--- title: "Classification and installation of power cables" subject: "Power System Analysis I" chapter: 2 key: "semester_4/power_system_analysis_i:cable-classification-installation" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/cable-classification-installation site: BCT Engineering --- # Classification and installation of power cables Power System Analysis I · Chapter 2: Overhead and Underground Transmission - Asked in: 2072 Asoj, 2+2+2 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/cable-classification-installation - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2 ## Question Classify power cables and give reasons for their usage. Briefly discuss cable installation techniques. ## Short answer Power cables are classified by voltage (LT to EHT) and construction (belted, screened, pressure, XLPE/PVC). They are used for underground supply in cities, substations, and crossings due to safety and fewer faults. Cable laying methods include direct trench burial, draw-in through ducts with manholes, and solid trough systems. ## Answer A power cable is an insulated conductor (or group of conductors) with a protective covering, used to carry power underground or through places where overhead lines are not possible. #### Classification of power cables **By voltage:** | Type | Voltage | |---|---| | Low tension (LT) | up to 1 kV | | High tension (HT) | up to 11 kV | | Super tension (ST) | 22–33 kV | | Extra high tension (EHT) | 33–66 kV | | Extra super voltage | above 132 kV | **By construction and insulation:** - **Belted cables (up to 11 kV):** Cheap and simple. Above 11 kV, tangential stress and voids cause breakdown. - **Screened cables, H-type and S.L.-type (up to 66 kV):** Each core is screened with metallised paper, so the stress is purely radial and there are no voids or sheath losses. - **Pressure cables, oil-filled and gas-pressure (above 66 kV):** Pressure prevents the formation of voids and ionisation at high stress. - **Modern solid-dielectric cables, PVC and XLPE:** Light, dry-type and easy to joint. XLPE is now used up to EHV levels. #### Reasons for using cables - Lines in cities, substations, airports and river crossings where overhead lines are unsafe or not allowed - Less exposure to lightning, storms and accidents, so fewer faults - No effect on appearance, and better public safety - Lower inductive reactance, so less voltage drop - Main drawback: they cost 5–10 times more than overhead lines, and fault location and repair are difficult. #### Methods of laying cables 1. **Direct laying:** An armoured cable is laid in a trench about 1.5 m deep on a sand bed and covered with bricks and soil. It is cheap and has good heat dissipation, but extension and repair are difficult. 2. **Draw-in system:** Ducts or pipes are laid with manholes at intervals, and cables are pulled through them. Repairs and additions are easy without digging, but the cost is high and cooling is poorer. This system is used in cities. 3. **Solid system:** The cable is laid in troughs (of wood or earthenware) and filled with bitumen. It gives good mechanical protection, but cooling is poor and the cost is high. It is now rarely used. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are belted cables not used above about 11 kV? A. They cannot be armoured B. Tangential stress and voids in the insulation cause breakdown C. Their conductors are too thin D. Their sheath loss is zero Answer: B. In belted cables the stress is not purely radial; tangential stress and voids lead to breakdown above 11 kV. (Key point: Belted cable limit) 2. What makes screened (H-type, S.L.-type) cables suitable up to 66 kV? A. Oil under pressure in the dielectric B. Each core is screened with metallised paper so stress is purely radial and there are no voids C. They use a single common belt D. They use rubber insulation Answer: B. Screening each core gives radial stress and removes voids and sheath losses. (Key point: Screened cables) 3. Pressure cables (oil-filled or gas-pressure) are used above 66 kV because the pressure A. Reduces conductor resistance B. Prevents formation of voids and ionisation at high stress C. Allows thinner insulation to be avoided D. Cools the sheath Answer: B. Maintaining pressure stops voids forming, so ionisation is avoided at high stress. (Key point: Pressure cables) 4. Which laying method uses ducts with manholes and is common in cities, with easy repair but poor cooling and high cost? A. Direct laying B. Draw-in system C. Solid system D. Aerial bunching Answer: B. In the draw-in system cables are pulled through ducts between manholes, so repairs need no digging. (Key point: Draw-in system) ## Related in Power System Analysis I - [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 - [What a power cable is](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Apower-cable-brief) — Power System Analysis I, chapter 2 - [Underground cables vs overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acables-vs-overhead-lines) — Power System Analysis I, chapter 2 - [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 - [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 - [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× ## Related in other subjects - [How cable size is chosen in an installation](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-size-chosen-installation) — Industrial Power Distribution and Illumination, chapter 5, asked 2× - [Cable types and installation methods](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-types-installation-methods) — Industrial Power Distribution and Illumination, chapter 3 - [Cable classification and 3½ core construction](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-classification-construction) — Industrial Power Distribution and Illumination, chapter 3 - [Splicing techniques of HV cable](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Ahv-cable-splicing) — Industrial Power Distribution and Illumination, chapter 3