Power System Protection MCQs Part 18
Contents
- 1 The arc voltage in circuit breaker
- 2 A fuse wire of cross-section has a radius of 0.6 mm. The wire blows off at a current of 5 A.The radius of wire that will blow off at a current of 1 A is
- 3 Which of the following device detects fault in power system
- 4 If the fault current is 2000 A, relay setting is 50% and the CT ratio is 500/5, the PSM is
- 5 The operating time of fuse is around
The arc voltage in circuit breaker
The arc voltage in circuit breaker:
- Lags arc current by 90°
- Leads arc current by 90°
- Lags arc current by 180°
- Is in phase with arc current
Correct answer: 4. Is in phase with arc current
A fuse wire of cross-section has a radius of 0.6 mm. The wire blows off at a current of 5 A.The radius of wire that will blow off at a current of 1 A is
A fuse wire of cross-section has a radius of 0.6 mm. The wire blows off at a current of 5 A.The radius of wire that will blow off at a current of 1 A is:
- 0.110 mm
- 0.205 mm
- 0.388 mm
- 0.455 mm
Correct answer: 2. 0.205 mm
Solution
I² ∝ r³
(I2/I1)² ∝ (r2/r1)³
r2 = r1 (I2/I1)^2/3
r2 = 0.6 * (1/5)^2/3 = 0.205 mm
Which of the following device detects fault in power system
Which of the following device detects fault in power system
- Relay
- Circuit breaker
- Isolator
- None of these
Correct answer: 1. Relay
If the fault current is 2000 A, relay setting is 50% and the CT ratio is 500/5, the PSM is
If the fault current is 2000 A, relay setting is 50% and the CT ratio is 500/5, the PSM is:
- 2
- 4
- 8
- 10
Correct answer: 4. 10Solution:
Pick-up current = Rated seocndary current of C.T * Current setting = 5 * 0.5 = 2.5 A
———————
Fault current in relay coil = 2000 * 5 / 500 = 20 A
——————–
P.S.M = 20/2.5 = 8
The operating time of fuse is around
The operating time of fuse is around:
- 0.002 s
- 0.005 s
- 0.2 s
- 5 s
Correct answer: 1. 0.002 s