Top 60+ Electrical Engineering Interview Questions & Answers [2025 Edition]

1. What is Electrical Engineering?

Electrical Engineering is the field of engineering that generally deals with the study and application of electricity, electronics, and electromagnetism. It generally involves high voltage AC above 110V or 220V. Electrical components and devices typically use AC voltages and tend to be larger, requiring 230V (UK) or 110V (US) single-phase AC voltages. In industries and power stations, voltages can be up to 11kV, and for transmission, higher than 400kV. For a deeper understanding of industrial systems, you can explore our electrical system assessment services.

2. Why are Transformers Rated in kVA, Not in kW?

Transformers are rated in kVA because the manufacturer doesn’t know what kind of load will be connected. The load can be resistive, inductive, capacitive, or mixed, leading to different power factors. Real power output varies with the power factor. Therefore, manufacturers rate transformers based on the voltage and current they can handle (VA) rather than real power (W).

3. What is an Ideal Transformer?

An ideal transformer is a theoretical transformer with no losses, meaning input power equals output power (100% efficiency). In reality, all transformers have some losses.

• Transformer Input Power = Transformer Output Power
• 

4. What are Transformer Losses & Their Types?

Real transformers have the following losses, which are critical to understand for effective transformer maintenance:

• Copper Losses (Winding Resistance): Due to current flowing through windings causing resistive heating ().
• Core or Iron Losses:
  • Hysteresis Losses: Due to magnetization and demagnetization of the core as the magnetic field reverses.
  • Eddy Current Losses: Due to currents generated inside the core causing resistive heating.
• Other Losses: Stray losses (leakage flux), dielectric loss, magnetostriction losses, mechanical losses. For specialized oil treatment, see our transformer oil filtration service.

5. What is Transformer Efficiency & All-Day Efficiency? Condition for Maximum Efficiency?

• Transformer Efficiency (): Output Power / Input Power.
• All-Day Efficiency: Ratio of energy delivered (kWh) to energy input (kWh) over 24 hours. = Output in kWh / Input in kWh.
• Condition for Maximum Efficiency: Copper Loss = Iron Loss (). To ensure your assets maintain peak efficiency, consider our comprehensive transformer maintenance services.

6. What is Voltage Regulation of a Transformer & Why is it Important?

Voltage regulation is the percentage change in secondary voltage from no-load to full-load. Ideally, it’s zero. It indicates the transformer’s efficiency; a lower voltage regulation is preferred.

7. Why Should a Current Transformer’s Secondary Not Be Open When Primary Current Flows?

A current transformer (CT) acts as a step-up transformer for voltage. If the secondary is open while primary current flows, the primary current becomes magnetizing current, generating a dangerously high secondary voltage that can damage insulation and harm personnel.

8. How to Identify Inductive, Capacitive, or Purely Resistive Circuits?

• Inductive: Imaginary part of total impedance is positive. Current lags voltage.
• Capacitive: Imaginary part of total impedance is negative. Current leads voltage.
• Resistive: Imaginary part of total impedance is zero. Current is in phase.

9. What is a Unilateral & Bilateral Circuit?

• Unilateral Circuit: Properties change with the direction of current/voltage (e.g., diode).
• Bilateral Circuit: Properties do not change with the direction of current/voltage (e.g., transmission line).

10. What is a Linear & Non-Linear Circuit?

• Linear Circuit: Current and voltage have a linear relationship (directly proportional). Parameters (R, L, C, frequency) remain constant.
• Non-Linear Circuit: Current and voltage do not have a linear relationship. Parameters change with voltage/current. Understanding these parameters is essential when deploying harmonic filtering solutions in complex networks.

11. What are the Limitations of Ohm’s Law?

Ohm’s law applies only when resistance and temperature are constant. It is not applicable to:

• Unilateral circuits
• Non-linear circuits
• Circuits where resistance varies with voltage/current or temperature changes.

12. What is the Maximum Power Transfer Theorem?

In a linear, bilateral network, maximum power is transferred from source to load when the external load resistance equals the internal resistance of the source (or Thevenin’s resistance).

13. What is Active, Reactive, Apparent & Complex Power?

• Active Power (P): Actual power delivered to the load and dissipated (Watts, W).
• Reactive Power (Q): Power bouncing back and forth between source and load (Volt-Ampere Reactive, VAR).
• Apparent Power (S): Product of voltage and current magnitude (Volt-Ampere, VA). .
• Complex Power (S): Vector sum of active and reactive power (VAR). Accurate measurement of these values is vital for energy monitoring solutions.

