVA. Determine the Thevenin’s equivalent reactance using a base
of 15 MVA.
a. 0.40 pu
c. 0.025 pu
b. 40 pu
d. 15 pu
19. In a short circuit study, the positive, negative, and zero sequence
reactances are 0.25 pu, 0.25 pu, and 0.30 pu respectively. The base MVA
is 100. Determine the fault current for a 3-phase fault at the 115 kV
level.
a. 2,204 A
c. 2,008 A
b. 2,410 A
d. 2,140 A
20. The estimated short circuit MVA at the bus bars of a generation
staton A is 1200 MVA and of another station B is 1000 MVA. The
generated voltage at each station is 33 kV. If these station are
interconnected through a line having a reactance of 1.2 Ω and negligible
resistance, calculate the new short circuit MVA at station A.
a. 1517 MVA
c. 1824 MVA
b. 1676 MVA
d. 2010 MVA
21. A generator havinga solidly grounded neutral and rated 50 MVA, 30
kV has a positive, negative, and zero sequence reactances of 20, 20, and
5% respectively. What reactance must be placed in the generator
neutral to limit the fault current for a bolted single line to ground fault
to that for a bolted three phase fault?
a. 0.90 Ω
c. 0.80 Ω
b. 0.50 Ω
d. 0.15 Ω
22. In a power system, the fault current at a point is 8.0 pu. What series
reactance must be included at the fault point to limit the short circuit
current to 5.0 pu?
a. 0.075 pu
c. 0.065 pu
b. 0.055 pu
d. 0.045 pu
23. A 3-phase, 20 MVA, 10 kV alternator has internal reactance of 5%
and negligible resistance. Find the external reactance per phase to be
connected in series with the alternator so that steady current on short
circuit does not exceed 8 times the full load current?
a. 0.125 Ω
c. 0.265 Ω
b. 0.375 Ω
d. 0.075 Ω
24. If symmetrical components of phase voltage Van are: positive seq. =
50; negative seq. = j20; zero seq. = -10 V. Determine the phase voltage
Vbn.
a. 40 + j20
c. -52.32 – j53.3
b. -17.68 + j33.3
d. -22.63 – j17.2
25. At a certain point of a power system network, the positvie, negative,
and zero sequence impedances are 0.25 pu, 0.25 pu, & 0.30 pu
respectively. The base MVA is 100. The voltage level at the point is 34.5
kV. Determine the current for a one line to ground fault.
a. 6275 A
c. 9536 A
b. 8302 A
d. 7845 A
26. At 34.5 kV substation, the available fault current is 10 pu. What is
the available fault MVA if the base MVA is 50?
a. 500 MVA
c. 50 MVA
b. 5 MVA
d. 5000 MVA
27. A 3-phase, 4-wire system supplies loads which are unequally
distributed in the thee phase lines. An analysis of the circuit shows that
the positive and the negative sequence components of the current in
line A are as follows: 𝐼𝑎1 = 48.02∠ − 87.6° 𝐴; 𝐼𝑎2 = 163.21∠ −
40.45° 𝐴; 𝐼𝑛 = 156.6∠112.7° 𝐴. Determine the line current at line A.
a. 150∠ − 45° A
c. 100∠45° A
b. 160∠60° A
d. 140∠50° A
28. A transformer 4.5 MVA, 13.8/6.24 kV with 5% impedance is tapped
to a 13.8 kV infinite bus with available fault MVA of 500. A generator of
2 MVA, 6.24 kV with 10% impedance is connected to the transformer
secondary bus bar. Determine the fault current delivered for a 3-phase
fault near the common bus bar.
a. 9,102 A
c. 8,907 A
b. 8,500 A
d. 10,105 A
29. A short 3-phase transmission line operating at 11 kV has an
impedance of 1.5 + j2 ohms per conductor and is connected through a
5000 kVA transformer having 5% reactance to 6.6 kV busbar to a
generating station. Two generators are connected to the bus bars, each
having a capacity of 10,000 kVA and 15% reactance. Calculate the short
circuit current if a symmetrical fault occurs at the far end of the
transmission line.
a. 1,490 A
c. 1,450 A
b. 1,520 A
d. 1,550 A
30. What is the Thevenin’s equivalent reactance of a certain electric
system rated 50 MVA at 34.5 kV if the available short circuit MVA is
1000 MVA?
a. 0.025 pu
c. 0.05 pu
b. 0.04 pu
d. 0.0625 pu
31. When a single line-to-ground fault occurs, the current in the faulted
phase is 100 A. The zero sequence current in this case is
a. 0 A
c. 66.66 A
b. 33.33 A
d. 100 A
32. The sequence components of phase a current are: zero sequence =
0.47 + j1.49; positive sequence = 18.4 cis (-31.6); negative sequence =
3.23 cis (168.2). Determine the phase c current.
a. 31.36 A
c. 18.97 A
b. 22.51 A
d. 25.25 A
33. A 10-MVA, 13.8 kV/440 V transformer has a subtransient reactance
of 0.05 pu. This transformer is connected to a 13.8 kV line where the
Thevenin’s equivalent reactance is 0.3 Ω. A symmetrical 3-phase fault
occurs at the secondary side of the transformer. Solve the fault current
in the primary side of the transformer.
