The item is in accordance with standards of VDE 0664, CEE27 and BS4293 etc, applying to the circuit of AC 50/60Hz,240V single phase, 415V three phases and below it for industrial and mining enterprise, building trade, commerce and family. To mainly prevent electric fire and personal casualty accident caused by personal electric shock or leakage of electrified wire net , this ELCB is a currentoperated, fast leakage protector of pure electromagnetic type, which can bread off fault circuit rapidly in order to avoid occurrence of accident. The item is precise in structure,less elements,without auxiliary power and high working reliability.The function of the switch won't be influenced by ambient tempe-rature and lightning.The mutual inductor of the item is used to text vecto differential value of passing current,and produces a relevant output power and adds it to the transformer in secondary winding.If the current of vector differential value of protested circuit of perating current,the trip will act and cut off quickly so that the item will take effect of protection.