80+ Electrical interview questions

1. Why do we use a three-phase power supply for heavy loads

During each half cycle, the voltage will drop to zero in a single-phase and because of this the amount of power won’t be constant and this would badly affect for heavy loads. While in case of three-phase power there is never a time where all the voltages go to zero. Three-phase power will be constant throughout the whole cycle.

2. How grounding is different from earthing

Earthing is done to electrical equipment so that in case of any fault the equipment will be protected. While grounding means the connection of the load to the ground by neutral.

3. What is the full form of KVAR

KVAR stands for kilovolt amps with the reactive component

4. What is the electric power system

It is the combination of generating, transmission and distribution system

5. How is electricity measured

Watt is the unit of power and electricity measurement is done in watts 1W = 1 J/s A kilowatt is 1000 watts, a kilowatt is the energy of 1000 watts working for 1 hour.

6. When will be the efficiency of transformer maximum

The efficiency of a transformer will be maximum when the copper loss is equal to iron loss.

7. Describe electricity and its types

Electricity is the process, which is caused by the electric charge. The electricity could be in motion or static. The electricity which is not in motion is static and the one which is in motion is current electricity.

8. What is static electricity

Static electricity is the electricity without motion it is the electric charge that is at rest, it is also known as frictional electricity. It can be created by rubbing two substances, such as a comb can pick up tiny pieces of paper when it is charged.

9 Explain megger and contact resistance meter

The resistance of the cable and the continuity of the conductor can be measured by megger. The measuring of low resistance like relays and contactor can be done with the help of low resistance meter

10. What is excitation

Applying external voltage to a DC shunt coil in a DC motor is called excitation.

11. Explain the connection of relay in a power system

In a power system, relays are connected with the help of a current and potential transformer

12. The direction of induced EMF in an electric generator can be find out by using which rule

Fleming’s left hand rule

13. What is the relation between induction motor and transformer

The induction motor is considered as a transformer, this is because the operation of a transformer and inductor motor is the same. The transfer of power from the primary to the secondary by mutual induction, in a motor the stator gives power to the rotor by using the same principle.

14. What is active and reactive power

The power which is generated in a power system is active power and can be used to do work. But the reactive power cannot be used to do work, it only helps the active power to flow. Without reactive power, there won’t be any flow of active power.

15. What is an electric motor

The conversion of electrical energy to mechanical energy can be done by using an electrical motor. Electric motors are of two types AC and DC motor, depending on the input type.

16. in an alternator the frequency of voltage generated is depended on

number of poles and rotative speed

17. How alternating current is different from direct current

In AC the charge flows and changes direction periodically, and the voltage level also reverses with the current. AC is used to deliver power to houses, industries, etc. DC can be considered as the movement of electric charge that carries an electron in a unidirectional flow.

18. What is voltage

Voltage is an electric pressure that causes the movement of electrons in a circuit. It is referred to as electromotive force and is measured in volts.

19. What are the characteristics of series circuit

The total resistance of a series circuit would be the sum of all resistance and at all points of the circuit, the current will be the same. If the resistor value is different, then the voltage drop across each resistor will be different.

20. What are the characteristics of parallel circuit

If the resistance is different, then the current flow through each leg will be different. The total sum of the current in each leg is equal to the total current of the parallel circuit.

21. What is isolators

An isolator is a switch which can be operated manually or automatically it is a switch which isolates the faulty section or a portion of a circuit of substation which is meant for repair from healthy section so that more faults can be avoided

22. What is capacitor banks and why is it used in substation

The capacitor bank is a set of identical capacitors which is connected in series or parallel. They are used in the substation for power factor correction and basic protection of substation.

23. What is electric traction

Electric traction can be defined as the usage of electric power for traction systems such as railways and trolleys. DC motors are used for the traction system.

24. What could happen if the transformer is operated at 100Hz frequency

The winding voltage is directly proportional to the transformer frequency. So if we double the frequency the winding voltage will be doubled and because of this flux density increases and it could destroy the core.

25. What is regenerative braking

Due to inertia the motor will run after the supply is cut off, so in order to stop it quickly a load is placed across the armature winding and the motor would maintain continuous field supply. The EMF voltage is applied across the resistor and due to the load the motor will stop and this is known as regenerative braking.

