30 Most Common Interview Questions with Answers

1. Q: What is a hazardous area?
   A: A hazardous area is a location where flammable gases, vapors, or dust may exist in sufficient quantities to produce explosive or ignitable mixtures.

2. Q: What are the main types of hazardous materials considered?
   A: Gases/vapors and combustible dusts.

3. Q: What are the different methods of hazardous area classification?
   A:

   • IEC/ATEX (Zone classification)

   • NEC (Class/Division system – USA)

4. Q: What is Zone 0, Zone 1, and Zone 2?
   A:

   • Zone 0: Continuous presence of explosive atmosphere

   • Zone 1: Likely under normal operation

   • Zone 2: Unlikely or short-duration presence

5. Q: What are the IEC classifications for dust?
   A:

   • Zone 20: Continuous presence of combustible dust

   • Zone 21: Likely during normal operations

   • Zone 22: Not likely but possible

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✅ NEC CLASSIFICATION (NORTH AMERICA)

6. Q: What is Class I, II, III as per NEC?
   A:

   • Class I: Gases and vapors

   • Class II: Dusts

   • Class III: Fibers or flyings

7. Q: What are Division 1 and Division 2?
   A:

   • Division 1: Explosive atmosphere present during normal operation

   • Division 2: Present only during abnormal conditions

8. Q: How do Class I and Zone systems correlate?
   A:

   • Class I Div 1 ≈ Zone 0 & Zone 1

   • Class I Div 2 ≈ Zone 2

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✅ GAS GROUPS AND TEMPERATURE CLASSES

9. Q: What are the gas groups under IEC?
   A:

   • Group I (mining)

   • Group II (surface industries – subdivided into IIA, IIB, IIC)

10. Q: What is the significance of Group IIA, IIB, and IIC?
    A:

• IIA: Least ignitable gases (e.g., propane)

• IIB: Intermediate (e.g., ethylene)

• IIC: Most ignitable (e.g., hydrogen, acetylene)

11. Q: What are temperature classes?
    A:

• Classification based on maximum surface temperature (T1–T6)

12. Q: Match temperature class T6 with ignition temperature.
    A:

• T6: Max surface temp = 85°C; suitable for gases with ignition temp > 85°C

13. Q: What temperature class is used for hydrogen atmosphere?
    A:

• Usually T1 or T2 due to high ignition temp.

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✅ STANDARDS & CODES

14. Q: Name the key standards for hazardous area classification.
    A:

• IEC 60079 series

• NEC 500, 505

• API RP 500 / 505

• IS 5572 (India)

15. Q: What is API RP 500 and RP 505?
    A:

• RP 500: Class/Division based classification

• RP 505: Zone-based classification

16. Q: What is IS 5572?
    A:

• Indian Standard for area classification for electrical installation in hazardous areas.

17. Q: What is IEC 60079?
    A:

• A standard series covering explosive atmospheres and equipment requirements.

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✅ CLASSIFICATION PROCEDURE & PRACTICAL CONSIDERATIONS

18. Q: What factors affect hazardous area classification?
    A:

• Type of flammable material

• Release source and frequency

• Ventilation

• Quantity of release

• Equipment location

19. Q: What is a release source?
    A:
    A point where flammable gas or vapor can be released under normal or abnormal conditions.

20. Q: What are primary, secondary, and continuous release sources?
    A:

• Continuous: Always present

• Primary: Normally present during operation

• Secondary: Present only under fault conditions

21. Q: How is ventilation considered in area classification?
    A:

• Good ventilation reduces the extent of hazardous zones.

• Poor ventilation increases risk and zone size.

22. Q: What is the typical hazardous zone extent around a flange?
    A:

• Zone 2, with a radius of 1m (varies based on standards and conditions)

23. Q: What are the tools used for hazardous area classification?
    A:

• DNV software, Shell DEP guidelines, API/IEC zone calculation worksheets.

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✅ PRACTICAL IMPLEMENTATION & EQUIPMENT SELECTION

24. Q: What is Ex d protection?
    A:

• Flameproof enclosure that contains any explosion within the device.

25. Q: What is Ex e protection?
    A:

• Increased safety; prevents arcs, sparks, and hot surfaces.

26. Q: What is Ex i protection?
    A:

• Intrinsically safe; limits energy below ignition level.

27. Q: Can an Ex d device be installed in Zone 2?
    A:

• Yes, Ex d is suitable for Zone 1 & 2.

28. Q: Can an Ex nA or Ex ec device be used in Zone 1?
    A:

• No, typically suitable only for Zone 2.

29. Q: What are the marking requirements for Ex equipment?
    A:

• Must show Ex symbol, protection concept, zone, group, and temperature class (e.g., Ex d IIB T4)

30. Q: How is intrinsic safety maintained in instrumentation systems?
    A:

• Through the use of zener barriers or galvanic isolators that limit current, voltage, and energy.