# Switchgear and Protection MCQs Part 4

Switchgear and Protection MCQs Part 4

## A fuse performs

A fuse performs

1. Detection function only
2. Interruption function only
3. Both of these
4. None of these

Correct answer: 3. Both of these

## When oil circuit breaker operates under fault conditions, 1 cm³ of oil forms about

When oil circuit breaker operates under fault conditions, 1 cm³ of oil forms about:

1. 1 cm³ of gas
2. 10 cm³ of gas
3. 100 cm³ of gas
4. 1000 cm³ of gas

Correct answer: 3. 100 cm³ of gas

## A 50 Hz, 11 kV, 3-phase alternator with earthed neutral has reactance of 7 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 average rate of rise of re-striking voltage upto first peak is

A 50 Hz, 11 kV, 3-phase alternator with earthed neutral has reactance of 7 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 average rate of rise of re-striking voltage upto first peak is:

1. 89 * 10^3 kV/sec
2. 95.3 * 10^3 kV/sec
3. 102.9 * 10^3 kV/sec
4. 121 * 10^3 kV/sec

Correct answer: 4. 121 * 10^3 kV/sec

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

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Inductance per phase L = XL/2πf = 7/2π50 = 0.0222 H

Capacitance per phase, C = 0.01 µF = 0.01 * 10^-6 F

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Peak restriking voltage occurs at time t given by

t = 1/2fn = π√LC

= π√0.0222 H * 0.01 * 10^-6 F = 1.48 * 10^-4 Hz

———————–

Average rate of rise of re-striking voltage = Peak restriking voltage/Time upto first peak = 17.96/1.48 * 10^-4 Hz = 121 * 10^3 kV/sec

## The fuse in motor circuit provides protection against

The fuse in motor circuit provides protection against:

1. Open circuit
2. Short circuit
4. None of these