The operating principle of WHOCP (WH Overcurrent Protection Device) is based on the dynamic equilibrium of energy, with part of or all the head brought about by the current passing WHOCP component due to the reation with WHOCP sending forth into the ambience while the heat not sending out raising the temperature of WHOCP component. Temperature is rather low during normally working, while a dynamic equilibrium between the heat produced the that sent forth being reached. When WHOCP component being at low resistance state,WHOCP does not operate. And though the current passing WHOCP component increasest and the ambient temperature rises, if the heat produced and that sent forth reaching a equilibratory state, WHOCP will still not operate.When current and ambient temperature further raising ,WHOCP will come to a rather high temperature. If right now current and ambient temperature continue to further rise, then the quantity of heat produced will be larger than that sent forth, thus causing the tenperature of WHOCP component abruptly rising.In this stage, the least change of temperature will cause resistance  value substantially increasing and WHOCP component will be in the high resistance protection state.The increase of impedance will limit current,which will dramatically fall within a very short time, thus protecting circuit unit from damage. So provided for the quantity of heat produced by the applied voltage is enough to match that sent forth by WHOCP component, the WHOCP component under changing state will always remain in the operating state(high resistance)Once the applied voltage disappears, WHOCP will automatically reset itself.