Power System Protection MCQs Part 8

a. A power system contains two relays of 5 VA and 10 VA. The correct statement about the sensitivity of relays:

  1. 5 VA relay is more sensitive than 10 VA
  2. 10 VA relay is more sensitive than 5 VA
  3. Both are equally sensitive
  4. One can’t determine sensitivity from VA

Correct answer: 1. 5 VA relay is more sensitive than 10 VA

b. An overcurrent relay has setting of 200% and is connected to a transformer of 200/5. Find the pickup value.

  1. 2.5 A
  2. 5 A
  3. 10 A
  4. 20 A

Correct answer: 3. 10 A

c. The isolator is installed in power system to:

  1. Protect against high current faults
  2. Isolate a part of the system
  3. Protect the buses
  4. Separate a CT from PT

Correct answer: 2. Isolate a part of the system

d. The term inverted running represents an alternator:

  1. Which is protected via inversion circuit
  2. Which runs at a very high speed
  3. A turboalternator which uses inverter circuits
  4. Which behaves as a motor

Correct answer: 3. Which behaves as a motor

e. The making capacity of the circuit breaker is measured in terms of ______:

  1. R.M.S
  2. Peak
  3. Both
  4. None

Correct answer: 2. Peak

f. Mathematically the making capacity is defined by the equation:

  1. 1.88 × Symmetrical breaking capacity
  2. 2.55 × Symmetrical breaking capacity
  3. 3.87 × Symmetrical breaking capacity
  4. 4.99 × Symmetrical breaking capacity

Correct answer: 2. 2·55 × Symmetrical breaking capacity

g. The desired quantity in a fuse is:

  1. High conductivity
  2. Low conductivity
  3. High melting point
  4. High deterioration rate

Correct answer: 1. High conductivity

h. The correct relationship between the fusing current and diameter of the wire is:

  1. I = k d
  2. I = k d1.5
  3. I = k d2
  4. I = k d3

Correct answer: 2. I = k d1.5

i. The basic function of a relay is to:

  1. Detect the fault
  2. Interrupt the circuit
  3. Both of these
  4. None of these

Correct answer: 1. Detect the fault

j. A three phase 1000 MVA circuit breaker, 11 kVA has symmetrical breaking current:

  1. 90909 A
  2. 11000 A
  3. 52486 A
  4. 44088 A

Correct answer: 3. 52486 A

Solution: I = 1000 MVA / (11 kVA * √3) = 52486 A

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