Ques 81. The percentage of limiting error in the case of an instrument reading 0f 8.3 V with a 0 to 150V voltmeter having a guaranteed accuracy of 1% full-scale reading is
1.810%
0.181%
18.10%
0.0018%
Answer.3. 18.10%
Explanation:
Limiting Error = accuracy * full-scale reading
= (1 x 150) / 100 = 1.5 V
% limiting error when measuring 8.3 V
% error=(maximum error/scale reading) * 100
= (1.5 x 100) / 8.3
= 18.10 %
Ques 82. The surge impedance of a 100 KM long underground cable is 50 ohms. The surge impedance of a long similar cable would be
20 ohms
50 ohms
80 ohms
125 ohms
Answer.2. 50 Ohms
Explanation:
Surge Impedance of the cable does not depend on the length of the line, Hence for 100 km length the surge impedance of the line will remain same i.e 50 ohms
Ques 83. The low voltage winding of a 400/230 V, 1-phase 50 Hz transformer is to be connected to a 25 Hz supply. In order to keep the magnetization current at the same level as that for a normal 50 Hz, the supply voltage should be
230 V
460 V
115 V
65 V
Answer.3. 115 V
Explanation:
To maintain the magnetization current at the same level, flux should be same i.e V/f ratio should be same.
V2/F2 = V1/F1
V2 = (230 x 25)/50
= 115 V
Ques 84. Auto-transformer is used in transmission and distributions
When operator is not available
When iron losses are to be reduced
When efficiency consideration can be ignored
When the transformation ratio is small
Answer.4. When the transformation ratio is small
Explanation:
Auto-transformer are generally used for transforming one transformer voltage to another when the ratio is 2:1 or less
Note: An autotransformer is never used as a distribution transformer because the lack of insulation can cause dangerously high voltage in a customer location if the neutral is open.
Ques 85. The surge impedance of a 400 Km long overhead transmission line is 400 Ohms. For a 200 km length of the same line, the surge impedance will be
200 ohm
800 ohm
400 ohm
100 ohm
Answer.3. 400 ohms
Explanation:
Surge Impedance of the cable does not depend on the length of the line hence for the 200 km length surge impedance will remain same i.e 400 ohms.
Ques 86. If the fault current is 3000 A, for an I.D.M.T relay with a plug setting of 50% and C.T ratio of 400/5, then the plug setting multiplier would be
7.5
15.0
18.75
37.5
Answer.2. 15.0
Explanation:
The current transformer ratio is 400 / 5. So, the CT can be provided with a rated current of 5 Amps to the relay coil. Hence the rated relay current is 5 Amps.
If the relay setting is 50%, then the relay can be operated for 5 × ( 50 / 100 ) = 2.5 Amps
We know that the fault current is 3000 Amps. Therefore P.S.M is
=
Ques 87. For an existing ac transmission line, the string efficiency is 80%. If dc voltage is supplied for the same setup, the string efficiency will be
<80%
80%
90%
100%
Answer.4. 100%
Explanation:
When Dc is applied, then there will be no effect of line to ground capacitance. Hence string efficiency will be 100%
Ques 88. A lap wound dc machine has 400 conductors and 8 poles. The voltage induced per conductor is 2V. The machine generates a voltage of
100 V
200 V
400 V
800 V
Answer.1. 100 V
Explanation:
For lap wound number of parallel paths is equal to number of poles
Number of conductor in each parallel path
= 400/8 = 50
Generated E.M.F
= 50 × 2 = 100V
Ques 89. The self-inductance of a long cylindrical conductor due to its internal flux linkages is 1mH/m. If the diameter of the conductor is doubled, then the self-inductance of the conductor due to internal flux linkage would be
0.5 KH/m
1mH/m
1.414 KH/m
4 KH/m
Answer.2. 1mH/m
Explanation:
Self-inductance is independent of the shape and size of the conductor. Hence the self-inductance will remain the same.
Ques 90. The inductance of a three-phase transmission line is 1.2 mH/phase/km. If the spacing of conductors and the radius of the conductors are doubled then the inductance of the line will be