a. 6,368 A
c. 6,920 A
b. 6,640 A
d. 7,120 A
34. In a short circuit analysis, the bases chosen are: 34.5 kV and 100
MVA. What is the impedance base?
a. 11.9 Ω
c. 23.8 Ω
b. 7.5 Ω
d. 5 Ω
35. A station operating at 33 kV is divided into sections A and B. Section
A consist of three generators 15 MVA each having a reactance of 15%
and section B is fed from the grid through a 75 MVA transformer of 8%
reactance. The circuit breaker have each a rupturing capacity of 750
MVA. Determine the reactance of the reactor to prevent the breakers
from being overloaded if a symmetrical short circuit occurs on an
outgoing feeder connected to A.
a. 2.42 Ω
c. 1.26 Ω
b. 1.31 Ω
d. 2.88 Ω
36. A 3-phase, 22 kV, 100 MVA alternator has positive, negative, and
zero sequence reactances of 0.3 pu, 0.3 pu, and 0.05 pu respectively.
What is the negative sequence component for a line to line fault?
a. 5,343 A
c. 5,763 A
b. 4,374 A
d. 6,632 A
37. Given the following line currents: Ia = 60 + j0 A; Ib = -36 – j48 A; Ic = 48 + j36 A. Solve for the negative sequence component of Ia.
a. 8.641 – j1.543 A
c. 9.436 + j1.346 A
b. 9.751 – j1.464 A
d. 8.354 + j1.034 A
38. At a certain point of a power system network, the positvie, negative,
and zero sequence impedances are 0.25 pu, 0.25 pu, & 0.30 pu
respectively. The base MVA is 100. The voltage level at the point is 34.5
kV. Determine the zero sequence current for a one line to ground fault.
a. 3,045 A
c. 2,480 A
b. 3,210 A
d. 2,091 A
39. A station has a short circuit impedance at a given point of 0.21 pu
based on 100 MVA. Determine the interrupting capacity of the circuit
breaker needed.
a. 21 MVA
c. 476 MVA
b. 2267 MVA
d. 121 MVA
Page 4 of 11
40. The computed values of positive, negative, and zero sequence
reactances of a network at fault are: 0.15, 0.15, and 0.10 pu. Determine
the fault current if the fault is a double line-to-ground.
a. 6.23 pu
c. 8.57 pu
b. 7.15 pu
d. 9.11 pu
41. A 3-phase, 15 MVA, 10 kV alternator has internal reactance of 6%
and negligible resistance. Find the external reactance per phase to be
connected in series with the alternator so that steady current of 3-phase
short circuit does not exceed 6 times the full load current.
a. 0.589 Ω
c. 10.67 Ω
b. 1.672 Ω
d. 0.711 Ω
42. A generator rated 600 kVA, 2400 V, 60 cycles, 3-phase, 6-poles and
wye-connected has 10% synchronous reactance. If a 3-phase fault
occurs, what will be the short circuit current?
a. 1,443 A
c. 1,450 A
b. 1,422 A
d. 1,476 A
43. A 3-phase, 375 kVA, 480 V, wye connected alternator has an equal
positive and negative sequence reactance of 10%. Find the
unsymmetrical fault current, if a line to line occurs at the alternator
terminals.
a. 3906 A
c. 2250 A
b. 4210 A
d. 3820 A
44. Evaluate : (1 – a)2 / (1 + a)2.
a.8∠ − 180°
b. 1.5∠ − 30°
c. 1.5∠30°
d. 3∠180°
45. A 60 Hz turbo generator is rated 500 MVA, 22 kV. It is Y-connected
and solidly grounded and is operating at rated voltage at no load. It is
disconnected from the rest of the system. Its reactances are X1 = X2 =
0.15 and X0 = 0.05 per unit. Find the ratio of the subtransient current for
a single line to ground fault to the subtransient current for a
symmetrical 3-phase fault.
a. 1.286
c. 1.333
b. 1.125
d. 1.428
46. A 3-phase 34.5 kV, 100 MVA alternator has positive, negative, and
zero sequence impedances of 0.25 pu, 0.25 pu, and 0.30 pu respectively,
Determine the fault current for a line to line fault?
a. 5797 A
c. 5804 A
b. 5234 A
d. 5932 A
47. Three-phase unbalanced currents are Ia = 10 cis (-30) A; Ib = 0 A; Ic
= 10 cis 150 A. What is the phase b positive sequence component?
a. 5.77 cis 180
d. 5.77 cis 0
b. 5.77 cis 240
c. 5.77 cis 120
48. A 60-Hz turbo generator is rated 500 MVA, 22 kV. It is Y-connected
and solidly grounded and is operating at rated voltage at no load. It is
disconnected from the rest of the system. Its reactances are X1 = X2 =
15%, X0 = 5%. Find the ratio of the subtransient line current for a line to
line fault to the subtransient current for a symmetrical 3-phase fault on
the generator.
a. 0.636
c. 0.86
b. 0.577
d. 0.725
49. A 30 MVA, 13.8 kV, 3-phase ac generator is grounded through a
reactance of 0.162 ohm has a positive, negative, and zero sequence
reactance of 0.2, 0.2, and 0.05 per unit respectively. Determine the
short circuit current for a single line to ground fault.