14. What is Leading & Lagging Power Factor & How Can You Improve It?

• Leading PF: Current leads voltage (capacitive load).
• Lagging PF: Current lags voltage (inductive load).
• Correction Methods:
  • Static Capacitor: Provides leading current to neutralize lagging component.
  • Synchronous Condenser: Over-excited synchronous motor running at no load.
  • Phase Advancer: AC exciter connected to an induction motor’s shaft.

15. Why Do We Improve Power Factor?

• Reduce large line losses ().
• Decrease kVA rating and size of electrical equipment (PF inversely proportional to kVA).
• Reduce conductor size and cost.
• Improve voltage regulation and reduce voltage drop.
• Increase system efficiency.

16. Why Are Motors Rated in kW Instead of kVA?

Motors have a defined power factor and convert electrical energy into mechanical power (active power) at the shaft. Therefore, they are rated in kW or HP (active power).

17. What is a Motor Starter / Magnetic Starter?

Motor Starter: A device connected in series with a motor to decrease starting current, start/stop the motor, and provide overload protection.

Magnetic Starter: A device for safely starting heavy-load electric motors, including a contactor for power cut-off, under-voltage, and overload protection. For advanced motor protection, consider exploring motor monitoring solutions.

18. Why is Battery Rating in Ah (Ampere-hour) and Not in VA or Watts?

Batteries store chemical energy converted to DC electricity. Rating in Ah indicates the current supplied over time. Since DC has no phase/frequency (no reactive power), VA/Watts isn’t used.

19. What is a Primary & Secondary Cell?

• Primary Cell: Non-rechargeable (disposable). Used in toys, remotes.
• Secondary Cell: Rechargeable. Used in phones, cars.

20. What is the Difference Between Circuit Breaker & Isolator?

• Circuit Breaker: Protective electromechanical device (ON/OFF load) that automatically breaks circuit on fault (overload/short circuit). Can be operated manually.
• Isolator: Mechanical switch (OFF load) used to isolate power supply for maintenance. Operated only when power is off. For high-voltage distribution, specialized recloser maintenance service is often required.

21. What is the Difference Between MCB & MCCB?

• MCB (Miniature Circuit Breaker): Rated <100A, interrupting rating <18kA. Fixed tripping characteristics. Domestic use.
• MCCB (Molded Case Circuit Breaker): Rated up to 2500A, interrupting rating 10k-200kA. Adjustable tripping characteristics. Industrial use. These are critical components in low voltage electrical cabinet maintenance.

22. What is the Difference Between a Generator and an Alternator?

• Generator: Stationary magnetic field, rotating armature.
• Alternator: Rotating magnetic field, stationary armature (stator).

23. Define Capacitance and Inductance

• Capacitance (C): Ability to store charge between plates with a potential difference (Farads, F).
• Inductance (L): Ability of a conductor to oppose change in current, generating a magnetic field (Henry, H).

24. What Do Different Colors on Wires Indicate?

Colors indicate phases, neutral, and earth. Codes vary globally, but earth is typically green with yellow stripes.

25. How Do You Decide What Size of Electrical Wire You Need?

Depends on load current, voltage, and cable length. Voltage drop shouldn’t exceed 2.5%.

1. Measure supply voltage.
2. Calculate allowable drop (2.5%).
3. Measure cable length.
4. Calculate max allowable voltage drop per meter.
5. Match load current and drop/meter with cable charts.

26. What are the Types of Semiconductors?

• Intrinsic: Pure form, equal electrons and holes.
• Extrinsic: Doped with impurities.
  • N-Type: Doped with electrons (majority carriers).
  • P-Type: Doped with holes (majority carriers).

27. What is the Role of a Transistor in a Circuit?

Composition: Combinations of P-type and N-type semiconductors (e.g., PNP, NPN).

• Switching: ON/OFF control of current.
• Amplification: Increasing signal power.

28. Current in Series Circuit if Resistance Doubles?

What Could Be the Reason for Current to Double in a Linear Circuit?

• Total resistance halved.
• Supply voltage doubled.

29. What is Meant by Reverse Polarity and How Can It Be Fixed?

Neutral connected to hot terminal. Creates shock hazard as switch cuts neutral, leaving appliance live. Fix by swapping wires. For general home and office safety, check our guide on Socket vs Outlet vs Receptacle.

30. Explain Rectifiers and Their Types

Rectifier converts AC to DC.

