Preventing Low Insulation Resistance (IR) Value Assignment Help

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Preventing Low Insulation Resistance (IR) Value:

The insulation resistance might be lowered because of moisture content in the oil and in the winding insulation, and might cause transformer failure. To avoid this, the entire transformer core with windings should be placed in hot air chamber until the moisture content is erased from the core and winding insulation resistance could be measured along with the help of a Megger. The minimum safe insulation resistance for various voltage ratings of windings is given in Table .

                       Table : Minimum Safe Insulation Resistance in M? (Mega-ohm)



Rated Voltage of the



Minimum Safe Insulation Resistance in M? at

Different Temperatures


30°C             40°C          50°C         60°C


66 kV and above


600               300            150           75


22 kV and 33 kV


500               250            125           65


6.6 kV and 11 kV


400               200            100           50

Below 6.6 kV

200               100            50             25

            Ensuring suitable insulation resistance enhances the life of transformer.

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