a. 6152 A
c. 7390 A
b. 8736 A
d. 7150 A
50. The bus bars of each of the two alternators of 15% reactance each
are interconnected through the tie-bar reactors of 15% each. The
equivalent impedance to fault current for a 3-phase fault in any
alternator bus bar will be ___.
a. 8.75%
c. 11.25%
b. 10%
d. 15%
51. A 15 MVA, 6.9 kV, Y-connected generator has X1 = X2 = 0.15 pu and
X0 = 0.05 pu. The neutral of the generator is grounded through a reactor
is operating at rated voltage without load. Determine the fault current in
the line for a. The fault current in the line for a single line-to-ground
fault is __ A.
a. 7825
b. 8011
c. 8345
d. 8513
52. What is the Thevenin’s equivalent reactance of a certain electric
system rated 20 MVA at 34.5 kV if the available short circuit MVA is 950
MVA?
a. 1.24%
c. 2.62%
b. 2.10%
d. 1.32%
53. A 3-phase transmission line operating at 33 kV and having a
resistance and reactance of 6  and 24  respectively is connected to
the generating station bus bar through a 6 MVA step up transformer
which has a reactance of 6%. Connected to the bus-bar are two
synchronous generators, one 12 MVA having a 10% reactance and
another 6 MVA having 7.5% reactance. Calculate the MVA at short
circuit fault between phases occuring at the high voltage terminals at
load end of the line.
a. 25.2
c. 26.7
b. 23.8
d. 24.6
54. The positive, negative, and zero sequence reactances of a 20 MVA,
13.2 kV synchronous generator are 0.3 pu, 0.2 pu, and 0.1 pu
respectively. The generator is solidly grounded and is not loaded.
Neglect all resistances. Determine the fault current if a double line to
ground fault occurs at the generator terminals b and c.
a. 5620 A
c. 6250 A
b. 5180 A
d. 4770 A
55. In a power system with negligible resistance, the 3-phase fault
current at a point is 8 pu. The series reactance to be inserted at the fault
point to limit the short circuit current to 5 pu is ___.
a. 0.125 pu
c. 0.075 pu
b. 0.333 pu
d. 0.20 pu
56. A 20 MVA, 13.8 kV generator has sub-transient reactance of 0.25 pu.
The negative and zero sequence reactances are respectively 0.35 and
0.10 pu. The neutral of the generator is solidly grounded. Determine the
fault current when a double line to ground fault occurs at the terminals
of the generator operated without load.
a. 5956 A
c. 4872 A
b. 6253 A
d. 4017 A
57. In a certain electrical system, the Thevenin’s equivalent impedance is
0.02 per unit. What is the fault current if the base power is 50 MVA and
the base voltage is 34.5 kV?
a. 44,423 A
c. 40,211 A
b. 45,422 A
d. 41,837 A
58. When a generator has terminal a open and the other two terminals
are connected to each other with a short circuit from the connection to
ground, typical values for the symmetrical components of current in
phase a are positive seq. = -j600 A; negative seq. = j250 A, and zero seq.
= j350 A. Find the current into the ground.
a. j700 A
c. j1050 A
b. j920 A
d. j1180 A
59. The sequence component of phase b current are: Ib0 = 0.77 + j1.88;
Ib1 = 17.4 cis (-31.8); Ib2 = 4.72 cis (178.2). What is the phase a line
current?
a. 12.96 cis (-33.4) A
c. 23.6 cis (80.7) A
b. 22.45 cis (78.2) A
d. 10.21 cis (59.3) A
60. A 15 MVA, 34.5 kV/6.24 kV transformer is connected at an infinite
bus. The percent impedance of the transformer is 2.5%. What is the
current at the 34.5 kV side for a 3-phase short at the 6.24 kV side?
a. 10,040 A
c. 12,450 A
b. 10,120 A
d. 11,240 A
61. A 3-phase, 20 MVA, 13.8 kV, 60 Hz altenator has a positive, negative
and zero sequence reactance of 0.30, 0.30, and 0.15 respectively. The
generator is unloaded when a double line to ground fault occurs at lines
b and c. Determine the ground current.
a. 5120 A
c. 4184 A
b. 4821 A
d. 5518 A
62. A 15 MVA, 6.6 kV, wye-connected generator has X1 = X2 = 0.2 and X0
= 0.1. The neutral of the generator is grounded through a reactor of 5%
reactance based on the generator rating. A double line-to-ground fault
occurs at terminals b and c. What is the current in the ground?
