# 100+ Electrical Engineering Interview Questions

1. What is Electrical Engineering?

It is the study and application of electricity, electronics, and electro mechanism.

2. What is a capacitor?

It is an electrical component that opposes the flow of current and behaves as a passive element. It also accumulates some kind of charge when an electric potential is applied.

3. What is Capacitance?

Capacitance is referred to as an amount of charge stored inside a capacitor at a specifically given voltage

4. What is an inductor?

This is an electrical component used to create electrical circuits and store energy in the form of magnetic fields. The inductor is also called a choke or a coil.

5. What is Inductance?

It is a coil’s property to resist any changes in the electric current passing through it. Mutual conductance occurs while the secondary coil opposes the current change in the primary coil.

6. Name some tools essential for the measurement of electrical parameters in an electrical circuit?

• Voltmeter

• Ammeter

• Ohmmeter

• Multi-meter

• Power meter

• Microwave meter

• Cathode Ray Oscilloscope

• Signal Generators and Analyzers

• Wattmeter

• Sweep Generator etc.

7. What is Ohm’s law?

Ohm’s law states that the voltage across a conductor is directly proportional to the current flowing through it

V=IR

Where

V= Voltage

I= Current

R= Resistance

8. What are the limitations of ohm’s law?

Ohm’s law condition is resistance always be constant which also depends on temperature. In the non-linear and unilateral circuits, the resistance value varies depending on the temperature. so, we cannot apply Ohm’s law to unilateral circuits and non-linear circuits.

9. Can you apply ohms law to all types of electrical conductors?

No, Ohm’s law can be applied only if the conductor is metal or in electrolytic conduction. It cannot be applied in the case of conduction through ionized gases or semiconductors.

Ex: Vacuum tubes, Discharge tubes.

10. What is a linear circuit?

The relation between the current and voltage is directly proportional to each other i.e. linear. The parameters of the circuit such as resistance, frequency, capacitance, and inductance remain stable with the varying current & voltage

11. What is a non-linear circuit?

In the non-linear circuit, there is no linear relationship between current and voltage. The electrical parameters vary with the change of voltage and current value.

12. What is active power?

It’s the power that actually drives the device and does useful work. Power is measured in kilowatts (kW) or MW.

13. What is reactive power?

The continuous bouncing power between source and load is called reactive power. It is denoted by Q and its unit is the reactive volt VAR.

14. What is apparent power?

It is the product of voltage and current without reference to the phase difference between them. It is the active reactive power combination. It is rated S & its unit is VoltAmpere (VA).

15. What is complex power?

It is the product of the potential difference and the amperage versus the phase difference between them. it is the complex sum or vector sum of the active and reactive power. It is represented by S & measure in VAR.

16. What is a starter?

A starter is a device that is used to on or off an electric motor or motor-controlled electrical equipment while providing overload protection.

17. What is a motor starter?

It is a device that is used to reduce the starting current to protect the motor’s windings and gradually increase current after starting the motor. It connects series with the motor and it also provides protection from overload issues.

18. What is a magnetic starter?

It is a starter that is designed to give protecting method while starting electrical motors with a heavy load. A contactor is considered as the main component during power-cutoff, low voltage, and protection from overload.

19. What is armature reaction?

It is a magnetomotive force set up by the current induced in the armature of a dynamo that results in altering as to both magnitude and direction of the flux due to the field magnet.

20. What is a circuit breaker?

It is a fuse-like device used to protect the device by controlling the current flow. It automatically cut off or breaks the circuit and stops the flow of current if any fault occurs such as a short circuit or overload. We can also control it manually. It can operate in both ON and Off conditions so it is referred to as On-load and Off-load devices.

21. What is an isolator?

It is a mechanical switch used for disconnecting the power supply in substations. It is operated when the power supply is off so, it is called an off-load device

22. What is the principle of motor?

Whenever a current-carrying conductor is placed in a magnetic field it produce turning or twisting movement which is called torque.

