---
title: "Underground cables vs overhead lines"
subject: "Power System Analysis I"
chapter: 2
key: "semester_4/power_system_analysis_i:cables-vs-overhead-lines"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/cables-vs-overhead-lines
site: BCT Engineering
---

# Underground cables vs overhead lines

Power System Analysis I · Chapter 2: Overhead and Underground Transmission

- Asked in: 2069 Bhadra, 4 marks
- Video: [Underground Vs Overhead Transmission: Why Cables Are More Challenging?](https://www.youtube.com/watch?v=Ny_OVk3Ti7g) — LifeAda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/cables-vs-overhead-lines
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2

## Question

Compare underground cables with overhead lines.

## Short answer

Underground cables have higher cost, better safety, lower inductive reactance, higher capacitance, and harder fault repair; they are used in congested areas. Overhead lines are cheaper, easier to repair, suited for long-distance high voltage, but are exposed to weather and faults.

## Answer

Overhead lines carry bare conductors on insulators and towers. Underground cables carry insulated, sheathed conductors buried in the ground.

| Point | Underground cable | Overhead line |
|---|---|---|
| Initial cost | High (5–10 times) | Low |
| Insulation | Solid or oil insulation all along the length | Air, plus insulators at the supports only |
| Working voltage | Limited (insulation and charging current) | Can be very high (765 kV and above) |
| Fault chances | Few (no lightning, storm or bird faults) | More (exposed to weather and accidents) |
| Fault location and repair | Difficult and slow (digging needed) | Easy and quick |
| Charging current | High (large capacitance) | Low |
| Inductive reactance | Low (conductors close together) | High |
| Heat dissipation and current rating | Poorer | Better |
| Appearance and safety | No visual impact, safe for the public | Spoils the view, with risk of contact |
| Flexibility and extension | Poor | Easy to tap and extend |
| Interference with communication lines | Negligible | Possible |

Cables are used in cities, substations, river and sea crossings, and airports. Overhead lines are used for long-distance, high-voltage transmission.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Compared with overhead lines, the initial cost of underground cables is
   A. Lower
   B. About the same
   C. 5-10 times higher
   D. Much lower but with higher losses
   Answer: C. Cables cost 5-10 times more than overhead lines. (Key point: Cost)

2. Why do cables have higher charging current than overhead lines?
   A. Their conductors are far apart
   B. Their large capacitance, because conductors are close together
   C. They have higher resistance
   D. They carry higher voltage
   Answer: B. Close conductors and solid insulation give large capacitance and therefore high charging current. (Key point: Charging current)

3. Which statement about inductive reactance is correct?
   A. Cable reactance is high
   B. Cable reactance is low because conductors are close together
   C. Both are equal
   D. Overhead lines have lower reactance
   Answer: B. Closely spaced conductors in cables give low inductive reactance. (Key point: Reactance)

4. Cables compared with overhead lines have
   A. More faults but easy repair
   B. Fewer faults but difficult, slow repair
   C. More faults and difficult repair
   D. Fewer faults and easy repair
   Answer: B. Cables avoid weather and accident faults, but locating and repairing faults needs digging. (Key point: Faults and repair)

## Related in Power System Analysis I

- [Proximity effect in cables vs overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aproximity-effect-cables-vs-lines) — Power System Analysis I, chapter 4
- [Conductor materials for lines and cables](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconductor-materials-selection) — Power System Analysis I, chapter 2
- [Locating faults in underground cables](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acable-fault-location-method) — Power System Analysis I, chapter 2, asked 3×
- [Requirements of conductors for overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconductor-requirements) — Power System Analysis I, chapter 2, asked 2×
- [Proximity effect in overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aproximity-effect) — Power System Analysis I, chapter 4

## Related in other subjects

- [Underground vs overhead distribution systems](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aunderground-vs-overhead-distribution) — Transmission and Distribution System Design, chapter 5, asked 4×
- [Surface vs underground powerhouse conditions](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apowerhouse-surface-underground-conditions) — Hydropower Engineering, chapter 8