Answer / tapas

* In REF scheme 4 nos of CTs are used. 3 for phase and 1 for Neutral.
* It operates in the principle of balance current between phase and neutral and only inside In-zones. i.e. it protects Transformer, Busduct, LV switchgear.
* In this protection phase CTs are connected in such a way that it forms a close path between phase CT secondaries and this close path is connected with neutral CT secondary.
* In between the close path connected to common terminals of phase CT secondary and Neutral CT secondary REF relay is connected.
* When a phase fault occurs there is also residual current in Neutral. So current between both paths oppose each other and hence there will be no current in REF relay. So it won't operate.

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