23. What is the difference between a synchronous generator & an asynchronous generator?

The asynchronous generator supplies both active and reactive power but the asynchronous generator supplies only active power and observes reactive power for magnetizing. These types of generators are used in windmills. The asynchronous generator is also called an induction generator.

24. What is MCB?

The MCB stands for “miniature circuit breaker” & it is used for current rating lower than 100 amps with interrupting ratings of below 18k Amps. Its tripping characteristics cannot be adjusted & they are used for domestic purposes.

25. What is MCCB?

The MCCB stands off “Molded case circuit breaker”. It has a high current rating of around 2500 Amps, where its interrupting ratings are in-between 10k to 200k Amps. Also, its tripping characteristics can be adjusted. They are used in industries.

26. What is the difference between generator and alternator?

A generator & an alternator both convert mechanical energy into electrical energy. In a generator, the armature rotates inside a fixed magnetic field. In an alternator, the magnetic field rotates inside conductor windings (stator).

27. What is a semiconductor?

A semiconductor is a substance neither conductor nor an insulator. It is a substance that shows the character in between them

28. What are the types of semiconductors?

• Intrinsic semiconductor

• Extrinsic semiconductor

29. What is an intrinsic semiconductor?

An intrinsic semiconductor is a semiconductor that has a same number of electrons & holes. They exist in their natural form.

30. What is an extrinsic semiconductor?

This semiconductor is made by adding foreign impurities either electrons or holes artificially. The process of adding impurities is called doping.

31. What is an N-Type semiconductor?

The number of free electrons is increased during the doping process in this type of semiconductor. Hence the majority carriers are electrons in N-type.

32. What is a P-type semiconductor?

The number of holes is increased during the doping process in this type of semiconductor. Hence the majority of carriers are holes in P-type.

33. What is a transistor composed of?

Basically, a transistor is a combination of P-type & N-type semiconductors. The doping, combination, and different shapes of semiconductors form different types of transistors with various electrical properties.

34. What is the role of a transistor in a Circuit?

Switching: To switch ON/OFF the flow of current in a circuit. This switching will occur based on the input voltage or current. Amplification: Transistor also works as an amplifier that amplifies the low power input signal into a high power signal.

35. What is a Zener diode?

Zener diode is a semiconductor device that allows current to flow in both forward and reverse directions when a specified voltage is reached. It is most commonly used as a voltage regulator in reverse biased mode. The breakdown voltage in this diode when connected in the reverse-biased mode is called Zener voltage.

36. What is a laser diode?

The Laser diode converts electrical energy into light energy like an LED but it creates coherent light. It is a PIN junction diode that creates a beam of light with the help of holes & electrons combined in the intrinsic region. These types of diodes are used in presentation pointer, optical communication, printer, etc.

37. Types of DC Generator

• Self-excited D.C Generator

• Separately excited D.C. Generator

38. What is the difference between a four-point starter and a three-point starter?

In a four-point starter, the shunt connection is provided separately from the line.

In a three-point starter, the shunt connection is connected with the line, this is a drawback in three-point starter

39. What is Marx Circuit?

Marx generator or Marx circuit is used to generating the high voltage from the low voltage source.

40. Working principle of Marx circuit

The circuit achieves high voltage current from low voltage by charging a large number of capacitors in parallel and discharging the current in series.

41. What is the difference between a surge arrestor and a lightning arrestor?

A lightning arrestor is used to ground the effect of lightning and it is installed outside of the building.

Surge arrestors are installed inside the panel to nullify the surge effect

42. Why do the capacitors work on ac only?

Usually, capacitors act as an infinite resistor to DC components that mean it blocks the flow of current from D.C components.

43. What is the frantic effect?

The Ferranti Effect is a voltage increase in the receiving end of a transmission line when it is operated in a low-load or no load condition. This results in a receiving end voltage value being higher than the sending end.

