Basic Training Course

Basic Training Course Cables & Joints Electrical

3-Insulation, Covering and Stress Relief

Natural rubber or oil-impregnated paper is no longer used for the

insulation of cables up to 3810/6600V; synthetic

materials are now

used. For high-vo1tage cables the insulation is ethylene propylene rubber

(EPR) and for low-voltage cables it is polyvinyl chloride (PVC). EPR has

good electrical properties and is resistant to heat and chemicals; it is

suitable for a conductor temperature up to 85oC. PVC is a thermoplastic

material, therefore care must be taken not to overheat it; it is suitable

for conductor temperatures up to 70°C. PVC insulated cables should not

be laid when the temperature is less than 0oC because it becomes brittle

and is liable to crack.

High-voltage cables have an earthed metallic screen over the insulation of

each core. This screen consists of a lapped copper tape or metallic foil,

and its purpose is to control the electric field within the insulation and

thus the voltage gradient across it, as shown in Figure 3. Also, it avoids

any interaction of the electric stresses due to the voltages on different

phase conductors within the same cable.