Switchgear and Protection MCQs Part 4

by

Switchgear and Protection MCQs Part 4

A fuse performs

A fuse performs

  1. Detection function only
  2. Interruption function only
  3. Both of these
  4. None of these

Correct answer: 3. Both of these

When oil circuit breaker operates under fault conditions, 1 cm³ of oil forms about

When oil circuit breaker operates under fault conditions, 1 cm³ of oil forms about:

  1. 1 cm³ of gas
  2. 10 cm³ of gas
  3. 100 cm³ of gas
  4. 1000 cm³ of gas

Correct answer: 3. 100 cm³ of gas

A 50 Hz, 11 kV, 3-phase alternator with earthed neutral has reactance of 7 ohms per phase and is connected to bus-bar through a circuit breaker. Distributed capacitance upto circuit breaker between phase and neutral is 0.01 µF. The average rate of rise of re-striking voltage upto first peak is

A 50 Hz, 11 kV, 3-phase alternator with earthed neutral has reactance of 7 ohms per phase and is connected to bus-bar through a circuit breaker. Distributed capacitance upto circuit breaker between phase and neutral is 0.01 µF. The average rate of rise of re-striking voltage upto first peak is:

  1. 89 * 10^3 kV/sec
  2. 95.3 * 10^3 kV/sec
  3. 102.9 * 10^3 kV/sec
  4. 121 * 10^3 kV/sec

Correct answer: 4. 121 * 10^3 kV/sec

Solution:

Maximum value of recovery voltage (phase to neutral – Emax is)

Emax = √2 * 11 / √3 = 8.98 kV

————————–

Peak restriking voltage = 2 Emax = 2 * 8.98 = 17.96 kV

————————-

Inductance per phase L = XL/2πf = 7/2π50 = 0.0222 H

Capacitance per phase, C = 0.01 µF = 0.01 * 10^-6 F

————————

Peak restriking voltage occurs at time t given by

t = 1/2fn = π√LC

= π√0.0222 H * 0.01 * 10^-6 F = 1.48 * 10^-4 Hz

———————–

Average rate of rise of re-striking voltage = Peak restriking voltage/Time upto first peak = 17.96/1.48 * 10^-4 Hz = 121 * 10^3 kV/sec

The fuse in motor circuit provides protection against

The fuse in motor circuit provides protection against:

  1. Open circuit
  2. Short circuit
  3. Over-load
  4. None of these

Correct answer: 2. Short circuit

The breaking capacity of fuse as compared to circuit breaker is

The breaking capacity of fuse as compared to circuit breaker is:

  1. Less
  2. More
  3. Almost same
  4. None of these

Correct answer: 1. Less

Leave a Reply