Heat is a critical factor in the reduced life of electrical equipment. The type of insulation is classified depending on the operating temperature that the motor will experience.

Today we will learn about the meaning of insulation class and how important it is.

The insulating material is used in the motor basing on the maximum temperature which an insulating material can undergo or in other words, the maximum temperature up to which the insulating materials functions reliably without failure.

The insulating material is likely to fail if the temperature rises above its rated maximum temperature capacity.

The life of the insulating material reduces to half with every 10-degree Celcius rise above its specified class temperature. If the insulating material of temperature class of 90 degrees Celcius is operated at 100 degrees celsius its life will get halved of its specified life.

The temperature classes of insulating materials are classified to use for electrical applications as follows:


 Insulation Class                   

Maximum Temperature withstanding
capacity (

 Class -Y


 Class -A


 Class- E


 Class -B


 Class -F


 Class -H


 Class -C

 Above 180

Insulation Class of Transformer Oil

The mineral oil is commonly used in the transformer with the purpose of cooling and providing necessary insulation between the winding and the tank of the transformer. The transformer oil can undergo temperature rise up to its flashpoint. The flashpoint of the transformer gets 140 degrees Celcius. Theoretically, transformers can be operated at temperatures below the flashpoint of the oil. However, this is not true. Why? 

The transformer can be operated safely up to the temperature of the lowest safe operating temperature of part of the transformer inside the tank. The winding and its insulation are parts of the transformer inside the tank. The pressed wood and impregnated paper insulation are used to make the insulation of the winding. Impregnated paper insulation has the maximum allowed temperature of 105 degrees Celcius.

Therefore, the transformer can be operated safely if the temperature rise of the winding is up to 105C., and does not exceed above the flashpoint of the transformer oil. The impregnated insulation paper with the maximum operating temperature of 105C is classified as a class-A insulating material. 

Thus, the insulation class of the oil-immersed transformer is class-A, and the temperature of the winding must not be up to 105C. 


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