44. What is meant by insulation voltage in cables?

It is the property of a cable up to which it can withstand the applied voltage without getting damaged. The voltage level at which the cable gets damaged is known as the insulation voltage of the cable

45. Why do we use 3 phase supply? Why not 4 or something else?

Already we use the 3 phase power generation and delivery of 3 phase power is more efficient than 2 phase, If the number of phases increases, the efficiency also increases and it affects the installation cost of transmission.

46. What are the advantages of a star-delta starter with an Induction Motor?

Reduction of current during starting of motor,

Starting current reduced to 3-4 times,

Voltage Drop during the starting of the motor is reduced.

47. Why are Delta-Star-Transformers used for lighting loads?

The primary winding is Delta & the Secondary winding is Star.

In lighting loads, a neutral conductor is a must and hence the secondary winding is star winding, and this lighting load is always unbalanced in all three phases. To minimize the unbalanced current in the primary we have to use delta winding.

48. What is 2 phase motor?

A two-phase motor system consists of two voltages that are set 90 degrees apart.

The alternator is composed of two windings placed 90 degrees apart from each other. They require 2 live and one ground wire that works in two phases.

1st live wire increases the current up to 240v for the motion, and 2nd live wire maintains the fluidity of the current for the use of the motor.

This type of motor is no longer used in the industry.

49. What is AVR (Automatic Voltage Regulator)?

It controls the output voltage of the generator and keeps stable output voltage by controlling its excitation current & also controls the output reactive power of the generator.

50. What is the function of anti-pumping in circuit breakers?

The anti-pumping is a function that is used to prevent multiple breaker closures. For instance, if the operator applies the close command to the breaker by pressing the close button and the breaker closes. However, some system failure causes the breaker to trip. Since the close command is still latched, there is a good chance of the breaker trying to close again and being tripped by the relay multiple times. This will rupture the closing mechanism of the breaker.

The anti-pumping relay prevents this by ensuring that the breaker close can happen only once for one close command issued from the control panel.

51. Types of cooling transformers?

ONAN (Oil Natural, Air Natural)

ONAF (Oil Natural, Air Forced)

OFAF (Oil Forced, Air Forced)

ODWF (Oil Direct, Water Forced)

OFAN (Oil Forced, Air Natural).

52. What do you mean by closed circuit?

The complete electric circuit where the current will flow, when the voltage is applied.

53. What is voltage Drop?

Voltage drop is defined as the decrease of electrical potential along a conductor or in an apparatus through which a current is passing against its resistance.

54. What happens when two positively charged material is placed together?

It will repel.

55. What is referred to as the electron in the outer orbit?

The outermost orbital shell of an atom is referred to as the valence shell, and the electrons that reside in the valence shell are called valence electrons

56. What is a Vacuum Circuit Breaker?

A circuit breaker breaks the circuit by opening the contact terminals. During the opening of a circuit breaker, an arc is generated between the terminals this can be quenched using various methods. In VCB, the medium for arc quenching is in a vacuum. The vacuum has a high voltage arc quenching ability as compared to air & they are used for high voltage circuits.

57. What is the composition of transformer oil?

Mineral oil is free from fatty acids and the chemical name is Hydrotreated light naphthenic distillate. It’s available in different grades like EHV GR-1, EHV GR2 depending upon values of dielectric strength, compositions, neutralization values, and flashpoint.

58. What is the function of transformer oil?

Provide cooling & proper insulation.

59. Why is thyristor SCR considered a charge-controlled device?

A thyristor is triggered into a forward conduction state where the charge is injected by applying the gate signal (voltage between gate & cathode terminal). As SCR is switched on by the current or charge it is known as charge controlled device.

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

This pin is longer because this pin will be the first pin to make a connection and this earth pin will be the last to disconnect, This assures the safety of the person.

It works on the R= pL/A principle

Where area (A) is inversely proportional to resistance (R ).

As per the above formula when the area increases, resistance decreases. If resistance is less the leakage current will take a low resistance path and hence the earth pin should be thicker.

