Important properties of electrical insulation materials
- Volume and surface resistivity – surface resistivity is the insulating material’s resistance to leakage current on its surface while volume resistivity is its resistance to leaking current along its body. Generally, the higher the volume/surface resistivity, the lower will be leakage current and the less conductive will be the insulating material.
- Relative permittivity – or dielectric constant is the ratio of the electric flux density that is produced by the insulating material to that which is produced in vacuum by the same electric field strength.
- Dielectric loss – or electric dissipation factor is the ratio on the power loss of dielectric material to the total power that is transmitted through it. Dielectric strength is usually the most important property of electric insulation materials because it is the ability to withstand electric stress without breaking down.
Another significant factor in the choice for electrical insulation materials is the maximum temperature to which they can perform properly. It is typical for insulating materials to deteriorate more rapidly in high temperature and such deterioration can reach a point that can compromise the insulating material’s ability to perform its required function. This characteristic is usually referred to as ageing. If a reasonable lifetime is expected to be achieved, it is important to assign a maximum temperature beyond which the material will no longer be able to perform. When thermal class is used as a description for electrical equipment, it should represent the maximum temperature that is found within the product under rated load and other related conditions. However, it is important to take note that not all insulation is located at maximum temperature since insulation with lower thermal classification is also used in other parts of the equipment.
What does useful lifetime mean in electrical insulation materials?
Useful lifetime varies among different equipment and it is determined by factors like type and usage including the thermal stresses to which it is exposed to. For example, the ageing of an insulating material is different in a household appliance compared to a power station generator. The degree of insulation provided is not only dependent on chemical and physical properties but by the presence of and level of influence of mechanical, electrical and environmental stresses. In some cases, the way that the insulating material was processed during its manufacture can affect its useful lifetime. It is not uncommon for companies to test insulating systems by introducing different stresses. The focus should not be limited to material but the particular application whether it will be used as the primary insulating material or secondary insulating material to provide a line of defense in case the primary insulation is damaged.