Page 5 of 11
a. 6268 A
b. 6834 A
c. 5624 A
d. 5435 A
63. The positive, negative, and zero sequence reactances of a 20 MVA,
13.2 kV synchronous generator are 0.3 pu, 0.2 pu, and 0.1 pu
respectively. The generator is solidly grounded and is not loaded. A line
to ground fault occurs on phase a. Neglect all resistances, determine the
fault current.
a. 8747 A
c. 4374 A
b. 9120 A
d. 6214 A
64. Circuit breakers are essentially ___.
a. current carrrying contacts called electrodes
b. arc extinguisher
c. circuit to break the system
d. transformers to isolate the two systems
65. Magnetic circuit breaker has ___ trip action.
a. instantaneous
c. both a and b
b. delayed
d. none of these
66. Arc in a circuit behaves as ___.
a. a capacitive 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
67. In a circuit breaker, the basic problem is to ___.
a. maintain the arc
c. extinguish the arc
b. transmit large power
d. emit the ionizing electrons
68. The voltage appearing across the contacts after opening of the
circuit breaker is called __ voltage.
a. recovery
c. surge
b. operating
d. arc
69. The delay fuses are used for protection of ___.
a. motors
c. power outlet circuits
b. fluoresecent lamps
d. light circuits
70. Protective relays are devices that detect abnormal conditions in
electrical circuits by measuring ___.
a. current during abnormal conditions
b. voltage during abnormal conditions
c. constantly the electrical quantities which differ during normal and
abnormal conditions
d. none of these
71. Which of the following is the least expensive protection for
overcurrent in low voltage system?
a. re-wireable fuse
c. isolator
b. oil circuit breaker
d. air break circuit breaker
72. Which of the following is used in liquid fuses?
a. transformer oil
c. sulphur hexafluoride
b. distilled water
d. carbon tetrachloride
73. A fuse in a motor circuit provides protection against ___.
a. overload only
b. short circuit only
c. both short circuit and overload
d. open circuit, short circuit and overload
74. A fuse wire is never inserted in ___.
a. neutral wire
c. negative dc line
b. phase line
d. positive dc line
75. The single phasing relay are used for protection of ___.
a. single phase motors only
b. two phase motors only
c. two single phase motors running in parallel
d. three phase motors
76. The main factor in favor of the use of aluminum as bus bar material
is ___.
a. its low melting point
c. its high resistivity
b. its low cost
d. its low density
77. Which of the following circuit breakers has high reliability and
minimum maintenance?
a. air blast circuit breakers
c. circuit breakers with SF6
b. vacuum circuit breakers
d. oil circuit breakers
78. 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
79. Directional relays are based on flow of ___.
a. power
c. current
b. voltages
d. all of these
80. Burden of a protective relay is the power ___.
a. absorbed by the circuit of relay
b. required to operate the circuit
c. developed by the relay circuit
d. monitored by the relay
81. An efficient and well designed protective relaying should have ___.
a. good selectivity and reliability
c. economy and simplicity
b. high speed and selectivity
d. all of these
82. DC shunt relays are made of ___.
a. many turns of thin wire
b. few turns of thin wire
c. many turns of thick wire
d. few turns of thick wire
83. The line currents of a 3-phase Y-connected load are as follows: Ia =
10<-30; Ib = 12<215; and Ic = 15<82. Determine the positive
sequence component of phase b.
a. 15.456<-184.944 A
c. 18.704<251.209 A
b. 12.274<-151.621 A
d. 34.887<78.321 A
84. The sequence components of the current in phase a are as follows:
Ia0 = 1.56<72.5 A; Ia1 = 18.4<31.6 A; and Ia2 = 3.23<168.2 A.
Determine the phase c current.
a. 15.46<-14.44 A
c. 18.54<137 A
b. 22.274<51.62 A
d. 24.81<-98 A
85. A balanced delta-connected load is connected to a balanced 3-phase
supply and the line currents are equal to 20 A in magnitude. If the fuse
in line c melts, determine the symmetrical components of the current
Ia1.
a. 12.466<-98 A
c. 11.547<-30 A
b. 12.274<91 A
d. 14.841<-45 A
86. The per unit impedance value of an alternator corresponding to base
values 13.2 kV and 30 MVA is 0.2 per unit. The per unit value for the
base values 13.8 kV and 50 MVA is
a. 0.331 pu
c. 0.305 pu
b. 0.231 pu
d. 0.245 pu
87. A 500 MVA, 22 kV, 3-phase generator has a per unit reactance of
0.12 based on its rating. What is the per unit reactance at 750 MVA, 25
kV base?
a. 0.139
c. 0.239
b. 0.158
d. 0.234
88. Thermal circuit breaker has __.
a. delayed trip action
b. instantaneous trip action
c. both (a) and (b)
d. none of these
89. A thermal protection switch can protect against __.
a. overload
c. temperature
b. short circuit
d. over voltage
90. Fault diverters are basically ___.
a. fast switches
b. fuses
c. relays
d. circuit breakers
91. A differential relay measures the vector difference between ___.
a. two currents
c. current
b. two voltages
d. none of these
92. Air blast circuit breaker is used for ___.
a. short duty
c. repeated duty
b. intermittent duty
d. instantaneous duty
Page 6 of 11
93. Thermal overload relays are used to protect the motor against
overcurrent due to ___.
