Personal Interview Questions

 Personal Interview Questions for Electrical Engineers

1. Where CSA sheath is used in cables?

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Corrugated seamless aluminums sheath is used in high voltage oil filled cables and telephone lines.

2. Why it is used?

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It is used because it is very flexible and easily by repeated bending the sheath is not distorted and it is not damaged. It has lesser weight and reduced thickness.

3. Why protective covering is done in cables?

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To protect the cables from mechanical damage, corrosion and electrolytic action when laid direct in the ground the protective covering is made.

4. By what material protective covering is made?

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Bitumen &Bituminized materials, PVC and layers of fibrous materials.

5. What is meant by serving of a cable?

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Layers of fibrous material permitted with waterproof compound applied to the exterior of the cable is called serving of a cable.

6. Why armoring is done in the cables?

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To protect the sheath from mechanical damage.

7. Why armoring is not done in single core cables?

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The presence of magnetic material within the alternating magnetic field of a single core cable produces excessive losses. Hence single core cables are left unarmored with non-magnetic materials like tin-bronze or silicon-bronze tapes or wires.

8. Why Al is used as an armour material?

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It has non-magnetic properties, high conductivity and mechanical strength.

9. What is meant by grading of cables?

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The method of equalizing the stress in the dielectric of the cable is called the grading of cables.

10.Mention the methods of cables.

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Capacitance grading and intersheath grading.

11.Why the capacitance of the cable is very high than the capacitance of the overhead lines?

The distance between the conductors are small. The distance between the cores and the earthed sheath is also small. The permittivity of the cable insulation is 3 to 5 times greater than that of air insulation.

12.Write the expression of the capacitance of a single core cable.

( (2πε)
----------- = C=q/v

( ln(R/r)

13.What is meant by charging current of a cable?

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The capacitance of a cable determines the charging current. The charging current restricts the use of cables on EHV lines. The current carrying capacity of an a.c cable is also reduced by the charging current.

14.Why power loss occurs in the dielectric of a cable?

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-due to conductivity of insulation
-Dielectric hysteresis or dielectric absorption.
-Ionization or corona

15. Why all transmission and distribution systems are 3 phase systems?

  # A 3 phase a.c circuit using the same size conductors as the single phase circuit can carry three times the power which can be carried by a 1 phase circuit and uses 3 conductors for the 2 phases and one conductor for the neutral. Thus a 3 phase circuit is more economical than a 1 phase circuit in terms of initial cost as well as the losses. Therefore all transmission and distribution systems are 3 phase systems.