B curve, the tripping current is 3-5In,it mainly protects for people and big length cables in TN and IT; C curve, the tripping current is 5-10In, it mainly protects for resistive and inductive loads with low inrush current; D curve, the tripping current is 10-20In,it protects for circuit which supply loads with high inrush current at the circuit closing, such as LV transformaers, breakdown lamps.