--- title: "Cable classification and 3½ core construction" subject: "Industrial Power Distribution and Illumination" chapter: 3 key: "semester_6/industrial_power_distribution_and_illumination:cable-classification-construction" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/cable-classification-construction site: BCT Engineering --- # Cable classification and 3½ core construction Industrial Power Distribution and Illumination · Chapter 3: Power Carrying Devices - Asked in: 2069 Bhadra, 8 marks - Video: [Construction of Underground Cable । Use of Protective Layers in UG Cables । XLPE Cable.](https://www.youtube.com/watch?v=y7eYB6XwAZI) — Industrial Solutions - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/cable-classification-construction - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-3 ## Question State the classification of cables and discuss the general construction features of 3½ core armoured cable. ## Short answer Cables are classified by voltage (LT/HT/ST/EHT), insulation (PVC, XLPE, paper, MICC), mechanical protection (armoured/unarmoured), number of cores (single to multi-core), conductor (Cu/Al), and construction type. A 3½-core armoured cable contains three full-size phase and one reduced neutral conductor, stranded and sector-shaped, with PVC/XLPE insulation, fillers, inner sheath, galvanized steel wire/strip armour, and an outer PVC sheath; it provides a complete 3-phase 4-wire circuit with earth path. ## Answer A cable is one or more insulated conductors enclosed in a protective covering. Cables are classified by voltage, insulation, construction and number of cores. #### Classification of cables 1. **By voltage rating** - Low tension (LT): up to 1 kV (1.1 kV grade, used for 400/230 V) - 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 (oil-filled, gas-pressure cables) 2. **By insulation**: PVC, XLPE, EPR/rubber, paper (PILC), mineral insulated (MICC, fire survival). 3. **By mechanical protection**: armoured (steel wire/strip) and unarmoured. 4. **By number of cores**: single core, 2-core, 3-core, 3½-core, 4-core and multi-core control cables. 5. **By conductor material**: copper and aluminium. 6. **By construction of HT cables**: belted, screened (H-type, S.L. type), pressure (oil-filled, gas-pressure) cables. 7. **By use**: power, control, instrumentation, flexible/trailing, fire-resistant cables. #### General construction of 3½-core armoured cable It has three phase conductors of full section and a neutral of about half section, and is used for LT 3-phase 4-wire distribution in industries. ```text outer sheath (PVC) armour (GS wire / strip) inner sheath / bedding fillers ____________________ / ( R ) ( Y ) \ | | | ( B ) (N½) | \____________________/ cores: Cu/Al conductor + insulation ``` 1. **Conductor**: stranded, compacted Cu or Al; sector-shaped phase cores reduce cable diameter. Neutral is smaller (e.g. 3½ × 50 mm² = 3 × 50 + 1 × 25 mm²). 2. **Core insulation**: PVC (70 °C) or XLPE (90 °C), coloured red, yellow, blue and black for identification. 3. **Fillers**: PVC or polypropylene to fill the gaps and keep the cable round. 4. **Inner sheath / bedding**: extruded PVC binding all cores; protects them from the armour. 5. **Armour**: galvanised steel round wires or flat strips wound helically; protects against mechanical damage (digging, crushing, rodents) and gives an earth path. It is earthed at both ends. 6. **Outer sheath**: black PVC or FRLS compound; resists water, chemicals and sunlight, and carries the printed marking (voltage grade, size, manufacturer). **Advantages**: suitable for direct burial, mechanically strong, saves copper/aluminium in the neutral, single cable gives a complete 3-phase 4-wire circuit. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which voltage classes are correct? A. LT up to 11 kV; HT up to 33 kV B. LT up to 1 kV; HT up to 11 kV; super tension 22-33 kV C. LT up to 440 V only; HT above 1 kV D. All cables above 1 kV are LT Answer: B. Cables are classified LT, HT, ST, EHT by voltage rating. (Key point: Voltage classification) 2. What is the purpose of fillers in a 3.5 core cable? A. To provide the earth path B. To protect against rodents C. To fill the gaps between cores and keep the cable round D. To carry the neutral current Answer: C. Fillers are non-hygroscopic PVC or polypropylene. (Key point: Fillers) 3. What is the role of the inner sheath (bedding)? A. It carries the printed markings B. It is the main insulation of each core C. It resists sunlight and chemicals D. It binds the cores and protects them from the armour, providing a bed for it Answer: D. The outer sheath carries markings and resists environment; bedding sits beneath the armour. (Key point: Inner sheath) 4. How is armour treated and why? A. Galvanised steel wires or strips wound helically, earthed at both ends, giving protection and an earth path B. Plastic strips left isolated at both ends C. Copper tape earthed at one end only D. Aluminium sheath used only for HT Answer: A. Armour guards against digging, crushing and rodents and provides earth continuity. (Key point: Armour earthing) ## Related in Industrial Power Distribution and Illumination - [Construction of 3.5 core armoured cable](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aarmoured-cable-construction) — Industrial Power Distribution and Illumination, chapter 3, asked 3× - [Conductor sizing factors and armoured cable](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aconductor-size-factors-armoured) — Industrial Power Distribution and Illumination, chapter 3 - [3.5 core cable for 60 A over 200 m](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Acable-60a-200m) — Industrial Power Distribution and Illumination, chapter 5 ## Related in other subjects - [Classification and installation of power cables](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acable-classification-installation) — Power System Analysis I, chapter 2 - [Constructional features of belted cable](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Abelted-cable-construction) — High Voltage Engineering, chapter 1