Insulating Materials for Cables

Insulating Materials for Cables

AU: May-12

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, vulcanised rubber, PVC etc. are used for providing such an insulation.

1. Properties of Insulating Materials used for Cables

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 be very high.

3. To withstand the mechanical injuries, 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.

The main insulating materials which are in use are,

1. Poly Vinyl Chloride (PVC)

2. Paper

3. Cross Linked Polythene     

4. Vulcanised India Rubber (VIR)

2. Poly Vinyl Chloride (PVC)

It is thermo plastic synthetic compound. It is available in the powder form 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 75^oC.

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.

3. 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 insulating oil. After impregnating 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 80^oC.

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.

4. Cross Linked Polythelene

The cables 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 centimetres 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 90^oC.

XLPE cables are directly laid on soil bed and are used for the voltages upto and including 33 kV. 

5. Vulcanised 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 of 15 kV/mm.

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 now a days and is used for low and moderate voltage cables i.e. distribution systems only.

Review Questions

1. List out the properties of insulating materials used for the cables.

AU : May – 12,17, Marks 6

2. List the various insulating materials used for the cables and discuss their features.

AU : Dec– 15, Marks 8