Characteristics of Dielectrics (Insulation)

Dielectrics (insulation) for electrical equipment and apparatus is used for many different applications. It is expressed for a wide range of environmental conditions such as temperature, moisture, chemicals, other contaminants, and exposure to weather. One major factor affecting insulation life is thermal degradation, although moisture, contamination, voltage stress, and other factors may also contribute to its degradation. In addition, the life of an insulating material depends on the degree of loading, the type of service to
which the equipment is subjected, the care it receives during installation and operation, and mechanical vibration and forces to which it is subjected.

The properties of insulating materials that are necessary and desirable are surface leakage, resistance to moisture, chemicals, oils and other contaminants,and mechanical properties. The important electrical characteristics of insulation are volume resistivity, PF, DF, capacitivity, dielectric constant, and dielectric strength. These characteristics, except for dielectric strength, can be assessed by nondestructive testing. These tests are

1. AC dielectric loss
2. PF or DF (tan d )
3. Capacitance
4. AC resistance
5. Radio interference voltage (RIV)
6. DC insulation resistance
7. DC dielectric absorption