Types of Electrical Insulation Classes- Application

There is no single electrical equipment that does not depend on insulation material.

insulation required in electrical motor, transformer,between two live conducting wire, to avoid short circuit condition.insulating material has  many advantages in electrical equipment. 
in one form or other to maintain the flow of electric current in desired paths or circuits.If due to some reason the current changes its path, the potential will drop.


An example of this is a short circuit and this should always be avoided. This is done by using  proper insulation wherever there is a current or voltage

Electrical Insulating material and class of insulation,application of insulation

There are four main  areas where insulation must be applied. 

They are follows


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

    There are three main categories of insulating materials, gasesliquids and solids. The insulating materials are classified mainly based on their thermal capability.

  The insulation is the primary thing to resist electrical stresses.it should also be able to withstand to 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 degradation, and hence the lower will be its operating life.


if life of insulation is expected long, its operating temperature must be maintained low i.e in it’s operating range. Therefore, it is necessary to determine the limits of temperature for which insulation, will ensure safe operation over its expected life.

Thus the insulating materials are divided into different classes O, A, B, and C.

What are types of insulation/ classes of insulation, uses and temperature range:

Insulation Class Y : 9O°C: Paper, cotton, silk, natural rubber, poly vinyl chloride,

etc. formerly O
Insulation Class A: 105°C 
Class A insulation consists of materials such as cotton, silk and paper when suitably impregnated or coated or when immersed in a dielectric liquid such as oil. Other materials or combinations of materials may be included in this class if by experience or tests they can be shown to be capable of operation at the Class A temperature
Insulation Class B: 13O°C
Class B insulation consists of materials or combinations of materials such as mica, glass fibre, asbestos, etc., with suitable bonding, impregnating or coating substances.

Insulation Class C: Above 18O°C
Class C insulation consists of materials or combinations of materials such as mica, porcelain, glass, quartz with or without an inorganic binder . Other materials or combinations of materials may be included in this class ,Mica, fibreglass (alkali free alumino borosilicate), bitumenized asbestos, bakelite, polyester enamel

Insulation Class E: 12O°C
Polythylene,terephthalate , cellulose,triacetate, polyurethanes.

Insulation Class F: 155°C
As class B  Mica, fibreglass , bitumenized,asbestos, bakelite, polyester enamel. but with alkyd and epoxy based resins.

Insulation Class H: 18O°C
Class H insulation consists of materials such as silicone elastomer and combinations of materials such as mica, glass fibre, asbestos etc.

As class B with silicone resin binder, silicone rubber, aromatic polyamide, polymide film and estermide enamel.

 All the national standards allow  equipment to work up to these temperatures, but in
practice, certain differentials are allowed because of the over loads, other manufacturing advantages and economics.

Uses of insulating material in various equipment:


Transformers are the first to faced lightning and other high voltage surges. The

transformer insulation has to withstand very high surge voltages many times the
power frequency operating voltages. The transformer insulation is mainly divided

  1. conductor or turn-to-turn insulation,
  2. coil-to-coil insulation,
  3. low voltage coil-to-earth insulation,
  4. high voltage coil-to-low voltage coil insulation, and
  5. high voltage coil-to-ground insulation.



 Classes Y and C insulation find no application in rotating machines. Class E which was widely used in low voltage machines for over 20 years is now  replaced by class F which is useful  for the high voltage machines.

Also,Class F is widely  used in place of class B. Considerable development  has been done in recent years, in reducing the size of the electrical machines  by use of class H materials, particularly, for small machines. however,the  class H materials silicones, teflon is very expensive , and hence they are used only  in traction motors and mill motors.


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