Cable classification and 3½ core construction

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.

  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.