61. If one lamp connects between two-phase it will glow or not?

Case 1:

If the voltage difference between the two phases is equal to the specified lamp voltage, then the lamp will glow.

Case 2:

If the voltage difference between the two phases is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

62. Two bulbs of 100 watts and 40 watts respectively connected in series across a 230V power supply, which bulb will glow bright and why?

Two bulbs are connected in series they will get an equal amount of electrical current but the supply voltage is constant across the bulb, So the resistance of the 40Watt bulb is greater and the voltage across 40W is more, so the 40W bulb will glow brighter.

63. How to check the capacitor with the use of a multimeter?

A shorted capacitor will clearly show very low resistance

An open capacitor will not show any movement on Multimeter.

A good capacitor will show low resistance initially and resistance gradually increases this shows that the capacitor is not bad.

While shorting two ends of the capacitor, it momentarily can give a weak spark, to know the value and other parameters we need better instruments.

64. Which type of AC motor is used in the fan?

A Single-phase induction motor is used, in which mostly squirrel cage rotor and a capacitor start and run AC motors.

65. Why transmission line 11KV or 33KV, 66KV not in 10KV, 20KV?

When an alternator generates voltage, we always use a multiple of 1.11 for a pure sine wave.

Form factor = Voltage Rms/Average voltage = 1.11

And also, some voltage drops due to the resistance of the transmission line, for this reason, they transmit 10% extra voltage for example 10KV* 10%=11KV

66. Can a 3-phase motor be run on a single-phase line?

Yes, it can be but a phase splitter is essential.

67. 1ton is equal to how many watts?

1ton=12000 BTU/Hr.

12000BTU/Hr to HP (Horse Power) =4.7161hp

4.7161hp to watts= 3518.82watts (1hp = 746watts)

68. What are the various kind of cables used for transmission?

Low tension cables transmit voltage up to 1000volts

High tension cables transmit voltage up to 23000volts

Super Tension cables transmit 66 KV to 132KV.

69. What is the power factor of an alternator at no load?

The synchronous impedance of the alternator will create an angle difference at no load, so it should be zero lagging like an inductor.

70. Why series motors cannot be started in no-load conditions?

Series motor can’t be started without load, because high starting torque will happen. It is used in trains & cranes (with load only).

71. What is Excitation?

Excitation can be defined as the application of an external voltage to a DC shunt coil in DC motors.

72. What are the different types of bus bar arrangement schemes in substations?

• Single Bus Scheme

• Double Bus Single Breaker Scheme

• Main and Transfer Bus bar scheme

• Breaker and half Scheme

• Ring main Bus Scheme

73. What are the main circuits involved in thermal power plants?

Four main circuits

1. Feedwater and Steam flow circuit

2. Coal and Ash circuit

3. Air and Gas circuit

4. Cooling water circuit.

75. Difference between UPS and inverter?

UPS means uninterrupted power supply, It stores electrical power in a battery during normal conditions when a power failure occurs it provides power to the load (as per load required).

Inverter – It converts DC supply into AC supply.

76. Does the frequency change with increasing voltage 220V to 11KV?

It doesn’t affect the frequency it remains the same because when the voltage increases the magnitude varies, but there is no effect in frequency.

78. What is meant by line loss how do you prevent the losses?

Line loss occurs during transmission of electric power, itis mainly due to copper loss, It can be reduced by increasing voltage by a step-up transformer.

79. What is the difference between AC drive & DC drive?

DC drive consists of a chopping Circuit,

AC drive consists of a converter & Inverter.

80. Difference between Earthing & Neutral?

Earthing is installed in every system to prevent shock to the human body, earthing helps to send leakage current to earth through a low resistance path.

Neutral is used to complete the circuit, it is a non-potential wire, Neutral comes from a transformer which is common to all loads.

81. What is meant by HRC fuse?

HRC means High rupture capacity, it’s a protective device acts as a fuse.

82. What is the form factor?

The ratio of maximum value to its RMS value.

