In impedance earthed networks which subjected to earth leakage concerning, the earth fault current is limited to a value in the range app. 10 to 1000 ampere. In this systems, the feeders is equipped with earth fault protection which fed either from a core-balance CT or from the residual connection of the 3 line CTs.When the residual connection of 3 line CTs is applied, the operating level should not be set below %6 of the CT nominal current. This is due to the fact that in the event of an inter-phase short-circuit , the current transformers may start to saturate in a non-symmetrical manner, and this could cause spurious operation of a relay whose operating level is set too low. 
If a relay supplied from a ring (core-balance) CT, the pick