Switchgear and Protection MCQs Part 10
Switchgear and Protection MCQs Part 10
Contents
An arc is produced when switch having high voltage and large current circuit is
An arc is produced when switch having high voltage and large current circuit is
- Closed
- Opened
- Either of above
- None of above
Correct answer: 2. Opened
A circuit breaker is rated as 1500 A, 1000 MVA, 33 kV, 3 second, 3 phase oil circuit breaker. The short time rating is
A circuit breaker is rated as 1500 A, 1000 MVA, 33 kV, 3 second, 3 phase oil circuit breaker. The short time rating is:
- 15236.66 A for 3 seconds
- 20994.55 A for 3 seconds
- 22342.48 A for 3 seconds
- 25669.55 A for 3 seconds
Correct answer: 2. 20994.55 A for 3 seconds
Solution: Rated symmetrical breaking current = (1200 * 10^6)/(√3 * 33 * 10^3) = 20994.55 A for 3 seconds
The rated secondary current of current transformer for relays is usually
The rated secondary current of current transformer for relays is usually:
- 2.5 A
- 5 A
- 10 A
- 20 A
Correct answer: 2. 5 A
Which of following is correct about back-up protection
Which of following is correct about back-up protection:
- It operates when primary protection fails
- It is second line of defense
- When back-up protection operates a large portion of system disconnects
- All of these
Correct answer: 4. All of these
A 60 Hz, 33 kV, 3-phase alternator with earthed neutral has reactance of 5 ohms per phase and is connected to bus-bar through a circuit breaker
A 60 Hz, 33 kV, 3-phase alternator with earthed neutral has reactance of 5 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 frequency of oscillations is:
- 12500 Hz
- 13852 Hz
- 14990 Hz
- 15672 Hz
Correct answer: 2. 13852 Hz
Solution:
Inductance per phase L = XL/2πf = 5/(2π * 60) = 0.0132 H
Capacitance per phase, C = 0.01 µF = 0.01 * 10^-6 F
————————
Frequency of oscillations
f = 1/2π√LC
f = 1/2π√0.0132 H * 0.01 µF = 13852 Hz