a. heavy loads
c. short circuit
b. grounds
d. all of these
a. voltage at the faulty point
b. total impedance up to the fault
c. both A and B
d. none of these
94. Overload relays are of ___ type.
a. induction
b. thermal
105. Merz-price protection is used on ___.
a. substations
c. capacitor bank
b. induction motor
d. generators
c. solid state
d. all of these
95. Relays can be designed to respond to changes in
a. resistance, reactance, or impedance
b. voltage and current
c. temperature
d. all of these
96. The main function of a fuse is to ___.
a. prevent excessive currents
c. open the circuit
b. protect the line
d. none of these
106. Which of the following relays is used on transformers?
a. Buchholz relay
c. MHO relay
b. Merz-price relay
d. none of these
107. Which relay is used for feeders?
a. MHO relay
b. translay relay
c. Merz-price protection
d. Buchholz relay
97. With which of the following a circuit breaker must be equipped for
remote operation?
a. inverse time trip
c. shunt trip
b. time delay trip
d. all of these
108. The breaking capacity of a circuit breaker in MVA (3-phase) is given
by ___.
a. rated service voltage x rated symmetrical current
b. 1.1 x rated service voltage x rated symmetrical current
c. sq rt of 2 x rated service voltage x rated symmetrical current
d. sq rt of 3 x rated service voltage x rated symmetrical current
98. Which of the following protective devices can be used against
lightning surges?
a. horn gap
c. sure arresters
b. lightning arresters
d. all of these
109. Ionization process during arc is generally accompanied by emission
of ___.
a. light
c. heat
b. sound
d. all of these
94. Over voltage transients may occur due to ___.
a. lightning
c. switching
b. arcing grounds
d. all of these
110. Which statement is correct?
a. SF6 gas is non-toxic
b. SF6 gas has pungent smell
95. Which of the following is the voltage wave of magnitude higher than
desirable value?
a. over voltage
c. porcelain
b. surges
d. copper tungsten alloy
111. Fault diverters ___.
a. divert the current to earth in the event of short circuits
b. neutralize the surges by resistors
c. modify the surge wave shapes
d. none of these
96. The acting contacts for a circuit breaker is made up of ___.
a. stainless steel
c. porcelain
b. hard pressed carbon
d. copper tungsten alloy
97. In which starting method of starting a motor, the starting current is
maximum?
a. auto-transformer
c. stator rotor starter
b. star-delta starter
d. direct on line
98. For the contact and their material, which of the following should
have low value ___.
a. contact resistance
c. thermal capacity
b. thermal conductivity
d. all of these
99. Overcurrent protection for motor is provided by ___.
a. cartridge fuse
c. over current relay
b. high resistance fuses
d. all of these
100. The isolator is interlocked with circuit breaker and an earthing
switch. While opening the circuit, the ___ opens first, then ___ and
finally the ___ can close.
a. isolator, circuit breaker, earthing switch
b. earthing switch, isolator, circuit breaker
c. circuit breaker, earthing switch, isolator
d. circuit breaker, isolator, earthing switch
101. Insulation resistance of HV circuit breaker is more than ___.
a. 100 Ω
c. 1 MΩ
b. 500 kΩ
d. 100 MΩ
102. The material used in bus bars should have ___.
a. low resistivity
c. higher softening temperature
b. low cost
d. all of these
103. The advantage of neutral earthing is ___.
a. simplified design of earth fault protection
b. over-voltage due to lightning can be discharged to the earth
c. freedom from persistent arcing grounds
d. all of these
c. SF6 gas is lighter than air
d. SF6 gas is yellow in color
112. In star-connected system without neutral grounding, zero sequence
currents are __.
a. same as peak value of phase current
b. same as rms value of phase current
c. vector sum of phase currents
d. zero
113. Solid grounding is used for voltages __.
a. above 220 kV
c. below 660 V
b. above 11 kV
d. below 115 V
114. Switching over voltages are more hazardous than lightning surges
in case of ___.
a. low voltage systems
c. 11 kV systems
b. unbalanced systems
d. EHV and UHV systems
115. Resistance grounding is used for voltages __.
a. below 220 V
c. up to 660 V
b. between 3.3 kV to 11 kV
d. above 66 kV
116. The disadvantage offered by ungrounded systems is ___.
a. frequent arcing grounds
c. voltage oscillations
b. difficult earth fault relaying
d. all of these
117. Differential protection principle is used in the protection of ___.
a. generators
c. transformers
b. feeders
d. all of these
118. Which of the following is not a relay using electromagnetic force?
a. Buchholz relay
c. balanced beam relay
b. induction cup relay
d. attracted armature type relay
119. A bimetal strip consists of two metal strips have different ___.
a. thermal diffusivity
c. specific heat
b. thermal conductivity
d. coefficient of expansion
120. Thermal relays are often used in ___.
a. generator protection
c. motor starters
b. transformer protection
d. house wiring
104. The values of fault current depend on ___.
Page 7 of 11
121. Buchholz relay is operated by ___.
a. eddy currents
c. electro-magnetic induction
b. gas pressure
d. electro-static induction
122. Electro-magnetic relays maybe operated by
a. electro-magnetic attraction
c. electro-magnetic induction
b. thermal effect
d. any of these
b. power limiting device
d. pf limiting device
138. In a circuit breaker, the time duration from the instant of fault to
the instant of energizing of the trip coil is known as __.
a. lag time
c. lead time
b. protection time
d. operation time
123. The actuating quantity for the relays can be __.
a. magnitude
c. frequency
b. phase angle
d. any of these
139. Normally the fuse elements are in parts which are connected in the
middle by tin bridge. The melting point of tin bridge is __.
