Breaking into the electrical engineering field requires not only strong academic knowledge but also practical awareness of how systems work in real projects. Whether you are preparing for a substation, OHTL, UGC, or utility-based role in Saudi Arabia and the Gulf, these 25 commonly asked technical interview questions will help you sharpen your fundamentals.
Q1. Difference between MCB, MCCB, ACB, and VCB?
Answer:
- MCB (Miniature Circuit Breaker): For low current applications.
- MCCB (Moulded Case Circuit Breaker): For higher currents, adjustable settings.
- ACB (Air Circuit Breaker): High-capacity LV protection.
- VCB (Vacuum Circuit Breaker): Medium voltage switching in vacuum.
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Q2. How do you select cable size for a motor or load?
Answer: Based on current, voltage drop, short-circuit rating, and derating factors.
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Q3. Difference between LT and HT systems?
Answer:
- LT (Low Tension): < 1kV (e.g., 400V).
- HT (High Tension): > 1kV (e.g., 11kV, 33kV).
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Q4. What is power factor and why is it important?
Answer: Ratio of active power to apparent power. High power factor improves efficiency and reduces losses.
Q5. Explain star-delta starter.
Answer: Motor starts in star connection (low current), then shifts to delta connection (full voltage) once running.
Q6. Formula to calculate 3-phase motor current?
Answer: I=P3×V×PF×ηI = \frac{P}{\sqrt{3} \times V \times PF \times \eta}I=3×V×PF×ηP
Where P = power (W), V = line voltage, PF = power factor, η = efficiency.
Q7. Difference between isolator and circuit breaker?
Answer:
- Isolator: No-load switching, used for isolation.
- Circuit Breaker: Operates under load or fault, interrupts current.
Q8. What are harmonics?
Answer: Distorted waveforms due to non-linear loads. Cause heating, losses, and equipment malfunction.
Q9. Why do we use capacitor banks?
Answer: To improve power factor and avoid utility penalties.
Q10. Difference between earthing, grounding, and bonding?
Answer:
- Earthing: Connecting equipment to earth.
- Grounding: Neutral to earth.
- Bonding: Interconnection of metallic parts for safety.
Q11. What are standard voltages in Gulf countries?
Answer:
- LV: 230/400V
- MV: 11kV
- HV: 33kV / 132kV / 220kV
Q12. How do you test a transformer at site?
Answer: IR test, ratio test, polarity test, oil BDV test.
Q13. What tests are done before commissioning a motor?
Answer: Insulation test, continuity test, rotation check, and no-load run.
Q14. How to identify underground cable faults?
Answer: Using megger, Time Domain Reflectometer (TDR), or cable fault locator.
Q15. What safety steps before working on live systems?
Answer: Lockout-tagout, discharge, PPE, and permit to work (PTW).
Q16. Procedure for motor megger testing?
Answer: Disconnect motor, apply 500/1000V DC, and check insulation resistance ≥ 1 MΩ.
Q17. What protections are used for motors?
Answer: Overload, short-circuit, earth fault, and thermal relay.
Q18. Precautions before HV panel maintenance?
Answer: Isolate, earth, discharge, use interlocks, and follow PTW system.
Q19. How to check phase sequence?
Answer: Use a phase sequence meter or check by motor rotation direction.
Q20. How to handle load balancing in a distribution panel?
Answer: Measure phase currents and redistribute loads across phases.
Q21. Which standards are used in Gulf countries?
Answer: IEC, BS, NEC, and local utility standards (e.g., DEWA, SEC, Kahramaa).
Q22. What is the earthing resistance limit as per IEC?
Answer:
- Substations: < 1 ohm
- Normal installations: < 5 ohms
Q23. Difference between TN-S, TN-C, and TT systems?
Answer:
- TN-S: Neutral and earth are separate.
- TN-C: Neutral and earth are combined.
- TT: Direct connection to earth electrode.
Q24. What is IP rating?
Answer: Ingress Protection rating, defines protection against dust and water (e.g., IP55 = dust protected + water jet protected).
Q25. Cable color coding as per IEC?
Answer:
- Phase: Brown / Black / Grey
- Neutral: Blue
- Earth: Green-Yellow
✅ Conclusion:
These 25 questions are the most common in electrical interviews for Gulf-based projects. Mastering them will help engineers confidently face entry-level and mid-level interviews in substations, OHTL, and industrial projects.