Constructional features of belted cable

A belted cable is a three-core, paper-insulated cable used for voltages up to about 11 kV (sometimes 22 kV). It gets its name from an extra layer of paper insulation, the belt, wound around the three insulated cores together.

Constructional features

        lead sheath
     ____________________
    /  belt insulation   \
   |   ( C1 )    ( C2 )   |
   |     fillers (jute)   |
   |        ( C3 )        |
    \____________________/
     bedding, armour, serving

1. Conductors: three stranded copper or aluminium conductors, usually sector (D) shaped to reduce overall size.
2. Core insulation: each conductor is wrapped with impregnated paper.
3. Fillers: the gaps between the cores are filled with jute or paper fillers to give a round shape.
4. Belt: an impregnated paper belt is wound over the three cores and fillers together.
5. Lead sheath: keeps moisture out and protects insulation.
6. Bedding, armouring and serving: bedding of jute/paper, steel tape or wire armour for mechanical protection, and outer serving of jute.

Limitations (why not used above about 22 kV)

• The electric field is not purely radial; it has a tangential component along the paper layers. Paper is weak along its layers, so leakage current flows tangentially and heats the insulation.
• Heating and load cycles create voids in the insulation (between fillers and belt), leading to ionisation, partial discharge and breakdown.

For these reasons, screened (H-type, SL-type) and pressure cables are used for higher voltages.