Electrical Maintenance Engineering Interview Questions and Answers -1


comprehensive list of 100 Electrical Maintenance Engineering Interview Questions and Answers — ideal for job preparation in industrial, commercial, and utility sectors.

💡 100 Electrical Maintenance Engineering Interview Questions & Answers

⚙️ Section 1: Basic Electrical Concepts

  1. What is Ohm’s Law?
    → ; Voltage is directly proportional to current for a given resistance.

  2. Define Power and its formula.
    → Power (P) = Voltage (V) × Current (I); measured in Watts.

  3. What is the difference between AC and DC?
    → AC changes direction periodically; DC flows in one direction.

  4. What is a power factor?
    → Ratio of real power to apparent power; indicates efficiency.

  5. What causes low power factor?
    → Inductive loads like motors and transformers.

  6. How to improve power factor?
    → By installing capacitor banks or synchronous condensers.

  7. What is apparent, active, and reactive power?
    → Apparent (kVA), Active (kW), Reactive (kVAR).

  8. What is frequency?
    → Number of cycles per second, measured in Hertz (Hz).

  9. What is an electrical load?
    → Any device that consumes electrical power.

  10. What is insulation resistance?
    → Resistance offered by insulation material to current leakage.

🔌 Section 2: Transformers

  1. What is the working principle of a transformer?
    → Electromagnetic induction between windings.

  2. What are the types of transformers?
    → Power, Distribution, Instrument, and Auto-transformers.

  3. What is vector group in transformer?
    → Indicates phase displacement between primary and secondary.

  4. What is Buchholz relay used for?
    → Gas detection and protection in oil-filled transformers.

  5. What are common transformer losses?
    → Copper loss and core (iron) loss.

  6. Why neutral earthing is required for transformers?
    → For system protection and fault current path.

  7. What is percentage impedance?
    → Voltage drop at full load expressed as % of rated voltage.

  8. What causes humming noise in transformers?
    → Magnetostriction effect in the core.

  9. How to test transformer insulation?
    → Using Megger (Insulation Resistance Test).

  10. What is polarity test in transformer?
    → To determine relative direction of windings.

⚡ Section 3: Motors

  1. What is the principle of an induction motor?
    → Electromagnetic induction between stator and rotor.

  2. Difference between squirrel cage and slip ring motor?
    → Slip ring has external resistance; squirrel cage is closed rotor type.

