Questions and Answers for Electrical Interview


#1

1.Explain different types of fault?

There are basically two types of fault.

a)Shunt faults - Basically shunt faults are short circuits.When the path of the load current is cut short because of breakdown of insulation,we say that a ‘short circuit’ has occurred which is nothing but a shunt fault.

b)Series Fault - Series faults are nothing a break in the path of current.It is observed in practice that most of the open conductor faults sooner or later develop into some or other short-circuit fault

2.What are the various states of operation of a power system?

Normal alert,Emergency,In extremis and restorative are various states of operation of power system.

3.Explain the working of current transformer ?

The current transformer has two jobs to do.Firstly,it steps down the current to such a level that it can be easily handled by the relay current coil.Secondly,it isolates the relay circuitry from the high voltage of EHV system. A CT whose primary is in series with the line in which the current is to be measured.

4.Why the secondary of a CT should not be open ?

If the secondary of a CT is open current through it is zero and hence the ampere turns produced by secondary which generally oppose the primary ampere turns becomes zero.As there is no counter m.m.f,primary m.m.f produces high flux in the core which leads to excessive core losses and overheating.

5.Explain the working of a potential transformer ?

The voltage transformer or potential transformer steps down the high voltage of the line to a level safe enough for relaying system and personnel to handle.A PT primary is connected in parallel at the point where a measurement is desired.


#2

6.Explain the working principle of circuit breaker ?

The circuit breaker is an electrically operated switch,which is capable of safely making,as well as breaking,short-circuit current.it has two contacts namely fixed contact and moving contact.Under normal conditions the moving contacts comes in contact with fixed contact thereby forming the closed contact for the flow of current.

During abnormal & faulty conditions,(when current exceeds the rated value) an arc is produced between the fixed and moving and thereby it forms the open circuit arc is extinguished by the arc quenching media like air,oil,vacuum etc

7.What are the different types of circuit breaker ?

a)Air circuit breaker
b)Oil Circuit breaker
c)Minimum oil circuit breaker
d)Air blast circuit breaker
e)Vacuum circuit breaker
f)SF6 circuit breaker

8.What is Sf6 circuit breaker ?Its advantages ?

If SF6 (Sulpher Hexa Fluoride) gas is used as arc quenching medium in a circuit breaker,it is termed as SF6 CB.SF6 gas has superior arc quenching capacity and high dielectric strength.So they have very short arcing time and can interrupt high currents.