83. What is meant by the power factor?

Power factor is defined as the cosine of the phase difference between the voltage and current.

84. What is a series/parallel connection?

In series connection - current is constant

In parallel connection -voltage is constant.

85. What is an open circuit (or) short circuit?

An open circuit is a faulty condition, where the circuit element offers a resistance approaching infinite.

A short circuit is a faulty condition, where the circuit element offers a resistance approaching zero.

86. What are the parameters that affect the resistance of the wire?

Length of the wire

Area of the wire

Resistivity depends on the type of material/Temperature.

87. If a current of 5A flows for 2 minutes find the quantity of electricity transferred?

Hence formula, Q=I* t Coulombs

Current =5A, Time=2 minutes, convert 2minutes in to seconds (1minute=60seconds)

2 minutes*60 = 120 seconds.

Q= 5*120 = 600 Coulombs.

88. Difference between conductors and insulators?

The conductor is a material having a low resistance which allows electric current to flow in it,

All metals are conductors (Copper, Aluminum, Brass, Platinum, Silver, Gold, and Carbon)

An insulator is a material having a high resistance that doesn’t allow the electric current to flow in it. Insulators include Plastic, Rubber, Glass, Porcelain, Air, Paper, Cork, Mica, Ceramics and certain oils.

89. List four ways to maximize the reliability of your rotating equipment?

Be vigilant of changes

Perform regular walk through or rounds.

90. What is one psi?

Psi means pound-force per square Inch

Approximately equal to 6895 pascals.

Approximately 0.43 feet of water.

91. What will happen if a DC power supply is given on the primary of a transformer?

Mainly transformer has high inductance and resistance is low, In case DC power supply is given, there is no inductance, only resistance will act in the circuit, so high electrical current will flow through the primary side of the transformer, so, for this reason, coil and insulation will burn out.

92. What is Autotransformer?

In this transformer, only one winding is used in primary and secondary. Also primary and secondary are conductively coupled.

93. What happens to the resistance of a series circuit if the current is reduced by half?

If the current in a series circuit is reduced to half then, the resistance doubles.

94. What do you mean by “Eddy current”?

Eddy currents are loops of electrical current that can be induced within conductors by a modifying magnetic field in the conductor according to Faraday’s law of induction.

95. Why, when birds sit on transmission lines or current wires don’t get shocked?

When Birds touch the Single line (phase or neutral) they don’t get an electrical shock, If a bird touches both lines then the circuit is closed and they get an electrical shock, so if a human touches a single line phase then he doesn’t get shocked if he is in the air(not touching /standing) on the ground. If he is standing on the ground without insulation and touching the line he will get shocked because the ground on what we standing is like a line(ground bed-like neutral) and in most of the electric lines the neutral is grounded so that means a human who touches the line closes the circuit between phase and neutral.

96. Which MCB is best for 1.5ton A.C?

97. What can trip an MCB?

Three reasons to trip an MCB

• Short Circuit

• Ground Fault

98. How to select an MCB?

• Rated current

• Tripping characteristics

• Number of poles

• Breaking Capacity

• Rated operational voltage

• Insulation voltage

• Energy class

• Dimensions

• Mechanical and electrical life.

99. What is the advantage of Fuse?

• The cost of this protective device is very low

• It requires no maintenance

• It interrupts heavy current without noise or smoke

• Smaller size fuse element will imply a current-limiting effect under short circuit.

• The minimum time of operation can be determined by selecting the proper material of the fuse wire.

• The inverse time-current characteristic makes it suitable for overcurrent protection.

100. What are the advantages of speed control using thyristor?

Some of them are low cost, highly accurate, fast switching characteristics than BJT, IGBT.

101. Difference between core type and shell type?

102. Why tappings are required on transformers?

Tappings are required to compensate for changes in the applied voltage on the bulk supply transformer.

It is required to compensate for regulation within the transformer and maintain the output voltages constant.

103. What is Slip?

Slip is defined as the difference between an electrical induction motor’s synchronous and asynchronous speed.

