UNDER GROUND CABLE

Table of contents

1. Quick one-liner bullets (exam-focused) 2. Download PDF (Bullets) 3. Donate / Community 4. SEO Keywords

Quick one-liner bullets (most important — AE/JE style)

Short, memory-friendly one-liners — useful for quick revision and objective exams.

1. Underground (UG) cable is a complete electrical system consisting of conductor, insulation, metallic screen, bedding, armour, and sheath — memorize layers left-to-right as: Conductor → Insulation → Screen → Bedding → Armour → Sheath. Trick: CISBAS
2. Common insulation materials: PVC for low voltage, XLPE for medium/high voltage — XLPE resists water and has better thermal rating. Exam tip
3. Permissible conductor temperature: design ratings depend on insulation (e.g., XLPE typically rated for 90°C continuous). Remember: XLPE → 90°C
4. Current carrying capacity is higher underground than overhead for same conductor size due to better cooling of buried cable only if backfill and spacing are correct. Example: good backfill = higher rating
5. Skin effect is negligible in LV underground cables; in HV it affects reactance — use short trick: skin ↑ with freq and conductor size. One-liner
6. Positive sequence reactance and earth return reactance both influence fault currents for buried cables; use equivalent circuit models for calculations. Exam cue
7. Armouring provides mechanical protection and path for fault current — steel tape for flex, steel wire for high mechanical stress. Remember: armour ≈ strength
8. Cable derating factors: grouping, depth of burial, ambient ground temperature, backfill thermal resistivity — multiply derating factors to get net derate. Derating = product
9. Minimum bending radius depends on construction — typically 8× overall cable diameter for single-core and 12× for multicore (check manufacturer). Rule of thumb
10. Voltage drop: ΔV = I × (R cosφ + X sinφ) × length — keep within statutory % limits for supply. Formula compact
11. Types of faults in UG cable: sheath fault, insulation breakdown, mechanical damage; sheath test (DC) checks integrity before commissioning. Test step
12. Cable marking: follow IS/IEC standards for cores and sheath marking — helps rapid identification on site. Site tip
13. Lightning/Surge protection: use surge arresters and proper earthing for cable terminations in HV systems. Priority at terminations
14. Safe minimum earth cover: depends on voltage and traffic — typically 0.6–1.0 m; deeper under roads. Remember: roads need deeper
15. Heat dissipation path is soil — thermal resistivity of soil critical; moist sand backfill reduces resistivity. Backfill = key
16. Cable accessories (joints & terminations) are weakest thermal/electrical points — use factory-made kits where possible. Weak point = joints
17. Short-circuit rating depends on conductor material, cross-section and duration; choose protection to clear faults within rated time. Match protection & cable
18. Screen/armor continuity is essential for fault return — always bond at specified points (TN vs TT earthing schemes). Bonding rules
19. Trench layout: maintain separation from water, gas, other services; use warning tape and markers above cable route. Safety on site
20. Testing during installation: insulation resistance (Megger), sheath continuity, and HV test as per standards — record test sheets. Always document
21. Reactive charging current for long HV cables is significant — affects reactive power balance; series capacitors sometimes used. Think capacitive
22. Selection tip: for urban MV feeders prefer XLPE over oil-filled cables because of simplicity and lower maintenance. Urban choice
23. Cable life influenced by cyclic thermal loading — avoid frequent overloads to extend lifespan. Avoid thermal cycling
24. Color codes for cores help quick fault location in exams — learn the current IS/IEC color scheme applicable in your region. Color memory
25. Earthing of cable screens: solidly earthed screens limit circulating currents but consider EMC and induced voltages for long runs. Screen earthing tradeoff
26. Reinstatement after laying: compact backfill, markers and as-built drawing reduce future excavation damage. Always draw route
27. Cost drivers: conductor size, insulation type, armouring, trenching difficulty — specify for life-cycle cost, not just capex. LCC > capex

PDF includes "Electrical Zindagi" on every page footer.

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