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A new current limiting technique called "ISTAC" (Impulsive Slot Type Accelerator) has been developed for MCCBs (Low-Voltage Molded Case Circuit Breakers). The ISTAC is a technique where the magnetic flux density around the fixed contact in an arc chamber is enhanced using a new fixed conductor which is formed in a J-shape. This increases the arc driving force and the opening torque of the movable conductor at the beginning of the current-breaking motion.During this development, the magnetic fields in arc chambers were analyzed by 3-dimensional nonlinear niumerical analysis and the usefulness of the analysis was confirmed by measurement. A new arc chamber configuration was designed and optimized using this analysis. It became clear that at the beginning of the currentbreaking motion the magnetic arc driving force and the opening torque of the movable conductor in the new arc chamber with the J-shaped conductor were larger than those in old arc chambers with a U-shaped fixed conductor. In a high current-breaking test, an experimental MCCB with the new arc chamber showed 1.6 times the rising rate of arc voltage of the old one.By use of the ISTAC, a new type of MCCB has been developed, having a high level of current limiting performance in spite of its small external dimensions. Moreover, free spaces in the MCCB due to the reduction in size of the arc chamber could be used for supplementary functions.