Some basic concept for REF setting is noted above, also generally the “Threshold 10% of In and no delay” is recommended for 64REF setting, but for applying suitable setting, you must refer to relay manufacturer guide. For example in Micom protective relay (biasing restricted earth fault) two different operating modes with different properties are available by choice, biasing by residual current and biasing by maximum phase current, with related tripping characteristics forms which shall be set according to electrical system requirements and relay instruction.For more information please refer to MICOM 30 Series APPLICATION GUIDE, issued by ALSTOM