  3. What is motor slip?
    → Difference between synchronous and actual speed.

  4. What causes motor overloading?
    → Mechanical overload, phase imbalance, or voltage drop.

  5. What is a DOL starter?
    → Direct-on-Line starter connects motor directly to supply.

  6. Why use star-delta starter?
    → To reduce starting current.

  7. What is insulation class of motor?
    → Indicates max operating temperature (Class B, F, H).

  8. Why does motor trip on overload?
    → Excess current heating up windings.

  9. What is megger test for motor?
    → Measures insulation resistance between windings and body.

  10. How to check motor direction?
    → Swap any two supply phases.

🔋 Section 4: Generators

  1. Working principle of alternator?
    → Electromagnetic induction — rotor induces voltage in stator.

  2. What is AVR?
    → Automatic Voltage Regulator maintains output voltage.

  3. What causes generator hunting?
    → Sudden load variation or governor instability.

  4. What is synchronization of generator?
    → Matching voltage, frequency, and phase before connecting to busbar.

  5. How to test generator insulation?
    → Megger test and polarization index test.

  6. What are prime movers?
    → Engines that drive generators — diesel, steam, turbine, etc.

  7. Why use neutral grounding resistor (NGR)?
    → Limits fault current during earth fault.

  8. What is exciter in generator?
    → Provides DC excitation to rotor winding.

  9. What are common causes of generator vibration?
    → Misalignment, unbalance, or bearing wear.

  10. What is load sharing?
    → Equal distribution of load among parallel generators.

⚙️ Section 5: Circuit Breakers & Protection

  1. What is MCCB?
    → Molded Case Circuit Breaker for overload & short circuit protection.

  2. What is MCB?
    → Miniature Circuit Breaker for small circuits.

  3. What is ELCB/RCCB?
    → Earth Leakage/Residual Current Circuit Breaker protects from shock.

  4. What is MPCB?
    → Motor Protection Circuit Breaker — protects against overload/phase loss.

  5. Difference between fuse and circuit breaker?
    → Fuse is one-time protection; breaker can be reset.

  6. What is relay?
    → Device that senses fault and triggers breaker.

  7. Types of protection relays?
    → Overcurrent, differential, distance, earth fault, etc.

  8. What is CT & PT?
    → Current Transformer and Potential Transformer for measurement/protection.

  9. What is tripping curve in MCB?
    → Defines response time to overload (B, C, D, Z curves).

  10. Why is earthing important in protection?
    → To ensure safety and limit voltage rise during faults.

🧰 Section 6: Electrical Maintenance

  1. What is preventive maintenance?
    → Regular inspection to prevent failure.

  2. What is predictive maintenance?
    → Using condition monitoring to predict failure.

  3. What is breakdown maintenance?
    → Repair after equipment failure.

  4. What is thermography test?
    → Detects hotspots in electrical panels using infrared cameras.

  5. What is IR value acceptable for motor?
    → Minimum 1 MΩ per kV of rated voltage.

  6. What is continuity test?
    → Checks if electrical path is complete.

  7. What is earthing resistance limit?
    → Below 1 Ω for substation; below 5 Ω for general installation.

  8. How to maintain capacitor bank?
    → Check kVAR output, terminal tightness, and insulation.

  9. What are common causes of cable failure?
    → Overloading, moisture ingress, or poor termination.

  10. What is the purpose of a maintenance schedule?
    → Ensures equipment reliability and safety.

⚙️ Section 7: Cables and Wiring

  1. What is cable derating?
    → Reducing current capacity due to temperature or grouping.

  2. What is cable sizing based on?
    → Current, voltage drop, and short-circuit rating.

  3. What is voltage drop?
    → Loss of voltage along the length of cable due to resistance.

  4. Why use armored cable?
    → For mechanical protection.

  5. What is XLPE cable?
    → Cross-linked polyethylene insulated cable.

  6. How to identify phase sequence?
    → Using phase sequence meter.

  7. What is cable meggering?
    → Insulation resistance test between conductors and earth.

  8. What is lugs and ferrules used for?
    → Proper termination and identification of cables.

  9. What is a busbar?
    → Conductive bar distributing power within panel.

  10. Why aluminum cables are less preferred than copper?
    → Lower conductivity and mechanical strength.

⚡ Section 8: Panels and Switchgear

  1. What is LT panel?
    → Low-tension panel up to 415 V distribution.

  2. What is HT panel?
    → High-tension switchgear above 1 kV.

  3. What is an ACB?
    → Air Circuit Breaker used for LT protection.

  4. What is VCB?
    → Vacuum Circuit Breaker used for 11kV systems.

  5. Why busbar insulation is important?
    → Prevent short circuits and flashovers.

  6. What is SF6 breaker?
    → Uses SF6 gas for arc extinction.

  7. Why control wiring uses 2.5 mm² wire?
    → Standard for control and auxiliary circuits.

  8. What is interlocking?
    → Prevents unsafe operations (e.g., closing breaker under fault).

  9. What is a mimic diagram?
    → One-line representation on control panels.

  10. What are control fuses?
    → Protect control circuits from short circuits.

🔋 Section 9: Power Systems & Safety

  1. What is earthing vs grounding?
    → Earthing for equipment safety; grounding for system stability.

  2. What is touch and step potential?
    → Voltage difference experienced during earth fault.

  3. What is lightning arrester?
    → Protects equipment from high voltage surges.

  4. What is surge protection device (SPD)?
    → Diverts transient voltages to earth.

  5. What is isolation?
    → Disconnecting equipment for maintenance.

  6. What is lockout-tagout (LOTO)?
    → Safety procedure to ensure equipment remains de-energized.

  7. What is arc flash hazard?
    → Energy release during fault — can cause burns or fire.

  8. Why PPE is used?
    → Personal protective equipment ensures worker safety.

  9. What are danger and caution voltages?
    → Danger: >50V AC; Safe: ≤50V AC.

  10. Why neutral and earth should not be connected together in distribution?
    → To avoid circulating currents and voltage rise.

