# MCQs on Power Systems

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01․ The line current IR flowing into balanced delta connected load is - j5 - 3 A. If the phase sequence is RYB the phase current IYB is
5.
10.
20.
55.1.

Line current is - j5 - 3 A, phase current is - j5 A.

02․ A fuse wire of circular cross-section has a radius of 0.8 mm. The wire blows off at an current of 8 Amp. Calculate the radius of wire that will blow off at an current of 1 amp?
0.2 mm.
0.4 mm.
4 mm.
2 mm.

03․ Which of the following material is not used as fuse element?
Silver.
Copper.
Aluminium.
Carbon.

The melting point of carbon is 3500°C. It is very difficult reach that high temperature when short circuit current flows through the fuse. Hence for short circuit or over current condition the fuse will not melt to disconnect the circuit if the fuse is made off carbon.

04․ Rating of fuse wire is expressed in
Watts.
Mho.
Ampere.

A fuse in electrical circuit is used to protect the circuit from over current and short circuit current. Actually fuse elements melt to disconnect the continuity when current beyond the predetermined value flows through them. So it is most appropriate to express the rating of fuse in Ampere.

05․ Fusing factor =
normal current rating of fuse element / minimum fusing current.
minimum fusing current / normal
both.
none of these.

06․ Switchgear equipment include
all of these.
switches.
fuses.
relays and CB.

07․ The rating of CB is in terms of
volt - ampere.
current.
voltage.
VAR.

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity to immediately discontinue electrical flow. Generally the rating of CB is in terms of voltage.

08․ MCB stands for
miniature circuit breaker.
minimum capacity breaker.
molded circuit breaker.
maximum capacity breaker.

MCB is miniature circuit breaker. Miniature circuit breaker construction is very simple, robust and maintenance free. Generally an MCB is not repaired or maintained, it just replaced by new one when required.

09․ Recovery voltage in CB is equal to
transient voltage.
arcing period voltage.
normal frequency 50 Hz rms voltage.
none of these.

Recovery voltage in circuit breaker is equal to rms voltage at 50 HZ frequency.

10․ A 3-phase breaker is rated at 2000 MVA, 33 kV, its making current will be
35 kVA.
49 kVA.
70 kVA.
89.22 kVA.

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