Switchgear and Protection MCQs Part 8

Switchgear and Protection MCQs Part 8

The recovery voltage in circuit breaker is maximum for a power factor of

The recovery voltage in circuit breaker is maximum for a power factor of:

1. Zero
2. 0.5
3. 0.707
4. Unity

Correct answer: 3. 0.707

A circuit breaker is rated as 2000 A, 1200 MVA, 33 kV, 3 second, 3 phase oil circuit breaker. The rated service voltage is

A circuit breaker is rated as 2000 A, 1200 MVA, 33 kV, 3 second, 3 phase oil circuit breaker. The rated service voltage is:

1. 11 kV RMS
2. 33 kV RMS
3. 66 kV RMS
4. None of these

Correct answer: 2. 33 kV RMS

The normal current in power line is at 100 A. All at once, the line current rises to 160 A. It is expected that which of the following condition happened

The normal current in power line is at 100 A. All at once, the line current rises to 160 A. It is expected that which of the following condition happened:

1. Open-circuit fault
2. Short-circuit fault
3. Over-loaded condition
4. None of these

Correct answer: 3. Over-loaded condition

Switchgear and Protection MCQs Part 8 MCQ 4

Certain power system has phase to ground capacitance and inductance values 0.02 µF and 4 H respectively. If a magnetizing current of 10 A is to be interrupted what is the value of resistance R that should be used across contacts of CB in order to eliminate voltage surges:

1. 2.3 kΩ
2. 7.07 kΩ
3. 9.53 kΩ
4. 11.33 kΩ

Correct answer: 2. 7.07 kΩ

Explanation:

From formula: R = 1/2 √(L/C)

R = 1/2 √4/0.02 * 10^-6) = 7071.06 Ω = 7.07 kΩ

Silver is used for fuse element since

Silver is used for fuse element since,

1. It has high conductivity
2. It is free from oxidation
3. It doesn’t deteriorate when used in dry air
4. All of these

Correct answer: 4. All of these