Answer / hanupowergrid

TAN DELTA TEST:

A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material, there is zero percent impurity. In pure capacitor, the capacitive electric current leads the applied voltage by 90°. In practice, the insulator cannot be made 100% pure. Also due to the ageing of insulator the impurities like dirt and moisture enter into it. These impurities provide the conductive path to the current. Consequently, leakage electric current flowing from line earth through insulator has also resistive component.

Hence, it is needless to say that, for good insulator, this resistive component of leakage electric current is quite low. In other way the healthiness of an electrical insulator can be determined by ratio of resistive component to capacitive component. For good insulator this ratio would be quite low. This ratio is commonly known as tanδ or tan delta. Sometimes it is also referred as dissipation factor.

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