• Uncontrolled: Output depends only on input (diodes).
  • Half-wave: Converts one half-cycle.
  • Full-wave: Converts both half-cycles (Bridge or Center-tap).
• Controlled: Output controlled by firing angle (SCR/Thyristors).
  • Half-controlled: SCR + Diodes.
  • Full-controlled: All SCRs.

31. What is a Zener Diode?

Diode that conducts in reverse direction upon reaching Zener voltage. Used as voltage regulator or for overvoltage protection.

32. Difference Between Analog and Digital Circuit

• Analog: Processes continuous signals.
• Digital: Processes discrete signals (binary 1/0).

Laser Diode: PIN junction diode converting electrical energy into coherent light.

33. Difference Between Fuse and Breaker?

• Fuse: Metal wire melts on overcurrent (one-time use).
• Breaker: Electromechanical switch opens on fault (reusable/resettable).

34. What is a Marx Circuit?

Circuit charging capacitors in parallel and discharging in series to generate high voltage from a low voltage source.

35. Principle of Motor & Highest Starting Torque Motor?

• Principle: A current-carrying conductor in a magnetic field experiences a mechanical force.
• Highest Torque Motor: DC Series Motor. Used in cranes, hoists.

36. What is ACSR Cable?

Aluminum Conductor Steel Reinforced. Aluminum outer strands for conductivity, steel center for strength. Used in overhead transmission.

37. What is a Vacuum Circuit Breaker (VCB) & Why use it for HT?

Circuit breaker using a vacuum as the arc quenching medium. Used in high voltage circuits due to high quenching ability. Proper medium voltage cabinet maintenance services are essential for these devices.

Why VCB vs ACB? VCB (Vacuum) has faster arc quenching/recovery than ACB (Air). VCB used for High Tension (HT), ACB for Low Tension (LT).

38. Difference Between Surge Arrestor and Lightning Arrestor?

• Surge Arrestor: Internal protection against voltage spikes.
• Lightning Arrestor: External protection (e.g., towers) against lightning strikes.

39. What is Insulation Voltage in Cables?

Rated voltage a cable can withstand for its lifespan without breakdown. Monitoring this can be done via insulation monitoring solutions.

40. Advantages of Star-Delta Starter?

• Reduces starting current (high starting current can damage windings).
• Eliminates voltage drop issues on consumer lines.
• Simple operation, low cost.
• Good torque/current performance.

41. Resistance Grounding vs Resistance Earthing?

• Resistance Grounding: Load neutral connected to ground via resistor to limit fault current (unbalanced conditions).
• Resistance Earthing: Protects equipment by carrying fault current through earth cable.

42. What is Critical Disruptive Voltage?

Minimum voltage for air insulation breakdown between phase and neutral (start of corona discharge). This can be detected early using online partial discharge measurement service.

43. Types of Faults in 3-Phase Power System?

• Open Circuit: Single/Double/Three-phase open.
• Short Circuit: LG (Line-Ground), LLG, LL, LLL (Three-phase short). Detecting these early often requires thermal scan electrical cabinet service.

44. What is Skin Effect?

AC current density is high near conductor surface, low in middle. Increases with frequency. Reason for using stranded conductors.

45. SCR: Latching vs Holding Current?

• Latching Current: Min initial current to maintain ON state after gate signal removal.
• Holding Current: Min current to maintain ON state (drop below turns OFF). Latching > Holding.
• Why Charge Controlled? Triggered into conduction by charge injection via gate signal.

46. What is Knee Point Voltage & Reverse Power Relay?

• Knee Point Voltage: Voltage where a small increase (e.g., 10%) causes a large increase in magnetizing current (e.g., 50%) in a CT.
• Reverse Power Relay: Protects generator from motoring (taking current from grid) which can damage it.

47. What Happens if DC Supply is Given to Transformer Primary?

Transformer acts as inductor (high L, low R). DC has zero frequency -> no reactance (). Only low resistance limits current -> huge current flows -> coil burns/explodes.

48. Motor Starting Methods & Cable Types?

Methods for Starting Induction Motor: DOL (Direct Online), Star-Delta, Autotransformer, Resistance, Series Reactor.

Types of Cables for Transmission:

• LT: <1000V
• HT: Up to 11kV
• Super Tension: Up to 33kV
• Extra High Tension: Up to 66kV
• Extra Super Tension: Up to 132kV

49. What is Slip of an Induction Motor?

Percentage difference between synchronous speed () and rotor speed (). . Rotor speed always < Synchronous speed.