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If you’re preparing for GIS substation estimation and costing roles, you already know one thing: companies don’t want theory. They want someone who understands drawings, quantities, rates, and how a project actually runs on site.
GIS substations are expensive, compact, and highly engineered. That means even a small mistake in estimation can cost crores. So interviewers don’t just check your knowledge, they check your thinking.
Below are 100 practical interview questions with clear answers, exactly the way they are discussed in real interviews.
1. What is a GIS substation?
A Gas Insulated Substation uses SF6 gas for insulation instead of air, making it compact and suitable for urban areas.
2. Why GIS instead of AIS?
Less space, higher reliability, lower maintenance, better for polluted environments.
3. Main components of GIS?
Circuit breaker, disconnector, earthing switch, busbar, CT, PT, enclosure.
4. What is SF6 gas role?
It acts as an insulating and arc-quenching medium.
5. What is a bay?
A bay is a functional unit in a substation like feeder bay, transformer bay.
6. Types of bays?
Line bay, transformer bay, bus coupler, bus sectionalizer.
7. What is a single line diagram (SLD)?
A simplified drawing showing electrical connections.
8. What is estimation?
Calculating quantities and cost before execution.
9. What is costing?
Assigning rates to quantities to find project value.
10. What is BOQ?
Bill of Quantities listing items, quantities, and units.
11. What is takeoff?
Extracting quantities from drawings.
12. What is scope of work?
Defines what is included in project execution.
13. What is earthing system?
System to safely discharge fault current to ground.
14. What is cable trench?
Pathway for laying cables underground.
15. What is control room?
Building for panels, protection, and monitoring.
16. What is switchyard?
Area where electrical equipment is installed.
17. What is foundation?
Concrete base supporting equipment.
18. What is structural steel role?
Supports GIS equipment and cable trays.
19. What is lightning protection?
Protects substation from lightning strikes.
20. What is insulation coordination?
Ensures insulation levels match voltage requirements.
21. How do you start estimation?
Study SLD, layout, and GA drawings.
22. How to calculate earthwork?
Length × width × depth of excavation.
23. Foundation concrete quantity?
Volume from drawing dimensions.
24. Reinforcement quantity?
From BBS or thumb rule per cubic meter.
25. Cable trench quantity?
Length × cross-section.
26. Cable quantity?
From layout plus wastage (usually 5–10%).
27. Earthing conductor quantity?
Total grid length from earthing layout.
28. How to measure GIS equipment?
Usually as lump sum or per bay basis.
29. Steel structure quantity?
From fabrication drawings.
30. Painting quantity?
Surface area of steel structures.
31. Backfilling quantity?
Excavation minus concrete volume.
32. PCC quantity?
Base layer under foundations.
33. Road quantity?
Area × thickness.
34. Drain quantity?
Length × section.
35. Control building quantity?
Based on architectural drawings.
36. Cable tray quantity?
Length of tray routes.
37. Termination quantity?
Number of cable ends.
38. Jointing quantity?
Based on cable length and drum size.
39. Lighting system quantity?
Number of poles and fixtures.
40. Fire protection quantity?
Pipes, hydrants, detectors.
41. What is rate analysis?
Breaking down cost into material, labour, machinery.
42. Main cost components?
Material, labour, equipment, overhead, profit.
43. What is overhead?
Site expenses like staff, electricity.
44. What is profit margin?
Contractor’s expected earning.
45. What is contingency?
Extra provision for uncertainties.
46. What is escalation?
Increase in cost due to time delay.
47. What is mobilization cost?
Initial setup cost at site.
48. What is preliminaries?
Temporary works and site setup.
49. What is lump sum item?
Fixed cost item without detailed breakup.
50. What is unit rate item?
Cost per unit quantity.
51. How to price cable laying?
Per meter including labour and accessories.
52. How to price foundation work?
Concrete + steel + shuttering + labour.
53. What affects GIS cost most?
Equipment cost and import factors.
54. What is vendor quotation?
Supplier price for equipment.
55. What is comparison statement?
Comparing multiple vendor quotes.
56. What is cost control?
Monitoring actual vs estimated cost.
57. What is variation?
Change in scope or quantity.
58. What is billing?
Claiming payment based on work done.
59. What is cash flow?
Money inflow and outflow during project.
60. What is break-even point?
Where cost equals revenue.
61. Common mistake in estimation?
Missing items from drawings.
62. How to avoid underestimation?
Cross-check drawings and BOQ.
63. What if drawing is incomplete?
Take assumptions and mention clearly.
64. How to handle design changes?
Revise quantities and inform client.
65. What is wastage allowance?
Extra quantity for losses.
66. How to check BOQ accuracy?
Recalculate key quantities.
67. What is as-built quantity?
Actual executed quantity.
68. What is reconciliation?
Comparing estimated vs actual.
69. What is site measurement?
Physical verification of work done.
70. What is billing cycle?
Monthly or milestone-based billing.
71. What is retention money?
Amount held till project completion.
72. What is defect liability period?
Time for fixing defects after completion.
73. What is testing cost?
Cost for equipment and commissioning tests.
74. What is commissioning?
Making system operational.
75. What is shutdown work?
Work done during power shutdown.
76. What is safety cost?
PPE, safety training, barricading.
77. What is logistics cost?
Transportation of equipment.
78. What is storage cost?
Warehouse and material handling.
79. What is crane cost?
Heavy lifting equipment charges.
80. What is manpower planning?
Estimating required labour.
81. Why GIS is costly?
High-end equipment and gas insulation.
82. Biggest cost risk?
Design change and import delays.
83. How to reduce cost?
Optimize design and reduce wastage.
84. What is value engineering?
Improving function at lower cost.
85. What is critical item?
Item impacting project timeline.
86. What is lead time?
Time required to procure equipment.
87. How to handle price fluctuation?
Include escalation clause.
88. What is contract type?
Lump sum, item rate, EPC.
89. What is EPC contract?
Engineering, procurement, construction.
90. What is risk allocation?
Division of risk between parties.
91. What is delay penalty?
Fine for late completion.
92. What is bonus clause?
Reward for early completion.
93. What is scope gap?
Missing work in contract.
94. What is interface issue?
Coordination problem between systems.
95. What is design error impact?
Rework and cost increase.
96. What is procurement strategy?
Plan for purchasing materials.
97. What is cost benchmarking?
Comparing with similar projects.
98. What is lifecycle cost?
Total cost from installation to maintenance.
99. What is documentation importance?
Avoid disputes and claims.
100. Final question: Why should we hire you?
Because you understand both drawings and ground reality, not just theory.
In real projects, estimation is not about just numbers. It’s about understanding how work will actually happen.
For example:
If you miss cable tray supports in estimation, your cost may look correct on paper but fail on site.
Same with GIS equipment foundations. If you don’t consider anchor bolts or grouting, you’ll face problems during execution.
Mon Mar 23, 2026
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