Power System Protection MCQs Part 18

Power System Protection MCQs Part 18

The arc voltage in circuit breaker

The arc voltage in circuit breaker:

1. Lags arc current by 90°
2. Leads arc current by 90°
3. Lags arc current by 180°
4. 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:

1. 0.110 mm
2. 0.205 mm
3. 0.388 mm
4. 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

1. Relay
2. Circuit breaker
3. Isolator
4. 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:

1. 2
2. 4
3. 8
4. 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:

1. 0.002 s
2. 0.005 s
3. 0.2 s
4. 5 s

Correct answer: 1. 0.002 s