Conductors, electromagnet core, insulating medium in the current and its magnetic field will produce heat, the degree of heat can be characterized by the physical quantity of temperature. Under normal circumstances electrical equipment or line protection devices, in the selection of the right type, set the correct value, can overload equipment or line from the power supply to remove, equipment and lines will not overheat, the temperature rises, it will not cause a fire hazard. However, the capacity of electrical equipment or wire cross-section selection of small, the use of the load will increase, the protective device refuses to understand, equipment and lines in a long-term overload fault state, the temperature will rise to about 160 degrees above the insulation softening, aging accelerated, the life of the shortened, formed by the overload of superheated temperatures may be insulated or the surrounding combustible materials ignite on fire.