Method for adjusting trip sensitivity of thermal overload protection apparatus

A method for adjusting a trip sensitivity of a thermal overload protection apparatus, the thermal overload protection apparatus comprising bimetals for providing a driving force for trip operation by being bent when an overcurrent is conducted in a circuit, a shifter mechanism for transferring the driving force from the bimetals by contacting the bimetals, a trip mechanism rotatable to a trip position at which the circuit is broken at a time of release, a trip latch mechanism movable to a position for releasing the trip mechanism from a position for restricting the trip mechanism by the driving force from the shifter mechanism, and an adjusting knob for adjusting a gap between the shifter mechanism and the trip latch mechanism, the method comprising:

• setting a position of an adjusting reference point for the adjusting knob;
• measuring a position of the bimetals when a normal current is conducted in the circuit;
• measuring a moving distance of the trip latch mechanism by arbitrarily moving the same in a direction of trip operation by the time of trip event;
• determining an assembling position for the shifter mechanism based on the measured moving distance when the trip latch mechanism performs the trip operation, information on a predetermined trip distance between the shifter mechanism and the trip latch mechanism, and information on a size of the shifter mechanism;
• assembling the shifter mechanism at the determined assembling position;
• conducting a predetermined overcurrent to the thermal overload protection apparatus;
• measuring a conducting time for the overcurrent until a trip event;
• calculating a difference between the conducting time measured in said measuring the conducting time for the overcurrent and a predetermined trip time by converting the difference into a rotation angle;
• and
• marking a graduation of the trip operation current from the position of the adjusting reference point initially set in said setting the position to a position adjusted by the rotation angle calculated in said calculating.