Nonconducting material that provides electric isolation of two parts at different voltages. To accomplish this, an insulator must meet two primary requirements: it must have an electrical resistivity and a dielectric strength sufficiently high for the given application. The secondary requirements relate to thermal and mechanical properties. Occasionally, tertiary requirements relating to dielectric loss and dielectric constant must also be observed. A complementary requirement is that the required properties not deteriorate in a given environment and desired lifetime.
Electric insulation is generally a vital factor in both the technical and economic feasibility of complex power and electronic systems. The generation and transmission of electric power depend critically upon the performance of electric insulation, and now plays an even more crucial role because of the energy shortage.
Important properties of insulating materials
- Resistivity (specific resistance) is the property of a material that quantifies how strongly the material opposes to flow of electric current. Resistivity of a good insulator is very high.
- Dielectric strength of a material is the ability to withstand electric stresses without breaking down. Dielectric strength is usually quoted in kilovolts per millimeter (kV/mm).
- Relative permittivity (or dielectric constant) is the ratio of the electric flux density produced in the material to that produced in vacuum.
- Electrical dissipation factor (dielectric loss) is the ratio of the power lost in the material to the total power transmitted through it. It is given by the tangent of the loss angle and, hence, also known as tan delta
Some of the commonly used electrical insulating materials are paper, mica, teflon, rubber, plastic, polyvinyl chloride (PVC), glass, ceramic, porcelain etc.
The insulating material should have the following properties.
- The material must have high mechanical strength so that it carries the tension and weight of the conductors.
- They must have high dielectric strength.
- The material is highly resistive for preventing the flow of leakage current from the conductor to earth.
- The material is non-porous and free from impurities.
- The electrical and chemical property of the material should not be affected by the temperature.