🧠 Section 10: Practical & Troubleshooting

  1. Motor not starting — what to check first?
    → Supply voltage, fuses, overload relay, contactor coil.

  2. Why MCB trips instantly?
    → Short circuit or ground fault.

  3. Why motor runs in reverse?
    → Two phase interchanged.

  4. Transformer overheating cause?
    → Overload, oil level low, or cooling fan failure.

  5. What if voltage dips during motor start?
    → Use soft starter or VFD.

  6. How to identify single phasing?
    → Unequal current in phases or abnormal motor sound.

  7. Panel buzzing sound — reason?
    → Loose connection or magnetic hum from contactor.

  8. Why earthing test performed periodically?
    → To ensure low resistance path is maintained.

  9. Why cable termination fails?
    → Loose connection or improper crimping.

  10. How to ensure system reliability?
    → Follow preventive maintenance schedule, log data, and perform testing.

brief explanation of all 100 Electrical Maintenance Engineering Interview Questions —

⚡ Electrical Maintenance Engineering – 100 Questions with Brief Explanations

⚙️ 1. Basic Electrical Concepts

  1. Ohm’s Law: Voltage = Current × Resistance (V = IR).
  2. Power: Electrical power = V × I (Watts).
  3. AC vs DC: AC changes direction; DC flows one way.
  4. Power Factor: Efficiency of power use (Cos φ).
  5. Low Power Factor Causes: Inductive loads (motors, coils).
  6. Power Factor Improvement: Use capacitors.
  7. Types of Power: kW (real), kVAR (reactive), kVA (apparent).
  8. Frequency: Number of cycles per second (50 Hz in India).
  9. Electrical Load: Any device consuming electricity.
  10. Insulation Resistance: Prevents leakage current; checked by megger.

🔋 2. Transformers

  1. Working Principle: Electromagnetic induction between windings.
  2. Types: Power, Distribution, Auto, Instrument transformers.
  3. Vector Group: Phase difference between primary and secondary.
  4. Buchholz Relay: Detects internal gas faults in oil-filled transformers.
  5. Losses: Copper (I²R) and Iron (hysteresis + eddy current).
  6. Neutral Earthing: Safety and fault current path.
  7. Percentage Impedance: Voltage drop at full load as % of rated voltage.
  8. Humming Noise: Magnetostriction of the core.
  9. Insulation Test: Check winding-to-earth resistance.
  10. Polarity Test: Identifies relative winding connections.

⚡ 3. Motors

  1. Principle: Rotating magnetic field induces rotation.
  2. Squirrel Cage vs Slip Ring: Slip ring allows external resistance control.
  3. Slip: Difference between synchronous and actual speed.
  4. Overload Causes: High load, low voltage, mechanical jam.
  5. DOL Starter: Direct supply connection for small motors.
  6. Star-Delta Starter: Reduces starting current.
  7. Insulation Class: Temperature limit of winding (B, F, H).
  8. Overload Trip: Excess current heating protection.
  9. Megger Test: Checks insulation health.
  10. Motor Direction: Change any two phases to reverse.

⚙️ 4. Generators

  1. Principle: Electromagnetic induction (mechanical → electrical).
  2. AVR: Maintains stable output voltage.
  3. Hunting: Speed oscillation due to load change.
  4. Synchronization: Match voltage, phase, frequency before paralleling.
  5. Insulation Test: Ensures stator and rotor insulation are healthy.
  6. Prime Mover: Engine or turbine drives generator.
  7. Neutral Grounding Resistor: Limits fault current.
  8. Exciter: Provides DC supply to rotor field winding.
  9. Vibration Causes: Misalignment, unbalance, worn bearings.
  10. Load Sharing: Balancing load between generators.

