Themaximum value of earth fault loop impedance (Zs) that will allow the protective device in a circuit to disconnect an earth fault within the prescribed time.
Example: if themaximum earth fault disconnection time for a circuit is 0.4 s and a fault current Ia = 320 A is required to operate the circuit protective device in that time: Max Zs = (Cmin x Uo)/Ia = (0.95 x 230)/320 = 0.7188 Ω
The earth fault loop impedance, i.e., the path through which fault current will flow for an earth fault in the circuit under consideration. It comprises the sum of:Ze, the external impedance; Z1, the line conductor impedance;Z2, the circuit protective conductor impedance.
Zs = Ze + (Z1 + Z2).