Power System Protection MCQs Part 17

Power System Protection MCQs Part 17

A circuit breaker is rated as 1800 A, 1200 MVA, 33 kV, 3 second, 3 phase oil circuit breaker. The rated making current is

A circuit breaker is rated as 1800 A, 1200 MVA, 33 kV, 3 second, 3 phase oil circuit breaker. The rated making current is:

1. 35232.20 A (peak)

• 66555.20 A (peak)
• 70000.55 A (peak)

Correct answer: 2. 53536.11 A (peak)

Solution:

Rated symmetrical breaking current = (1200 * 10^6)/(√3 * 33 * 10^3) = 20994.55 A (RMS)

Rated making current = (2.55 * 20994.55) = 53536.11 A (peak)

An overcurrent relay that has current setting of 150% is connected to a supply circuit though a CT of 400/5. The pick-up current is

An overcurrent relay that has current setting of 150% is connected to a supply circuit though a CT of 400/5. The pick-up current is:

1. 2.5 A
2. 5 A
3. 7.5 A
4. 10 A

Correct answer: 3. 7.5 A

Explanation:

Solution:

Current setting = 150/100 = 1.5

Pick up current = Rated secondary current of CT * Current setting

= 5 * 1.5 = 7.5 A

The number 7.5 implies that relay will actually operate for relay coil current equal to or greater than 7.5 A

Fuses are used in circuits for which

Fuses are used in circuits for which:

1. Frequent operations are expected
2. Frequent operations are not expected
3. Any of above
4. None of above

Correct answer: 2. Frequent operations are not expected

Which of the below mentioned CB is preferred for EHT applications

Which of the below mentioned CB is preferred for EHT applications:

1. Air break CB
2. Bulk oil CB
3. Low oil CB
4. SF6 CB

Correct answer: 4. SF6 CB

In HRC fuse, the time between cut-off and final current zero is termed

In HRC fuse, the time between cut-off and final current zero is termed:

1. Arcing time
2. Pre-arcing time
3. Total operating time
4. None of these

Correct answer: 1. Arcing time