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 –up may be set at 1.5 times the zero sequence capacitive current of the feeder. In this regard, the value of capacitive current of high voltage cables is app. 3 Amp/Km typically.
Also in order to achieve satisfactory sensitivity for resistive fault situations, the setting of earth-fault relays is considered less than or equal to %20 of the maximum capacitive current of the entire network.