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Tuesday, 13 August 2019

Mechanical Properties of Insulating Materials

Mechanical Properties of Insulating Materials

Mechanical properties such as impact strength, tensile strength, toughness, hardness, elongation, flexibility, abrasion, resistance etc., have to be considered when choosing an insulating material. Machinability and resistance of splitting are equally important. In the case of varnished products, the bonding property of varnish is important. Some of the mechanical properties which are to be taken care off while selecting the insulating material are dealt in the succeeding paragraphs.

1. Tension and Compression:

The conductors of transmission and distribution systems of overhead lines are supported by means of insulators to avoid leakage of current through the supports to the earth. When a tensile load exists on it, it should be able to withstand and should not give way mechanically.

2. Resistance to Abrasion, Tear, Shear and Impact:

Take the case of the insulator used between the commutator segments which are subjected to abrasive action during the running of the motor. The insulation qualities should be such that it should withstand this abrasive force. The insulating material should be hard and mechanical resistant. In the case of insulating material which is subjected to tear, shear or impact, the damage caused to them will completely put the system out of control and sometimes out of stability. In such cases the insulating material should be capable of withstanding all such forces and should not give way. Importance should be given to these factors while selecting the insulating material for the job.

3. Viscosity : 

In liquid insulators, viscosity plays an important role. It affects the manufacturing process. For example, in paper insulated cables, the temperature at which the oil will penetrate through the paper will depend on its viscosity. Low viscosity liquids are more mobile and hence helps in transmission of heat by easy circulation as in the case of Power transformers and switch gears. Liquid insulations should not contain impurities as it will also affect the viscosity and the performance. The methods used to purify the insulating oil will also depend on the viscosity of the oil.

4. Porosity :

In solid insulators like pole insulators used for transmission and distribution lines, high porosity will increase the moisture holding capacity which is not desired because it adversely affects the electrical properties of the insulator. Therefore, high porosity is not a desired factor. But in certain other applications like in paper which is to be impregnated with oil to increase the insulation capacity, high porosity is desirable and advantageous. Therefore, this factor should be visualised with respect to application and decided as to what should be the level of porosity required for the insulating material.

5. Solubility : 

Certain insulating materials like varnish should be applied only after it is dissolved in proper solvents like acetone. In such cases the insulating material should be soluble in certain suitable solvents, but it should not be soluble in water otherwise the moisture of the atmosphere will be able to remove the applied insulation and cause breakdown.

6. Moisture Absorption : 

The insulating material should not absorb moisture as moisture lowers the electrical resistance and dielectric strength. Due to moisture absorption certain chemical and mechanical effects like swelling warping, corrosion may also occur. Therefore, the insulating materials should not absorb moisture but even if it so, it should be as low as possible without disturbing the electrical characteristic limit.

7. Machinability and Mouldability : 

In the manufacturing of solid insulating material, it should have the property of being easily moulded and machined into the required shape and size.

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