26. Why star delta starter is used with induction motor

The major purpose of using a star-delta starter with an induction motor is to reduce the current during the starting of the induction motor. Starting current will be reduced to four times than the normal starting current. So if the starting current is reduced then the voltage drop during the starting of the motor is also reduced.

27. Why delta star transformer is used for lighting loads

The neutral conductor is required in case of lighting loads, so the secondary must be star winding. The lighting loads will be unbalanced in all three phases. So in order to reduce the current unbalance in the primary, delta winding is used in the primary and that’s why delta star transformer is used for lighting loads.

28. Why the earth pin is thicker and longer than the other pins in a three-pin plug

The area is inversely proportional to the resistance, so if the area increases the resistance increases and the leakage current will take the long resistance path and that’s why the earth pin is thicker. It is longer because the earth pin must be the first to make a connection and last to disconnect. So the person who uses the electrical device will be safe.

29. Why can’t we start the series motor on no-load

Series motor cannot be started on no-load because of its high starting torque.

30. What is the difference between MCB and MCCB, and where can it be used

A miniature circuit breaker can be used for short circuit protection, they are operated thermally and it is used for low current rating. Molded case circuit breaker operates thermally in case of overload current and it will operate magnetically in case of short circuit condition for the instant trip. MCCB is used for electric current more than 100A.

31. Where must be the lightning arrestor placed in a distribution line

The lightning arrestor should be placed near distribution transformer and outgoing feeders of 11KV and incoming feeders of 33Kv and near power transformers in substations.

32. What is IDMT relay

IDMT is inverse definite minimum time relay, its operation is inversely proportional. So tripping time will decrease as the magnitude of fault current increases.

33. What are the types of transformer losses

There are two types of transformer losses, such as copper loss and magnetic loss. The reason for the copper loss is the resistance of the wire and the magnetic loss is due to the eddy current and hysteresis loss.

34. What is Buchholz relay

This relay is used for the protection of the transformer from its internal faults, so if any internal fault occurs in a transformer this relay will make a sound. The sound will stop if the transformer is isolated from the circuit otherwise it will trip the circuit itself by its own circuit mechanism.

35. What is a semiconductor

The semiconductor is a device that is neither a good conductor nor a good insulator. But it has some of their features, examples for semiconductors are carbon, silicon….

36. What is current

Current is measured as the amount of electrons passes through a circuit at a given point in one second.

37. What is resistance

Resistance can be described as the opposition to the current flow and they are measured in ohms.

38. What is Ferranti effect

If the output voltage is greater than the input voltage or if the receiving end voltage is greater than sending end voltage.

39. What is SF6 circuit breaker

In this circuit breaker, the SF6 stands for sulfur hexafluoride gas and they are good in case of arc extinguishing. This circuit breaker is used for the protection of the electrical system.

40. What is power factor

If the power factor is close to 1 then the system will work better. The ratio of true and apparent power is PF. There will be losses in case of a low power factor. The power factor should be ideally 1. If it is greater than 1 then the load will act as a capacitor and it would start feeding the source and will cause tripping.

41. What will happen if a generator load is connected with a capacitor

The power factor of the generator will improve if a capacitor is connected across it. According to the engine capacity of the alternator, the capacitor will help, otherwise, the alternator will be overload because of the extra watts which are consumed due to the improvement in PF.

42. Why the capacitor works only on AC

Mostly capacitor will give infinite resistance to DC, but it allows AC to pass through

43. Describe stepper motor

It is an electrical machine that would act if an input pulse is applied to it. It is a type of synchronous motor which would run in steps instead of a complete cycle.

44. What is an exciter and what are its types

Exciter is used to supply the excitation DC voltage to the fixed poles of the generator. Static and rotary are the types of the exciter.

45. What are the difference between a four-point starter and a three-point starter

The shunt connection in a four-point starter is provided separately from the line, whereas in a three-point starter it is connected with a line.

46. Boosters are basically


47 In a high voltage transmission line why conductors are suspended from towers

They are suspended to increase clearance from the ground

48. When will be the transmission efficiency increase

The transmission efficiency will increase if the voltage and power factor both increase

49. What is skin effect

If a conductor carries more current on the surface than the core then it is called the skin effect.

50. How could be the effective resistance of a conductor is the same as the ohmic resistance

It will be the same when the current is uniformly distributed in the conductor cross-section.

