ST08 is a protector with double protection function of overcurrent and overtheating, and its built-in heating wire and bimetal form a series circuit. The overload current when the motor is blocked will make the heating wire heat up rapidly, which will heat the bimetal disc and accelerate the heating speed of it, so that the bimetal disc can reach the rated operating temperature faster and bounce open, cutting off the circuit. The heating wire with different resistance values has different heat generation, resulting in different trip times. By adjusting the resistance of the heating wire, it is possible to control the tripping speed of the protector when the current value is fixed.