🛡️ 5. Protection Devices

  1. MCCB: Protects high current circuits from overload and short circuit.
  2. MCB: Protects small circuits.
  3. ELCB/RCCB: Trips on earth leakage or shock current.
  4. MPCB: Motor-specific protection from overload and phase loss.
  5. Fuse vs Breaker: Fuse melts once; breaker can reset.
  6. Relay: Detects fault and commands breaker trip.
  7. Relay Types: Overcurrent, earth fault, differential, etc.
  8. CT & PT: Step down current/voltage for metering & protection.
  9. MCB Tripping Curve: Shows how fast it trips (B, C, D, Z).
  10. Earthing: Ensures fault current flows safely to ground.

🧰 6. Electrical Maintenance

  1. Preventive Maintenance: Routine checks to avoid breakdowns.
  2. Predictive Maintenance: Uses data (vibration, IR) to predict failures.
  3. Breakdown Maintenance: Repair after equipment fails.
  4. Thermography: Detects hot spots via infrared imaging.
  5. IR Value for Motors: Minimum 1 MΩ per kV of rating.
  6. Continuity Test: Ensures closed electrical path.
  7. Earthing Resistance Limit: <1Ω for substations, <5Ω for others.
  8. Capacitor Bank Maintenance: Check terminals, leakage, and kVAR.
  9. Cable Failure Causes: Overload, moisture, poor termination.
  10. Maintenance Schedule: Ensures reliability and record keeping.

🔌 7. Cables & Wiring

  1. Cable Derating: Reduce current capacity due to heat/grouping.
  2. Cable Sizing: Based on current, voltage drop, and fault level.
  3. Voltage Drop: Loss due to resistance over length.
  4. Armored Cable: For mechanical protection.
  5. XLPE Cable: Cross-linked polyethylene insulation.
  6. Phase Sequence: Order of supply phases (R-Y-B).
  7. Cable Meggering: Check insulation resistance between cores.
  8. Lugs/Ferrules: Proper connection and identification.
  9. Busbar: Conductive bar distributing power.
  10. Copper vs Aluminum: Copper has better conductivity.

⚙️ 8. Panels & Switchgear

  1. LT Panel: Controls power below 415V.
  2. HT Panel: Handles above 1kV, up to 33kV.
  3. ACB: Air Circuit Breaker for LT power.
  4. VCB: Vacuum Circuit Breaker for 11kV protection.
  5. Busbar Insulation: Prevents flashover and short circuit.
  6. SF6 Breaker: Uses gas for arc extinction.
  7. Control Wiring Size: Usually 2.5 mm² copper wire.
  8. Interlocking: Prevents unsafe operation sequence.
  9. Mimic Diagram: Panel diagram showing system layout.
  10. Control Fuses: Protect low-voltage control circuits.

⚡ 9. Power System & Safety

  1. Earthing vs Grounding: Equipment safety vs system reference.
  2. Touch/Step Potential: Voltage experienced on ground faults.
  3. Lightning Arrester: Diverts lightning surge to earth.
  4. Surge Protection Device: Protects from transient voltage spikes.
  5. Isolation: Disconnect equipment for safe maintenance.
  6. LOTO: Lockout-tagout prevents accidental energization.
  7. Arc Flash: Dangerous fault causing heat and light explosion.
  8. PPE: Personal Protective Equipment for safety.
  9. Danger Voltage: Anything above 50V AC.
  10. Neutral-Earth Separation: Avoids circulating current and shock risk.

🧠 10. Troubleshooting & Practical

  1. Motor Not Starting: Check voltage, fuses, contactor coil, overload.
  2. MCB Trips: Due to short circuit or earth fault.
  3. Motor Reverse Rotation: Swap two supply phases.
  4. Transformer Overheating: Overload, cooling issue, oil low.
  5. Voltage Dip at Start: Use soft starter or VFD.
  6. Single Phasing: Check fuse or supply line open.
  7. Panel Buzzing: Loose contactor coil or vibration.
  8. Earthing Test: Done regularly to ensure <1Ω.
  9. Cable Termination Failure: Loose lugs or poor crimping.
  10. System Reliability: Maintain schedules, logs, and testing.

✅ Summary:
Electrical maintenance engineering focuses on keeping equipment reliable, safe, and efficient through regular inspection, testing, protection devices, and troubleshooting.


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