a. 35 C
c. 88 C
b. 230 C
d. 540 C
124. Bulk-oil circuit breaker is suitable for voltages up to __.
a. 10 kV
c. 16 kV
b. 26 kV
d. 36 kV
140. Most of the fuses operate due to __.
a. heating effect of current
c. magnetic effect of current
b. electrostatic effect of current
d. all of these
125. Mininum arcing voltages will be least in case of __.
a. carbon
c. graphite
b. tungsten
d. silver
141. Which of the following is the demerit of SF6 circuit breakers?
a. sealing problem of gas
b. influx of moisturein the gas system is dangerous
c. deterioration of quality of circuit breaker affects reliability of circuit
breaker
d. all of these
126. Minimum arcing voltage for platinum is 16 V. It can be therefore
concluded that when the voltage is below 16 V,
a. it will not be possible to interrupt the circuit
b. it will not be possible to pass the current
c. it will be possible to interrupt any value of current without arcing
d. it will be possible to interrupt any value of current without bringing
contact closer to the another
127. Oil immersion of contacts is the method of arc __.
a. dispersion
c. de-ionization
b. prevention
d. shielding
128. For magnetic blowout of arc, the magnetic field is produced __.
a. in the load circuit
b. parallel to the axis of the arc
c. at right angles to the axis of the arc
d. none of these
129. In a circuit breaker the time duration from the instant of fault to
the instant of closing of contact is known as __.
a. recycle time
c. total time
b. gross time
d. re-closing time
130. Which of the following is an air-break switching device?
a. isolator
c. limit switch
b. earthing switch
d. all of these
131. Which of the following statement about SF6 gas is incorrect?
a. it is non-toxic gas
b. it has density 5 times that of air at 20 C
c. it is non-inflammable
d. it has dark yellow color
132. The over voltage surges in power systems maybe caused by __.
a. lightning
c. switching
b. resonance
d. any of these
133. In circuit breakers, the contact space is ionized by ___.
a. thermal ionization of gas
b. thermal emission from surface of contacts
c. field emission from the surface of contacts
d. any of these
134. Which of the is a conducting medium for electric current?
a. low temperature gas
c. high temperarture
b. dissociated gas
d. plasma
135. The protection against overvoltage due to lightning is provided by
a. use of surge diverters
c. low tower footing resistance
b. use of overhead ground wires
d. any of these
143. Cool gases are solids brought into the arc stream assist in
quenching the arc mainly by __.
a. reducing current density
c. de-ionization
b. providing arc shield
d. providing parallel paths
144. Sparking occurs when load is switched off because the circuit has
a. high resistance
c. high capacitance
b. high inductance
d. all of these
145. HRC fuses are __.
a. high resistance and capacitance fuses
b. heat reflecting cooled fuses
c. holding and resisting current fuses
d. high rupturing capacity fuses
146. SF6 gas is transported in __.
a. gas cylinders
b. solid form in boxes
c air cylinders
d. liquid form in cylinders
147. The heat produced at the contact point, due to passage of current,
will least depend on __.
a. contact resistance
b. time during which the current flows
c. temperature of the surrounding medium
d. current flowing
148. Which of the following circuit breaker will produce the least arc
energy?
a. minimum circuit breaker
c. air blast circuit breaker
b. plain oil circuit breaker
d. all will produce same energy
149.A circuit breaker will normally operate ___.
a. when the switch is put on
b. when the line is to be checked
c. when the power is to be supplied
d. whenever fault in the line occurs
150. Which of the following is not a part of the circuit breaker?
a. explosion pot
c. conservator
b. operating mechanism
d. fixed and moving contacts
151. If the pf is zero, the active recovery voltage will be __.
a. minimum
c. 0.5
b. 0.707
d. maximum
136. During arc extinction SF6 gas __.
a. decomposes into S and F ions
b. decomposes into SF4 and SF2
c. gets oxidized
d. reduces to SF3
137. A fuse is normally a ___.
a. current limiting device
142. Dielectric strength of SF6 is __.
a. less than that of air at atmospheric pressure
b. less than that of oil used in OCB
c. more than that of oil used in OCB
d. more at lower pressure and low at higher pressures
152. For a high speed circuit breaker, the clearing time is nearly __.
a. 1 to 2 cycles
c. 5 to 10 cycles
b. 10 to 15 cycles
d. less than 50 cycles
c. voltage limiting device
153. For high speed circuit breaker, the total time is nearly ___.
Page 8 of 11
a. half cycle
b. few cycles
c. one cycle
d. ten cycles
154. In a circuit breaker, the time duration from the instant of the fault
to the extinction of the arc is known as __.
