--- title: "Construction of 3.5 core armoured cable" subject: "Industrial Power Distribution and Illumination" chapter: 3 key: "semester_6/industrial_power_distribution_and_illumination:armoured-cable-construction" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/armoured-cable-construction site: BCT Engineering --- # Construction of 3.5 core armoured cable Industrial Power Distribution and Illumination · Chapter 3: Power Carrying Devices - Asked in: 2078 Chaitra, 4 marks; 2075 Bhadra, 8 marks; 2072 Asoj, 4 marks (asked 3 times) - 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/armoured-cable-construction - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-3 ## Question Sketch the 3.5 core armored power cable used in an industry and explain each of the material used in this cable. ## Short answer A 3.5 core armoured cable has three phase conductors and one half-size neutral, each insulated with PVC or XLPE. Fillers and an inner PVC bedding surround the laid-up cores, covered by galvanized steel wire or strip armour for mechanical protection and an earth path. An outer PVC sheath protects against moisture and corrosion, with standard colour coding (R,Y,B, black) for identification. ## Answer A **3.5 core armoured power cable** has three full-size phase conductors (R, Y, B) and one reduced-size neutral (about half the phase area, hence "0.5 core"). It is the standard cable for 400 V three-phase, four-wire distribution in industries, laid underground or on trays. #### Cross-section sketch ```text ┌───────────────────────┐ ← outer PVC sheath │ ░░░░ armour ░░░░░░░░░ │ ← steel wire / strip │ ┌───────────────────┐ │ ← inner sheath │ │ ( R ) ( Y ) │ │ (bedding) │ │ filler │ │ │ │ ( B ) (n) │ │ ← n = half-size │ └───────────────────┘ │ neutral └───────────────────────┘ Each core: conductor + insulation ``` #### Parts and materials 1. **Conductors:** stranded **copper** or **aluminium**, often sector-shaped to make the cable compact. Copper has higher conductivity; aluminium is lighter and cheaper and is common for large sizes. Stranding gives flexibility. 2. **Insulation:** each core is insulated with **PVC** (rated 70 °C) or **XLPE** (cross-linked polyethylene, rated 90 °C). XLPE has better thermal and electrical properties and allows higher current. Cores are colour-coded red, yellow, blue and black. 3. **Fillers:** non-hygroscopic PVC or polypropylene material that fills the gaps between the laid-up cores to make the cable round. 4. **Inner sheath (bedding):** extruded PVC layer over the laid-up cores. It holds the cores together and acts as a cushion so the armour does not damage the insulation. 5. **Armour:** galvanised steel **round wires** (for smaller cables, up to about 13 mm diameter under the armour) or **flat strips** (for larger multi-core cables), wound helically. It gives mechanical protection against crushing, digging and rodents, and also acts as an earth continuity path. Single-core AC cables use non-magnetic (aluminium) armour to avoid eddy-current heating. 6. **Outer sheath (serving):** tough extruded PVC (or FRLS/LSZH for fire-safe areas). It protects the armour from moisture, corrosion and chemicals and carries the cable markings (size, voltage, maker, standard). **Standard:** IS 1554 (PVC) or IS 7098 (XLPE) for 1.1 kV cables. **Designation example:** "3.5 × 95 mm² Al, XLPE, armoured, 1.1 kV". ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does a '3.5 core' cable mean? A. Three phase cores of full size and a neutral of about half size B. Three and a half phases C. Three cores plus an earth wire D. Three small cores and one large Answer: A. Three full-size phase conductors plus a reduced neutral of about half the phase area. (Key point: Meaning of 3.5 core) 2. Which statement about insulation is correct? A. PVC is rated 90 degC and XLPE 70 degC B. XLPE is rated 90 degC and PVC 70 degC, so XLPE allows higher current C. Both are rated 70 degC D. XLPE cannot be used in cables Answer: B. XLPE has better thermal and electrical properties, with a 90 degC rating. (Key point: XLPE vs PVC) 3. What does the inner sheath (bedding) do? A. Carries the current B. Acts as an earth C. Holds cores together and cushions the insulation from the armour D. Provides fire alarm function Answer: C. The extruded bedding binds the cores and prevents the armour from damaging the insulation. (Key point: Purpose of bedding) 4. What are the functions of the galvanised steel armour? A. Insulation between cores B. Mechanical protection and an earth continuity path C. Only to carry the neutral current D. To reduce conductor resistance Answer: B. Armour protects against crushing, digging and rodents, and also acts as an earth continuity path. 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