Insulating Materials Types:

There is no piece of electrical equipment that does not depend on electrical insulation in one form or an other to maintain the flow of electric current in desired paths or circuits. If due to some reasons the current deviates from the desired path, the potential will drop. An example of this is a short circuit and this should always be avoided. This is done by proper choice and application of insulation wherever there is a potential difference between neighboring conducting bodies that carry current. There are four Insulating Materials Types areas where insulation must be applied. They are

  • between coils and earth (phase-to-earth),
  • between coils of different phases (phase-to-phase),
  • between turns in a coil (inter-turn), and
  • between the coils of the same phase (inter-coil).

As we know, there are three broad categories of insulating materials, gases, liquids and solids. The Insulating Materials Types are classified mainly based on the thermal endurance. The insulation is primarily meant to resist electrical stresses. In addition, it should also be able to withstand certain other stresses which the insulation encounters during manufacture, storage and operation.

The performance of the insulation depends on its operating temperature. The higher the temperature, the higher will be the rate of its chemical deterioration, and hence the lower will be its useful life. If a reasonably long life of an insulation is expected, its operating temperature must be maintained’low. Therefore, it is necessary to determine the limits of temperature for the insulation. which will grouped into different classes O, A, B, and C with temperature limits of 90°C, 105°C and 130°C for the first three classes and no specific limit fixed for class C. Classes O and A cover the various organic materials without and with impregnation respectively, while classes B and C cover inorganic materials, respectively with and without a binder.

With the advent of newer insulating materials, namely, the plastics and silicones, during the middle of this century, a need was felt to reorganize the classification of the Insulating Materials Types. This led IEC (International Electrotechnical Commission) to come up with the new categories:

Class Y (formerly O): 90°C: Paper, cotton, silk, natural rubber, polyvinyl chloride, etc., without impregnation.

Class A: 105°C: Same as class Y but impregnated, and nylon.

Class E: 120°C: Polyethylene terephthalate (terylene fibre, melinex film), cellulose triacetate, polyurethanes, polyvinyl acetate enamel.

Clase B: 130°C: Mica, fibreglass (alkali free alumino borosilicate), bitumenized asbestos, bakelite, polyester enamel.

Class F: 155°C: As class B but with alkyd and epoxy based resins.

Class H: 180°C: As class B with silicone resin binder, silicone rubber, aromatic polyamide (nomex paper and fibre), polyimide film (enamel, varnish and film) and estermide enamel.

Class C: Above 180°C: As class B but with suitable non-organic binders; teflon (polytetrafluoroethylene), and other high temperature polymers.

The temperatures mentioned above cannot be regarded as the limiting operating temperatures but only as an index to compare the Insulating Materials Types. All the international standards permit the equipment to work up to these temperatures, but in practice, certain differentials are allowed because of the overloads, other manufacturing advantages and economics.