a. operation time
c. total clearing time
b. lag time
d. lead time
155. Air used in air blast circuit breaker __.
a. must have least carbon dioxide
c. must have oil mist
b. must be ionized
d. must be free from moisture
156. Flame proof switch gears are usually preferred ___.
a. on transmission lines of low voltage c. in mines
b. substations
d. in high MVA capacity circuits
157. For a circuit breaker, “break time” is __.
a. same as opening time
b. opening time + arc duration
c. opening time + arc duration + resistor current duration
d. opening time + closing time
158. Air blast circuit breakers are usually used for __.
a. instantaneous duty
c. intermittent duty
b. permanent duty
d. repeated duty
159. The pf of the arc in circuit breaker is __.
a. always zero
c. always unity
b. always lagging
d. always leading
160. In a circuit breaker, the arc is indicated by the process of __.
I. thermal emission
III. High temperature of air
II. ionization
IV. Field emission
a. I and II only
c. I, II, and III only
b. II, III, and IV only
d. I and IV only
170. Breaking capacity of a circuit breaker is usually expressed in terms
of __.
a. amperes
c. volts
b. MW
d. MVA
171. Air blast circuit breakers for 400 kV power system are designed to
operate in __.
a. 50 s
c. 50 ms
b. 500 ms
d. 50 s
172. “Kick fuse” has __.
a. square law characteristics
b. inverse characteristics
c. linear characteristics
d. logarithmic characteristics
173. In a circuit breaker, the current which exists at the instant of
contact separation is known as __.
a. re-striking current
c. breaking current
b. recovery current
d. surge current
174. Merz-price protection is suitable for __.
a. transformers
c. feeders
b. alternators
d. transmission lines
175. A material best suited for manufacturing of fuse wire is __.
a. aluminum
c. silver
b. lead
d. copper
176. When D is the diameter of the fuse wire, the fusing current will be
proportional to __.
a. 1/D
c. 1/D^2
b. D^3/2
d. D^1/2
177. In a circuit breaker, the active recovery voltage depends upon __.
a. power factor
c. armature reaction
b. circuit conditions
d. all of these
161. An automatic device that operates at present values is known as
a. mercury switch
c. relay
b. fuse
d. contactor
178. Which of the following contact point has the highest melting point?
a. silver
c. tungsten
b. gold
d. copper
162. The basic function of a circuit breaker is to __.
a. produce the arc
c. ionize the surrounding air
b. transmit voltage by arcing
d. extinguish the arc
179. Out of the following circuit breakers, which one has the lowest
voltage range?
a. air circuit breaker
c. tank type oil circuit breaker
b. air blast circuit breaker
d. SF6
163. Which of the following metals does not amalgate with mercury?
a. tungsten
c. molybdenum
b. nickel alloy
d. all of these
164. As the force on contact is increased, the contact resistance will
a. increase linearly
c. increase exponentially
b. remain unaltered
d. decrease
165. The arc votlage in ac circuit breaker is __.
a. leading the arc current by 90
b. lagging the arc current by 90
c. in-phase with the arc current
d. in phase opposition to the arc current
166. The contact resistance is least affected by __.
a. mechanical force applied
c. shape of contact faces
b. amount of surface contamination d. ambient temperature
167. Sulphur hexafluoride is a ___.
a. conductor of electricity
b. inactive gas
c. semi-conductor
d. dielectric
168. In a circuit breaker, if the insulation resistance between phase
terminal and earthed frame is less than the specified limit, the probable
cause could be __.
a. moisture
b. dirty insulation surface
c. carbon or copper particles sticking to the internal surface
d. any of these
169. The insulation resistance of high voltage circuit breaker is __.
a. 1 kΩ
c. 10 kΩ
b. 20 MΩ
d. 2000 MΩ
180. Sparking between contacts can be reduced by __.
a. inserting a resistance in the line
b. inserting a capacitor in series with the contacts
c. inserting a capacitor in parallel with the contacts
d. inserting a resistor in parallel with the contacts
181. Which of the following is not a method of arc dispersion?
a. oil immersion of contacts
c. magnetic blow out of arc
b. use of rectifier
d. de-ionization of arc path
182. The transient voltage that appears across the contacts at the
instant of arc extinction is called __.
a. recovery voltage
c. re-striking voltage
b. supply voltage
d. peak voltage
183. The normal frequency voltage that appears across the breaker
poles after the final arc extinction has occurred is __.
a. recovery voltage
c. re-striking voltage
b. supply voltage
d. peak voltage
184. If a circuit breaker does not operate, the probable reason could be
a. spring defective
c. trip latch defective
b. trip ciruit open
d. any of these
185. There is definite objection to use of which of the following medium
for extinguishing the arc of a circuit breaker?
a. air
c. SF6
b. vacuum
d. water
186. The contact resistance of a circuit breaker is of the order of __.
a. 20  10 Ω
c. 20  10 mΩ
b. 20  10 Ω
d. 200  10 Ω
Page 9 of 11
187. Which of the following circuit breaker does not use pneumatic
operating mechanism?
a. air blast circuit breaker
c. SF6 circuit breaker
b. air break circuit breaker
d. bulk-oil circuit breaker
188. Fusing factor for a HRC fuse is __.
a. minimum fusing current/current rating
b. minimum fusing current/minimum rupturing time
c. maximum fusing current/minimum fusing current
d. minimum fusing current/prospective current of circuit
189. In a HRC fuse, the time between cut-off and final current zero is
known as __.
