51. The relay operating speed depends upon
A. the spring tension
B. the rate of flux built up
C. armature core air gap
D. all of the above
Answer
D. all of the above
Explanation:
The relay operating speed depends upon
- the spring tension
- the rate of flux built up
- armature core air gap
52. The fuse blows off by
A. burning
B. arcing
C. melting
D. none of the above
Answer
C. melting
Explanation:
- when too many lights or plug-in appliances draw power from the circuit, it can overload the capacity of the fuse and cause the metal ribbon inside the fuse to melt through.
- The result is that all lights, outlets, and appliances powered by the circuit will go dead suddenly.
53. If the fault occurs near the impedance relay, the V/I ratio will be
A. constant for all distances
B. lower than that of if fault occurs away from the relay
C. higher than that of if fault occurs away from the relay
D. none of the above
Answer
B. lower than that of if fault occurs away from the relay
Explanation:
- If the fault occurs near the impedance relay, the V/I ratio will be lower than that of if fault occurs away from the relay.
- If the fault is away from the relay, then the impedance seen by the relay is more.
54. Oil switches are employed for
A. low currents circuits
B. low voltages circuits
C. high voltages and large currents circuits
D. all circuits
Answer
C. high voltages and large currents circuits
Explanation:
- As the name implies, the contacts of switches are opened under oil.
- Usually, transformer oil is used whose purpose is to cool and quench the arc formed on account of opening the contacts.
- These switches are used for circuits of high voltage and large current-carrying capacities.
- Air break switches are generally used outdoors for circuits of medium capacity such as lines supplying an industrial load from a main transmission line or feeder.
- Oil switches are employed for high voltages and large currents circuits
55. Which of the following circuit breakers has high reliability and minimum maintenance?
A. Air blast circuit breakers
B. Circuit breaker with SF6 gas
C. Vacuum circuit breakers
D. Oil circuit breakers
Answer
B. Circuit breaker with SF6 gas
Explanation:
- A circuit breaker in which SF6 under pressure gas is used to extinguish the arc is called SF6 circuit breaker. SF6 (sulphur hexafluoride) gas has excellent dielectric, arc quenching, chemical and other physical properties which have proved its superiority over other arc quenching mediums such as oil or air.
- SF6 gas has excellent insulating, arc extinguishing and many other properties which are the greatest advantages of SF6 circuit breakers.
- The gas is non-inflammable and chemically stable. Their decomposition products are non-explosive and hence there is no risk of fire or explosion.
- Electric clearance is very much reduced because of the high dielectric strength of SF6.
- Its performance is not affected due to variations in atmospheric condition.
- It gives noiseless operation, and there is no over voltage problem because the arc is extinguished at natural current zero.
- There is no reduction in dielectric strength because no carbon particles are formed during arcing.
- It requires less maintenance and no costly compressed air system is required.
- SF6 performs various duties like clearing short-line faults, switching, opening unloaded transmission lines, and transformer reactor, etc. without any problem.
56. For which of the following ratings of the transformer differential protection is recommended?
A. above 30 kV
B. equal to and above 5 MVA
C. equal to and above 25 MVA
D. none of the above
Answer
B. equal to and above 5 MVA
Explanation:
- The high cost of transformers justifies fast, reliable protection for all sizes, although additional protection is justified above about 5 MVA.
- The simplest form of protection for transformers below about 2 MVA and at 69 kV and below is the fuse. Although fuses are available for 115 kV, their low fusing current can make them unreliable.
- For transformers up to this size that is equipped with circuit breakers, phase-instantaneous and phase-ground inverse-time overcurrent relays (50/51) can provide good protection if the fault level is large enough to get satisfactory settings.
- For transformers rated at 5 MVA and above that are important to system operation, or where high sensitivity is necessary (generator transformers), differential protection is recommended in addition to high- and low-side overcurrent relays.
- Buchholz protection should be standard for all conservator-type transformers together with the winding-temperature trip for transformers of more than 5 MVA or just a temperature trip for the smaller unit.
57. When a wave propagates on a transmission line, it suffers reflection several times at
A. tapping
B. load end
C. sending end
D. sending and other ends
E. all of the above
Answer
E. all of the above
Explanation:
When a wave propagates on a transmission line, it suffers reflection several times at
- tapping
- load end
- sending end
- sending and other ends
58. Which of the following medium is employed for the extinction of arc in air circuit breaker?
A. Water
B. Oil
C. Air
D. SF6
Answer
C. Air
Explanation:
Type of circuit breaker Vs Medium Vs Operating voltages
- Air Circuit Breaker ------ Air at atmospheric pressure ---- Low voltage up to l000 V
- Tank type oil circuit breaker ------ Oil ------- Up to 33 kV
- Minimum oil circuit breaker ------ Oil ----- 36 kV, 1500 MVA & 132 kV, 3000 MVA
- Air - blast ------ Compressed air ---- 132 kV, 220 kV & 400 kV, 760 kV
- SF6 --- SF6 gas --- 132 k V, 220 kV & 400 kV, 760 kV
- Vacuum C.B. --- Vacuum ---- 11 kV. 33 kV
- High voltage direct current C.B. ---- Vacuum or SF6 gas --- ± 500 kV DC
59. 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 conditions
D. none of the above
Answer
C. constantly the electrical quantities which differ during normal and abnormal conditions
Explanation:
- It is a sensing device.
- It senses the fault or given situation when the current attains a certain predetermined value.
- It knows its position and finally, it gives the tripping command to the circuit breaker.
- The protective relay gives the command to the circuit breaker to disconnect the faulted element.
- Relays can be designed to respond to changes in resistance, reactance, impedance, voltage, current, light intensity and temperature.
- Protective relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions.
- The electrical quantities which may change under fault conditions are voltage, current, frequency, and phase angle.
60. A fuse is never inserted in
A. neutral wire
B. negative of D.C. circuit
C. positive of D.C. circuit
D. phase dine
Answer
A. neutral wire
- Explanation:
- The fuse is always connected in the live path because if the live wire gets short-circuited somewhere within the appliance and results into the high current then the fuse can blow and protect the appliance.
- In case if the fuse is connected into the neutral path then the appliance cannot be protected as current returns through the earth which is highly unsafe.
- Also, connecting fuse in live line isolates the load from source once the fuse is blown under fault.