We first discuss what it means by insulation class and what is its importance. The insulating material is classified according to the maximum temperature which an insulating material can sustain or in other words, the maximum temperature up to which the insulating material functions reliably without failure.
The insulating material is apt to fail if the temperature is increased above its rated maximum temperature capacity. The life of the insulating material gets halved with every 10 ∘C rise above its specified class temperature. If the insulating material of temperature class of 90 ∘C is operated at 100 ∘C its life will reduce to half of its specified life.
There are seven temperature classes of insulating materials which are widely used for electrical applications. These are;
| Insulation Class | Maximum Temperature withstanding capacity (∘C) |
| Class -Y | 90 |
| Class -A | 105 |
| Class- E | 120 |
| Class -B | 130 |
| Class -F | 155 |
| Class -H | 180 |
| Class -C | Above 180 |
Insulation Class of Transformer Oil
The mineral oil is used in the transformer for transformer cooling and for providing necessary insulation between the winding and the tank of the transformer. The transformer oil can sustain temperature rise up to its flash point. The flash point of the transformer is 140 ∘C. The transformer can be theoretically operated up to the temperature below the flash point temperature 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 any part of the transformer inside the tank. The part of the transformer inside the tank is the winding and its insulation. The insulation of the winding is made of press wood and impregnated paper insulation. The maximum allowed temperature for impregnated paper insulation is 105 ∘C.
Therefore, the transformer can be operated safely if the temperature rise of the winding is up to 105∘C., and not up to the flash point of the transformer oil. The impregnated insulation paper is classified as a class-A insulating material and its maximum operating temperature is 105∘C.
Thus, the insulation class of the oil-filled transformer is class-A, and the temperature of the winding must not exceed 105∘C.
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