51. What are the factors that the skin effect is depended on

It is depended on the size of the conductor, frequency of the current and resistivity of the conductor material.

52. How voltage drop can be compensated in a constant voltage transmission

By using synchronous motors it can be compensated

53. What is the disadvantage of constant voltage transmission

The system short circuit current will increase

54. At what voltage range does the pin insulators are normally used

Pin insulators are used up to 25kv voltage

55. What is the main effect of corona

The major effect is the increased energy loss

56. What is the full form of ASCR

Aluminum conductor steel reinforced

57. Where does the feeder feeds power in a transmission system

The feeder feeds power to the distributors

58. In a transmission line what is the standing wave ratio

The maximum voltage to minimum voltage ratio is the standing wave ratio

59. Why synchronous generators are used for the production of electricity

Synchronous generators are used because the synchronous machines have the ability to work on different power factor and that’s why they are used for electricity production.

60. How synchronous generator is different from asynchronous generator

Synchronous generators can provide active and reactive power, while the asynchronous generator will only provide active power.

61. What are the types of DC generator

DC generators are of two types separately excited and self-excited and the self-excited is divided into series, shunt, and compound.

62. What is called armature reaction

Armature reaction is the result of armature flu to the main flux. The armature flux may oppose or support the main flux.

63. What is a two-phase motor

A two-phase motor is a motor that has the starting winding and running and has a phase split. An example of this is the servo motor.

64. What is reverse power relay

They are used for the protection of generation, the function of generating station is to provide power to the grid. In some cases, the generation station may be off and there won’t be any power so the plant may take power from the grid. So to prevent this we use reverse power relay.

65. What is HRC fuses and where is it used

The full form of HRC is high rupturing capacity, this type of fuse is used in the distribution system for electrical transformers.

66. Why do we use high voltage for power transmission

To reduce transmission loss

67. What are the different method for starting an induction motor

The different method to start an induction motor is direct online starter, star delta starter, auto transformer starter, resistance starter, and series reactor starter.

68. What type of AC motor is used in the ceiling fan

Single-phase induction motor is used

69. Which motor has high starting torque and current DC, induction or synchronous motor

High starting torque is for DC motor, the synchronous and induction motor cannot be started on load,

70. What is ASCR cable and where is it used

ASCR means aluminum conductor steel reinforced and they are used for transmission and distribution purpose

71. Describe Marx circuit

It is used in generators so that, the capacitor in parallel can be charged and discharges them in series. They are used if the required voltage for testing is higher than the available.

72. What will be the power factor of an alternator if there is no load

Angle difference is created by the synchronous impedance of the alternator and because of this, the PF would be zero.

73. Why temperature test is needed for busbars and isolators

These devices are used in continuous power flow, so they carry heavy currents which will increase their temperature. So temperature test is needed.

74. Describe automatic voltage regulator

They are used in the synchronous generator, AVR can control the output voltage of the generator by controlling the excitation current. So it can control the output reactive power of the generator

75. If we give high volt DC to a bulb, what will happen

When the DC supply is applied to the bulb, because of the low resistance of the bulb the current through the bulb would be very high, so that it may damage the bulb.

76. Why bundling of conductors is done

The bundling of conductors is done to reduce reactance

77. The impedance characteristic of a transmission line is depended upon

It is depended upon the geometrical configuration of the conductor.

78. What is the function of guard ring transmission line

It reduces the earth capacitance of the lowest unit

79. What kind of relay would be used for long transmission lines

Mho’s relay

80. When could be the chances of corona will be maximum

The corona chances are maximum during humid weather

81. When will be the shunt capacitance neglected

It would be neglected during the analysis of short transmission lines

82. When will be the power transmitted is maximum

The power transmitted would be maximum, when sending end voltage is more

83. Why is alternating current is transmitted at high voltage

It is transmitted at high voltage to minimize the losses

84. What will happen if the induction motor runs at synchronous speed

There won’t be any induced EMF in the rotor circuit if the induction motor runs at synchronous speed. So there won’t be any rotor current and no torque is developed. So the induction motor will stop.

85. What is ground clearance and phase clearance in a substation

The minimum distance between the live conductor and the earth or the ground, while the phase clearance is the clearance between different phases in a circuit or the same phases in different circuits.