SSC JE Electrical Important MCQ Part 25

SSC JE Electrical Important MCQ PDF Part 25

1.Refer to the two magnetically coupled coils of Fig. The polarity of the mutual voltage is

A. Positive

B. Negative

C. Both of these

D. None of these

Answer : A (The reference polarity of the mutual voltage is depends on the current through the dotted terminal of the coil.Short Trick-If both side current enters or leaves the dotted terminal of coil at same terminal,then mutual voltage is positive.If both current enters or leaves the dotted terminal of coil at different terminal (Entering one terminal and leaving at others terminal),then mutual voltage is negative.In this question both current I1 and I2 are leaving same terminal so mutual voltage is positive)

2.The ideal transformer in Fig. has N2∕N1 = 10. The ratio V2∕V1 is

A. 10

B. 0.1

C. -0.1


Answer : D (The current I1 leaves from the dotted terminal of the coil.So, negative polarity of the induced voltage V1 & The current I2 entering from the dotted terminal of the coil.So, positive polarity of the induced voltage V2. Hence Transformation ratio n is negative n=-N2/N1=V2/V1=-10)

3.If the three-winding transformer is connected as in Fig. the value of the output voltage Vo is

A. 10

B. 6

C. -6


Answer : A

(The polarities of the two windings of the secondary side are same when it is series aiding.

From the Figure, the current entering through both dot,So both voltage is positive.

V1=2 & V2=8 Hence Vo=V1+V2=2+8=10 V )

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4.For reducing tower footing resistance it is better to use

A. Chemical and ground rods only

B. Chemical and counterpoise only

C. Ground rod and counterpoise only

D. Chemical, ground rods and counterpoise

Answer : C (Since the tower footing resistance is a function of the soil resistivity and critical breakdown gradient of the soil, therefore chemical treatment can be the option. But practically this is not feasible as it needs regular check up about soil conditions at each and every tower which runs is several miles. Hence we use only ground rod and counterpoise only)

5.Mho relay is normally used for the protection of

A. Long transmission lines

B. Medium length lines

C. Short length lines

D. No length criterion

Answer : A (A mho Relay is also known as the admittance relay.It is use for long transmission lines)

6.Reactance relay is normally preferred for protection against

A. Earth faults only

B. Phase faults only

C. All of these

D. None of these

Answer : A (Reactance type relay is very suitable as a ground relay for ground/earth fault because its reach is not affected by fault impedance)

7.Carrier current protection scheme is normally used for

A. HV transmission lines only

B. HV cables only

C. HV transmission and cables

D. None of these

Answer : A (Carrier-current protection is the most widely used scheme for the protection of Extra High Voltage (EHV) and Ultra High Voltage (UHV) power lines)

8.The ratio of reset to pick up current for an induction cup relay is approx

A. 0.99

B. 1.01

C. 0.75

D. None of these

Answer : A (The ratio of reset to pickup is inherently high in induction relays; because their operation does not involve any change in the air gap of the magnetic circuit. This ratio is between 95% and 100%)

9.If the time of operation of a relay for unity TMS is 10 secs., the time of operation for 0.5 TMS will be

A. 20 secs

B. 5 secs

C. 10 secs

D. None of these

Answer : B (Time of operation = Unity TMS X TMS = 10 X 0.5 = 5 secs)

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10.A reactance relay is

A. Voltage restrained directional relay

B. Directional restrained over-current relay

C. Voltage restrained over-current relay

D. None of these

Answer : B (A reactance relay is an overcurrent relay with directional restraint because operating torque is obtained from the current and the restraining torque by the current and voltage of the directional element)

11.For protection of parallel feeders fed from one end the relays required are

A. Non-directional relays at the source end and directional relays at the load end

B. Non-directional relays at both the ends

C. Directional relays at the source end and non-directional at the load end

D. Directional relays at both the ends

Answer : A

(If two feeders connected in parallel from source to load. Both of the feeders have non-directional over current relay at source end and directional relay or reverse power relay at their load end.These relays should be operated as soon as flow of power in the feeder is reversed)

12.A shunt fault is characterized by

A. Increase in current, frequency and p.f.

B. Increase in current reduction in frequency and p.f.

C. Increase in current and frequency but reduction in p.f.

D. None of these

Answer : B (The shunt fault involves short circuit between conductor and ground or short circuit between two or more conductors. The shunt faults are characterized by increase in current and fall in voltage and frequency)

13.The capacitor switching is easily done with

A. Air blast circuit breaker

B. Oil C.B.

C. Vacuum C.B.

D. Any one of these

Answer : C

14.Where voltages are high and current to be interrupted is low the breaker preferred is

A. Air blast C.B.

B. Oil C.B.

C. Vacuum C.B.

D. Any one of these

Answer : C

15.The most suitable C.B. for short line fault without switching resistor is

A. Air blast C.B.

B. M.O.C.B.

C. SF6 Breaker

D. None of these

Answer : C

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16.In a d.c. series motor the electromagnetic torque developed is proportional to

A. Ia

B. Ia²

C. 1/Ia

D. 1/Ia²

Answer : B (Ta ∝ ɸ.Ia, In series motor ɸ∝ Ia, So Ta ∝ Ia²)

17.In a 3 – phase induction motor running at slip s the mechanical power developed in terms of air gap power Pg is
A. (s-1)Pg

B. (1-s)Pg

C. sPg

D. Pg/(1-s)

Answer : B

18.The size of a conductor used in power cables depends on the

A. Operating voltage

B. Power factor.

C. Current to be carried.

D. Type of insulation used.

Answer : C

19.Out of the following methods of heating the one which is independent of supply frequency is

A. Electric arc heating

B. Induction heating

C. Electric resistance heating

D. Dielectric heating

Answer : C

20.A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. Thes peed of the rotor m.m.f. relative to the rotor structure is

A. 1500 r.p.m.

B. 1425 r.p.m.

C. 25 r.p.m.

D. 75 r.p.m.

Answer : D

(Ns= 120f /P =120×50 /4 =1500rpm

N = Ns( 1-s) = 1500 (1-0.05) = 1425

∴Relative speed = 1500 – 1425 = 75 rpm)

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