What is the Insulation Class of Transformer Oil?

Before we jump into the insulation class of Transformer Oil, it is imperative to discuss briefly about the insulation class. Insulation Class basically defines the maximum temperature at which an insulating material will perform its specified duty for the specified life on continuous basis.

There are six insulation classes. These are tabulated below:


Life of insulation depends on its temperature. As a thumb rule, an increase in the temperature of insulation by 10 degree Celsius above its specified class temperature, reduces the life of insulation by nearly half of its specified life.

Insulation Class of Transformer Oil:

As the maximum working temperature of Transformer is 105 degree Celsius, hence it is assumed that the insulation class of Transformer Oil is Class-A. However, it is not true. In fact, the maximum working temperature of this oil is decided by its Flash Point temperature not by the insulation class. Generally, the flash point of a new transformer oil is 140 degree Celsius. However, it also depends on service conditions and internal faults. In any case, it should not be allowed to go below 130 degree Celsius. For detail, kindly read “Flash Point of Transformer Oil and its Importance”.

In fact, transformer oil can perform its duty even at higher temperature, provided its flash point is taken care. However, the limitation is put by the press board and impregnated paper insulation used in the transformer winding. These belongs to Class-A insulation class and therefore transformer winding temperature shall not be allowed to be more than 105 degree Celsius. This restricts the maximum operating temperature of transformer oil to Class-A temperature (105 degree Celsius).

Further, unlike other insulating material, life of transformer oil not only depends on the temperature but it also depends on many other factors. Therefore, insulation class of Transformer Oil is not defined.

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