# Switchgear and Protection MCQs Part 9

Switchgear and Protection MCQs Part 9

## During which part of fault current wave, the fuse melts

During which part of fault current wave, the fuse melts:

1. When first peak is reached
2. Before first peak is reached
3. After first peak is reached
4. None of these

Correct answer: 2. Before first peak is reached

## During short circuit test on circuit breaker following readings were obtained as single frequency transient:

During short circuit test on circuit breaker following readings were obtained as single frequency transient:

• Time to reach peak restriking voltage = 100 µsec
• Peak restriking voltage = 120 kV

The frequency of oscillations is

1. 1000 Hz
2. 2500 Hz
3. 5000 Hz
4. 10000 Hz

Explanation:

Natural frequency of oscillations is

fn = 1/(2 * Time to reach peak value)

1/(2  * 100 * 10^-6) = 5000 Hz

## The making current of a 3-phase circuit breaker rated at 1000 MVA, 66 kV is

The making current of a 3-phase circuit breaker rated at 1000 MVA, 66 kV is:

1. 18955 A
2. 20050 A
3. 22306 A
4. 25382 A

Solution:

Symmeterical breaking current = MVA/√3 * V

= 1000 * 10^6/√3 * 66 * 10^3 = 8747.73 A

—————-

Rated making current = 2.55 * Rated breaking current

Rated making current = 2.55 * 8747.73 A = 22306.71 A

## Which of following is responsible for initiation of electric arc at the instant of contact separation in the circuit breakers:

Which of following is responsible for initiation of electric arc at the instant of contact separation in the circuit breakers:

1. Field emission of electrons
2. Thermionic emission of electrons
3. Both of these
4. None of these

Correct answer: 3. Both of these

## If relay setting is 80% and CT ratio is 400/5, and fault current is 2000 A, the plug setting  multiplier will be

If relay setting is 80% and CT ratio is 400/5, and fault current is 2000 A, the plug setting  multiplier will be:

1. 3
2. 5
3. 6.25
4. 8.75