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Construction of 3.5 core armoured 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.

Sketch the 3.5 core armored power cable used in an industry and explain each of the material used in this cable.

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

        ┌───────────────────────┐ ← 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".

✓ Answer checked by Claude Opus 5.5 (high effort)

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