104. When using a potential transformer, where should the voltmeter be connected?

The voltmeter should always be connected to the secondary winding.

105. Explain the Energy meter?

Electrical energy is generally measured in Kilowatt-hour, the meter used to measure electric energy is called the energy meter. An inductive type energy meter is mostly used for the measurement of energy in domestic and industrial AC Circuits.

106. What is a delta connection?

Delta connection is an arrangement for connecting a three-phase load in a three-phase network.

107. What are the effects of temperature on resistance?

When temperature varies, the resistance of materials varies accordingly. In most cases, when the temperature of a material goes up, its resistance also goes up but with some other materials if the temperature goes up their resistance goes down. When the resistance of material goes up by an increase in temperature, it has a positive temperature coefficient, In case of Silver, Copper, and Aluminum In certain alloys like Eureka, Manganin, etc., the increase in resistance by the increase in temperature is relatively less and irregular.

108. How is the three-phase or single-phase supply system selected?

The supply system is selected according to the connected load in the installation. If the connected load is below 3000 watts, then a single-phase supply will be enough. If the connected load exceeds 3000 watts, then a three-phase supply should be selected.

109. Explain briefly about the Star-Delta starter?

It is commonly used in a three-phase induction motor, in star-delta starting an induction motor is connected in through a star connection throughout the starting period, then once the motor reaches the required speed, the motor is connected in through a delta connection.

110. What is the function of flux in soldering?

• For cleaning the dirt on the soldering surface

• For lowering the melting temperature of the solder

• For the easy flow of the solder into the joint.

111. How does the Diesel generator startup system work?

DG (Diesel Generator) is normally controlled by DG Controllers which have manual and auto startup functions of DG.

Moreover, DG start-up through starter motor which will start the engine first and after engine reached rated speed starter motor cut off automatically.

112. How many running hours after DG should take maintenance?

The First DG maintenance should take after 50 hours from initial commissioning.

As a thumb rule, periodic DG maintenance should take place after every 400 running hours.

113. Specify the current rating mentioned below?

Machine Current Rating Further Information

A full load current is the largest current that a motor or other device is designed to carry under particular conditions.

115. How to select the motor overload relay range?

10 HP, 3-Phase Induction motor, convert 10Hp to KW=10*0.746=7.46KW

10 HP motor current rating =1.3*10=13A (or) Take nameplate mentioned rating if in site,

Range selection = ± 10% of current Rating.

FLC=Full load current, assume pf=0.85, efficiency=0.90(It will be in the nameplate of the motor)

Full load current = p (watt)/v1.732pfefficiency = 7.461000/4151.7320.85*0.90 =7460/549.86 =13.6Amps. (It is only approximate)

Safety Factor of the overload Relay = ± 10% * Full load current, Some say ± 20%* Full load current

So overload relay must be in the range of 13A-15A for 10HP motor for 10% Safety Factor

So overload relay must be in the range of 11A-17A for 10HP motor for 20% Safety Factor

For short cut: 1KW =1.75A, If the interviewer asks what is full load current of 10KW?

10KW *1.75 = 17.5A (it is only approximate).

116. Why do we use the 1.73 constant for three-phase?

In a three-phase circuit, the use of the constant 1.732 results from the fact that not all three-phase are producing the same amount of power at the same time, suffice it to say that correct power from a three-phase system at any point in time is found by multiply by the square root of 3.

117. How to calculate the current carrying capacity of the cable?

Current carrying capacity depends upon the conductor what you use

Example: - Assume conductor as copper wire cable

Note: Multiply Factor

Less than or equal to 16 sq. mm wire, current capacity =4.5*(cable size).

16-35sq.mm wire =3.5*(cable size).

35-50sq.mm wire =2.5*(cable size).

Greater than 100 sq.mm=1.5*(cable size).

118. What is a relay?

Relay are switch that open and close circuits electromechanically or electronically. Relay controls one electrical circuit by opening and closing contacts.