# Switchgear and Protection MCQs Part 6

Switchgear and Protection MCQs Part 6

## 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 = 60 µsec
• Peak restriking voltage = 120 kV

The average RRRV is

1. 0.5 * 10^6 kV/sec
2. 1 * 10^6 kV/sec
3. 2 * 10^6 kV/sec
4. 3.8 * 10^6 kV/sec

Correct answer: 3. 2 * 10^6 kV/sec

Explanation:

Average RRRV = Peak restriking voltage/Time to reach peak value = 120 kV/60 µsec = 2 kV/µ sec = 2 * 10^6 kV/sec

## An overcurrent relay that has current setting of 10% is connected to a supply circuit though a CT of 30. The pick-up current is:

An overcurrent relay that has current setting of 10% is connected to a supply circuit though a CT of 30. The pick-up current is:

1. 1 A
2. 2 A
3. 3 A
4. 4 A

Correct answer: 3 A

Explanation:

Plug setting multiplier = Fault current in relay coil/Pick-up current = 30/10 = 3 A

## The transient voltage that appears across the contacts of circuit breaker at the instant of arc extinction is known as

The transient voltage that appears across the contacts of circuit breaker at the instant of arc extinction is known as:

1. Recovery voltage
2. Re-striking voltage
3. Breaking voltage
4. Supply voltage

Correct answer: 2. Re-striking voltage

## A circuit breaker is rated as 2000 A, 1200 MVA, 66 kV, 3 second, 3 phase oil circuit breaker. The breaking capacity of breaker is

A circuit breaker is rated as 2000 A, 1200 MVA, 66 kV, 3 second, 3 phase oil circuit breaker. The breaking capacity of breaker is:

1. 2000 MVA
2. 1200 MVA
3. 300 MVA
4. 100 MVA

Correct answer: 2. 1200 MVA

## A 50 Hz, 11 kV, 3-phase alternator with earthed neutral has reactance of 10 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 value of peak restriking voltage across contacts of breaker is:

A 50 Hz, 11 kV, 3-phase alternator with earthed neutral has reactance of 10 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 value of peak restriking voltage across contacts of breaker is:

1. 10.5 kV
2. 17.96 kV
3. 25.22 kV
4. 30.32 kV

Correct answer: 2. 17.96 kV

Solution:

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

Emax = √2 * 11 / √3 = 8.98 kV

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