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Reply To: Protection Riddle No.38 – Restricted earth fault protection and Inrush current

#1898
K B Santharam

    Dear All, REF protection being a current balance scheme applied to only one side of the transformer, it would appear that no second harmonic restraint is required as in the case of differential relay. However, REF relay, usually being set very sensitive (about 10-15% of CT rating) and without any intentional time delay, it is imperative that CT secondary current balance is maintained fairly accurately for all through faults, in zone phase faults, system recovery transients (after external fault has been cleared by another relay) and transformer