a. total operating time
c. arcing time
b. pre-arcing time
d. any of these
190. In a vacuum circuit breaker, the vacuum pressure is of the order
a. 10 mm Hg
c. 10-6 mm Hg
b. 10-2 mm Hg
d. 10-9 mm Hg
b. sulphur hexafluorine
d. sulphur hexafluoride
204. While selecting a gas for circuit breaker, the property of gas that
should be considered is __.
a. high dielectric strength
c. non-inflammability
b. non-toxicity
d. all of these
205. The pressure of SF6 gas in circuit breakers is of the order of __.
a. 100 mmHg
c. 1 kg/cm2
2
b. 3 to 5 kg/cm
d. 30 to 50 kg/cm2
206. SF6 gas ___.
a. is yellow in color
b. has pungent odor
c. is highly toxic
d. is non-flammable
207. SF6 gas ___.
a. is lighter than hydrogen
b. is lighter than air
c. has density 2 times as compared to that of air
d. has density 5 times as compared to that of air
191. Which of the following circuit breaker can be installed on 400 kV
line?
a. tank type oil circuit breaker
c. miniature circuit breaker
b. vacuum circuit breaker
d. air blast circuit breaker
208. In air blast circuit breakers, the pressure of air is of the order of
a. 100 mmHg
c. 1 kg/cm2
b. 20 to 30 kg/cm2
d. 200 to 300 kg/cm2
192. Best protection is provided by HRC fuses in case of __.
a. open circuits
c. short circuits
b. overloads
d. all of these
209. Interrupting medium in a contactor may be __.
a. air
c. oil
b. SF6 gas
d. any of these
193. For motor circuit breakers, the time of closing the cycle is __.
a. 0.001 sec
c. 0.01 sec
b. 0.10 sec
d. 0.003 sec
210. Which of the following is not a type of contactor for circuit
breakers?
a. electromagnetic
c. pneumatic
b. vacuum
d. electro-pneumatic
194. The number of cycles in which a high speed circuit breaker can
complete its operation is __.
a. 3 to 8
c. 10 to 18
b. 20 to 30
d. 40 to 50
195. For extra high voltage lines, which circuit breaker is preferred?
a. bulk oil circuit breaker
c. vacuum circuit breaker
b. SF6 gas circuit breaker
d. minimum oil circuit breaker
196. Fuse wire protection system is usually not used beyond __.
a. 10 A
c. 25 A
b. 50 A
d. 100 A
197. A fuse wire should have __.
a. low specific resistance and high melting point
b. low specific resistance and low melting point
c. high specific resistance and high melting point
d. high specific resistance and low melting point
198. Which circuit breaker is generally used in railway traction?
a. SF6 gas circuit breaker
c. air circuit breaker
b. vacuum circuit breaker
d. minimum oil circuit breaker
199. Fuse protection is used for current ratings up to __.
a. 10 A
c. 20 A
b. 50 A
d. 100 A
200. A relay used for protection of motors against overload is __
a. impedance relay
b. thermal relay
c. electromagnetic attraction type
d. Buchholz’s relay
201. A circuit breaker is __.
a. a power factor correcting device
b. a device to neutralize the effects of transients
c. a waveform correcting device
d. a current interrupting device
202. The fuse current I in amperes is related with fuse wire diameter D
as __.
a. I inversely proportional to D
c. I directly proportional to D
b. I directly proportional to D3/2
d. I directly proportional to D2
203. SF6 gas is __.
a. sulphur fluorine
c. sulphur difluoride
211. For high voltage ac circuit breakers, the rate of short circuit current
is passed for __.
a. 0.001 s
c. 0.1 s
b. 3 sec
d. 30 s
212. Which of the following circuit breakers is preferred for EHT
applications?
a. air blast circuit breakers
c. bulk oil circuit breakers
b. minimum oil circuit breakers
d. SF6 oil circuit breakers
213. The fault clearing time of a circuit breaker is usually __.
a. few minutes
c. few seconds
b. one second
d. few cycles of supply voltage
214. When an HV ac circuit breaker is tested fo endurance, it is tested
for at least __ “opening-closing” operations.
a. 1000
c. 100
b. 10
d. 50
215. A relay in which the measurement is performed by a stationary
circuit and which has no moving parts is known as __.
a. static relay
c. differential relay
b. instantaneous relay
d. timed relay
216. Low voltage circuit breakers have rated voltage of less than __.
a. 220 V
c. 400 V
b. 1 kV
d. 10 kV
217. The function of protective relay in a circuit breaker is __.
a. to limit arcing current during the operation of circuit breaker
b. provide additional safety in the operation of circuit breaker
c. to close the contacts when actuating quantity reaches a certain predetermined value
d. to protect against stray voltages
218. In a circuit breaker, the current which exists at the instant of
contact separation is known as __.
a. restriking current
c. surge current
b. breaking current
d. recovery current
219. MHO relay is used for __.
a. rectifier
b. feeders
c. circuit breaker
d. transmission lines
220. The medium employed for extinction of arc in air circuit breaker is
Page 10 of 11
a. SF6
b. oil
c. air
d. water
221. Ionization in a circuit breaker is not facilitated by __.
a. high temperature of surrounding medium
b. material of contacts
c. increase of field strength
d. increase of mean free path
***END***
Page 11 of 11