---
title: "What a power cable is"
subject: "Power System Analysis I"
chapter: 2
key: "semester_4/power_system_analysis_i:power-cable-brief"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/power-cable-brief
site: BCT Engineering
---

# What a power cable is

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

- Asked in: 2068 Magh, 4 marks
- Video: [PS48 Introduction to Underground cables](https://www.youtube.com/watch?v=2UjGGI9OeLY) — Lectures in Electrical Engineering
- 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/power-cable-brief
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2

## Question

What is a power cable? Explain briefly.

## Short answer

A power cable is an assembly of one or more insulated conductors with protective layers (sheath, armour, serving) for underground or underwater power transmission. It typically has a stranded copper or aluminium core, insulation like XLPE, a metallic moisture barrier, and mechanical protection.

## Answer

A power cable is an assembly of one or more insulated conductors with an overall protective covering. It is used to carry electric power underground, under water, or inside buildings and substations where overhead lines are not possible.

#### Construction (from inside out)
```text
   +-------------------------------+
   |  serving (jute/PVC)           |
   |  armouring (steel tape/wire)  |
   |  bedding                      |
   |  metallic sheath (lead/Al)    |
   |  insulation (paper/PVC/XLPE)  |
   |  conductor (Cu/Al, stranded)  |
   +-------------------------------+
```

1. **Core (conductor):** Stranded copper or aluminium, for flexibility.
2. **Insulation:** Impregnated paper, PVC, XLPE or rubber. Its thickness depends on the voltage.
3. **Metallic sheath:** Keeps out moisture and gases.
4. **Bedding:** Jute or fibrous tape that protects the sheath from the armour.
5. **Armouring:** Steel tape or wire for mechanical protection.
6. **Serving:** An outer jute or PVC layer that protects the armour from corrosion.

#### Types
Cables are classified by voltage (LT, HT, ST, EHT) and by construction (belted, screened, and pressure cables, which may be oil-filled or gas-filled).

Cables are safer, less exposed to faults and invisible, but they cost much more than overhead lines and are hard to repair.

## Check yourself

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

1. What is the purpose of the metallic sheath?
   A. Carry the current
   B. Keep out moisture and gases
   C. Provide mechanical strength
   D. Reduce voltage
   Answer: B. The sheath (lead/aluminium) protects the insulation from moisture and gases. (Key point: Metallic sheath)

2. The layer that gives mechanical protection is the
   A. Conductor
   B. Bedding
   C. Armouring (steel tape or wire)
   D. Insulation
   Answer: C. Steel tape or wire armouring protects mechanically. (Key point: Armouring)

3. From the inside, the order of layers is
   A. Insulation, conductor, sheath
   B. Conductor, insulation, sheath, bedding, armour, serving
   C. Sheath, conductor, insulation
   D. Conductor, armour, insulation
   Answer: B. Conductor is at the centre, then insulation, sheath, bedding, armouring and serving. (Key point: Layer order)

## Related in Power System Analysis I

- [Murray loop test for cable faults](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Amurray-loop-test) — Power System Analysis I, chapter 2, asked 5×
- [Cable fault location techniques and Murray loop](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acable-fault-techniques-murray) — Power System Analysis I, chapter 2
- [Optimum conductor radius for minimum cable stress](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acable-optimum-conductor-radius) — Power System Analysis I, chapter 2
- [Cable insulation resistance and capacitance](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acable-insulation-resistance-capacitance) — Power System Analysis I, chapter 2
- [Locating phase-to-phase cable fault](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aphase-to-phase-cable-fault) — Power System Analysis I, chapter 2
- [Varley loop test for cable faults](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Avarley-loop-test) — Power System Analysis I, chapter 2

## Related in other subjects

- [Power/light sub-circuits and cable for 50 kW, 60 kW](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Asub-circuits-cable-50kw-60kw) — Industrial Power Distribution and Illumination, chapter 5, asked 4×
- [Numerical: received power with cable and mismatch loss](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Areceived-power-coax-mismatch) — Wireless Communication, chapter 3
- [Power vs lighting system and cable sizing factors](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Apower-vs-lighting-cable-factors) — Industrial Power Distribution and Illumination, chapter 5
- [Cable size MDB to SDB1 (80 kW) and SDB2 (10 kW)](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-mdb-sdb-80kw-10kw) — Industrial Power Distribution and Illumination, chapter 5, asked 5×