Polyvinyl chloride (PVC) (thermoplastic)
A basic plastic material which can be made in many forms. The Wiring Regulations now refer to PVC as a "thermoplastic" material, meaning that its form can be changed if its temperature exceeds a certain level. It is robust, chemically inert, has good aging and fire-resisting properties. It is more resistant to weathering and sunlight than rubber, but will harden and crack in the presence of oil or grease. High temperatures lead to softening and possible insulation failure, whereas at low temperature, PVC may become brittle. Very little absorption of water will occur, although PVC is inferior to polychloroprene in this respect. PVC is widely in use as a sheathing material as well as for insulation. It can only usually be used where conductor temperature does not exceed 70.C. The material emits dense smoke and corrosive fumes when it burns.
Vulcanised rubber (VR) (thermosetting)
A good electrical insulator with high flexibility. The Wiring Regulations now refer to VR as a "thermosetting" material, meaning that its form cannot be changed after manufacture. It is subject to rapid aging and cracking due to weathering and exposure to sunlight, and softens and becomes sticky in the presence of oils and greases.
It ages rapidly at high or low temperatures. It is not widely used but is available in a form which will operate safely up to 85.C.
Polychloroprene (PCP) (thermoplastic)
Polychloroprene, also known as Neoprene, is a plastic material of lower strength and lower insulation resistance than PVC. However, it is more resistant than PVC to weathering, and to attack by oils, acids, solvents, alkalis and water. PCPis more elastic than PVC, and this elasticity is not affected by increased temperatures. Although more expensive than PVC, PCP is used as a cable insulation and sheathing material for conditions where PVC would not be suitable.
Silicone rubber (thermosetting)
Silicone rubber is a synthetic material with many of the advantages of natural rubber. It has good weathering properties, and will resist attack from water and mineral oils, but not from petrol. It remains elastic over extremes of temperature (.70.C to 150.C).
Butyl rubber (thermosetting)
Butyl rubber is another synthetic material which is less expensive than silicone rubber but has similar advantages. It remains flexible over the range.40.C to 85.C, but will burn readily once ignited.
Propylene rubber (EPR) (thermosetting).
EP rubber is generally similar to butyl rubber, but with improved properties. It is resistant to heat, water, oil and sunlight, and is suitable for direct burial, exposure to weather and contaminated atmospheres. It is used to insulate power cables which have an increased current rating owing to its heat resistance.
Chlorosulphonated polyethylene (CSP) (thermoplastic)
This material is mainly used for sheathing cables insulated with other plastics. It is capable of operating over the temperature range.30.C to 85.C, and has excellent oil resisting and flame-retarding properties. It is very tough, and very resistant to heat and water. This is the sheathing material which is called "HOFR" (heat resisting, oil resisting and flame retarding) in the IEE Wiring Regulations.
This material is in the form of a white powder, and is invariably used in mineralinsulated cables. It also finds application in some heating elements. Magnesium oxide is non-aging and will not burn, but it is very hygroscopic (absorbs moisture from the air), when it loses its insulating properties. It is unaffected by high temperatures, and is a good conductor of heat.
Cross-linked polyethylene (XLPE) (thermoplastic)
This is a polymeric insulation (like EPR above) and is used particularly for power cables. In this application it shows cost advantages over oil-filled and pressurized cable insulation, not least in the simpler jointing techniques which may be used. The material does not burn easily, and when it does, only limited emission of smoke and fumes occurs.
Paper Dry or oil-impregnated paper, wound in long strips over the conductors, was the most common method of insulating underground cables but has now been overtaken by XLPE. As long as the paper is kept dry (usually by sheathing with lead alloy) its insulating properties are excellent.
Glass fiber, impregnated with high-temperature varnish, is used to insulate some flexible cords used for high-temperature lighting applications.