Thermal-magnetic trip units used within residential and commercial molded case circuit breakers are generally limited by geometric considerations from providing low current magnetic trip response. U.S. Pat. No. 4,513,268 describes a residential type molded case circuit breaker incorporating a thermal-magnetic trip unit in accordance with the prior art. U.S. Pat. No. 4,951,015 describes a movable core that is designed to move into the gap existing between the core and armature of a magnetic trip unit to reduce the primary air gap and increase the magnetic flux. The movable core effectively allows the circuit breaker to trip at lower current levels. U.S. Pat. Nos. 3,179,767, 3,278,707 and 3,278,708 each describe the use of an additional turn of wire around the magnet used within the thermal-magnetic trip unit to increase the magnetic forces on the armature at low currents.
Additionally, U.S. patent application Ser. No. 841,182 entitled "Thermal-Magnetic Trip Unit with Low Current Response" now U.S, Pat. No. 5,173,674 describes a molded case circuit breaker trip unit employing a pivotally-arranged magnet that moves unit employing a pivotally-arranged magnet that moves toward the armature to reduce the magnetic gap separation distance.
The aforementioned thermal-magnetic trip assemblies are found to add to the materials and assembly costs of the residential circuit breakers employing thermal-magnetic trip units.
The addition of supplemental magnets and armatures correspondingly increases the manufacturing tolerances that must be carefully controlled to insure compliance with the relevant industry standards.
One purpose of the invention accordingly is to provide a thermal-magnetic trip unit providing low current magnetic trip response with automatic tolerance compensation at relatively low cost.