Why Diffrential and Ref relays are used together although Diffrential relay ca

Why Diffrential and Ref relays are used together although
Diffrential relay can protect Pri and Sec of Transformer

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Why Diffrential and Ref relays are used together although Diffrential relay can protect Pri and Se..

The normal bias setting in a differential relay is 20%.
So, complete earth fault protection is not possible with
differential relay. That is why you need a seperate earth
fault relay with sensitive settings like 5%.

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