Number of layers of the various materials is used around the actual conductor in a cable. To isolate the conductor from the surroundings, the conductor is provided with an insulation around it. The materials like paper, vulcanized rubber, PVC etc. are used for providing such an insulation.
The material to be used as an insulation must have the following properties,
1. To prevent leakage current, its insulation resistance must be very high.
2. To avoid electrical breakdown, its dielectric strength must very high.
3. To withstand the mechanical injures, it must be mechanically very strong.
4. It should be flexible.
5. It should be non-hygroscopic so that it will not absorb the moisture from the surroundings.
6. It should be non-inflammable.
7. It should be unaffected by acids and alkalies.
8. It should be capable of withstanding high breakdown voltages.
9. It should have high temperature withstanding capability.
Practically it is not possible to have all these properties in a single material. Hence insulation material is selected depending upon the use of the cable and the quality of insulation required for it. Some changes are done at the time of design depending upon the nature of material selected. For example if the material is hygroscopic then a layer of a waterproof covering is provided around it so that moisture can not reach the insulation. The main insulating materials which are in used are,
1. Poly Vinyl Chloride (PVC)
2. Paper
3. Cross Linked Polythene
4. Vulcanized India Rubber (VIR)
1.1 Poly Vinyl Chloride (PVC)
1.1 Poly Vinyl Chloride (PVC)
It is thermo plastic synthetic compound. It is available in the powder from and is obtained from polymerisation of acetylene. This powder is chemically inert, non-inflammable, odourless, tasteless and insoluble. It is combined with plastic compound and a gel is used over the conductor to obtain the insulation.
It has following characteristics,
1. Good dielectric strength of 17 kV/mm
2. Chemically inert.
3. Non-hygroscopic.
4. Resistant to corrosion.
5. Maximum continuous temperature rating of 75oC.
6. High electrical resistivity.
The mechanical properties like elasticity of PVC are not as good as rubber so PVC cables are used for low and medium voltage domestic, industrial lights and power installations.
1.2 Paper
The paper is very cheap insulating material. Its dielectric strength is also high but it is hygroscopic in nature. When it is dry its insulation resistance is very high but a small amount of moisture reduces its insulation resistance to a very low value. Thus it is impregnated in an insulation oil. After impregnated also it has a tendency to absorb the moisture. Hence paper cables are never left unsealed and provided with the protective covering. When not in use, paper cable ends are temporarily covered with wax or tar.
The paper has following characteristics,
1. High dielectric strength of 20 kV/mm.
2. Higher thermal conductivity.
3. Low capacitance
4. High durability
5. Low cost
6. Maximum continuous temperature rating of 80oC.
7. High insulation resistance when dry.
It is used in high voltage power cable manufacturing. The paper cables are preferred when the cable route has very few joints and hence generally used for low voltage distribution in thickly populated areas.
1.3 Cross Linked Polythelene
The cable using cross linked polythelene as the insulating material are called XLPE cables.
The low density polythelene is treated specially due to which there occurs cross linking of carbon atoms in it. This results into a new material which has following properties,
1. High dielectric strength of 20 to 40 kV/mm
2. Non-inflammable : If at all the continuous flame is applied its burning stops after very few centimeters away from the flame.
3. Extremely high melting point.
4. Light in weight and flexible.
5. Mechanically strong.
6. High temperature withstanding capability.
7. Low moisture absorption.
8. Maximum continuous temperature rating of 90oC.
XLPE cables are directly laid on solid bed and are used for the voltage upto and including 33 kV.
1.4 Vulcanized India Rubber (VIR)
This is the most olden insulating material developed during 1880-1930. The pure rubber is very soft and it can not withstand high temperatures hence it is 20 to 40% of India rubber mixed with mineral matter such as zinc oxide, red lead etc. with a little bit of sulphur in it.
It has following characteristics,
1. Good dielectric strength.
2. Good mechanical strength.
3. Durable and wear resistant.
4. Good insulation resistance.
5. Remain more elastic than pure rubber.
But it has number of drawbacks such as,
1. It absorbs moisture, slightly.
2. It has low melting point.
3. The sulphur content attack the copper conductor and changes the VIR insulation colour. Hence copper conductors to be used with VIR insulation must be tinned.
4. Short span of life.
The use of VIR is very limited nowadays and is used for low moderate voltage cable i.e. distribution systems only.
Read also
Insulation Resistance of a Cable
Read also
Insulation Resistance of a Cable