Quiz: Electrical Engineering 9 May 2020

Quiz: Electrical Engineering
Exam: NLC
Topic: Miscellaneous

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. The no of PN junctions in a SCR has:
(a) Two
(b) Four
(c) One
(d) Three



Q2. The instantaneous voltage and current across a load are given by v = 60 sin (314 t-π /6) volts and I = 20 sin (314 t-π /2) amperes, respectively. The active power consumed by the load is:
(a) 200 watts
(b) 300 watts
(c) 600 watts
(d) None of these



Q3. In a three-phase half wave rectifier feeding resistive load, if the input source is a
three phase 4 wire system and line to line voltage is 100 V. The supply frequency is 300 Hz. The ripple frequency at the output is:
(a) 300 Hz
(b) 900 Hz
(c) 1200 Hz
(d) None of these


Q4. A 35 V dc supply is connected across a resistance of 600 ohm in series with an unknown resistance of 1.2 k Ω is connected across 600-ohm resistances and reads 5 V. The value of resistance R shall be:
(a) 1.2 kΩ
(b) 2.4 kΩ
(c) 120 kΩ
(d) 400 kΩ



Q5. An isolator is designed to open a circuit under
(a) full-load
(b) normal conditions
(c) no-load
(d) none of the above



Q6. An arc is produced when the switch of a high-voltage and large-current circuit is
(a) opened
(b) closed
(c) opened or closed
(d) none of the above



Q7. Current chopping mainly occurs in
(a) air-blast circuit breaker
(b) oil circuit breaker
(c) SF_6 circuit breaker
(d) vacuum circuit breaker



Q8. The earth fault current is generally ………… than the short-circuit current.
(a) less
(b) more
(c) equal to
(d) none of the above



Q9 Overheating of a transformer may be due to
(a) overloads
(b) short-circuits
(c) failure of cooling system
(d) all of the above



Q10. In case of slow developing faults in a transformer, the Buchholz relay
(a) sounds an alarm
(b) trips the circuit breaker
(c) both (i) and (ii)
(d) none of the above



Sol.1 (d)
SCR has three junctions
S2. Ans.(b)
Active power = V.I cos ϕ
= 300 watts

S3. Ans.(c)
In 3 – phase 4 -wire System of half wave rectifier,
ripple frequency (f_r) = 3 × input frequency
= 3 × 300
= 900 Hz

S4. Ans.(b)
R_eq of 600Ω and 1200Ω is
= 400Ω
So, current in the circuit is
I=V/R=5/400 A
R=30V/I=30/5×400  (As 5 volt drop is across the 600 ohm)
= 2400 Ω = 2.4 kΩ

S5. Ans.(c)
Sol. Sole main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while current is flowing in a line.

S6. Ans.(a)
Sol. Generally, the load contains reactive elements (inductance and capacitance). The sudden change of current in the circuit due to breaking results in the production of large induced e.m.f. (several thousand volts) which ruptures the insulation of air between the contacts, causing a spark. When switch is closed, again large e.m.f. is produced but it is distributed in the circuit.

S7. Ans.(a)
Sol. The current chopping is the phenomenon of current interruption before the natural current zero is reached. It mainly occurs in air-blast circuit breakers because they retain the same extinguishing power irrespective of the magnitude of current to be interrupted.

S8. Ans.(a)
Sol. An earth-fault usually involves a partial breakdown of winding insulation to earth. The resulting leakage current is considerably less than the short-circuit current.

S9. Ans.(d)
Sol. Overheating of the transformer is usually caused by sustained overloads or short-circuits and very occasionally by the failure of the cooling system.

S10. Ans.(a)
Sol. In case of slow developing faults within the transformer, the heat due to fault causes the decomposition of some transformer oil in the main tank. When a predetermined amount of gas gets accumulated, it exerts pressure on the float to complete the alarm circuit.

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