| Summary: | A molded case circuit breaker (MCCB) is one of the most important factors for safety to protect a load from overcurrent in a power distribution system. MCCB, which is mainly installed in switchboards and distribution boxes, may be affected by external temperatures and magnetic fields, but the above factors are still excluded from product standards and performance evaluation. This paper is the result of experimenting and studying the negative effects of these external temperatures and external magnetic fields on MCCB with short-circuit characteristic and dielectric recovery strength. As a result of temperature, it can be found that both short-circuit characteristic and dielectric recovery strength change linearly in accordance with the external temperature. The ratio of the values of 35 °C to 25 °C and 45 °C to 25 °C show the following results. t<sub>10</sub>, t<sub>21</sub>, and t<sub>32</sub> are 1.58, 1.53, and 1.79, respectively, in short-circuit characteristics and t<sub>i</sub>, t<sub>m</sub>, and t<sub>l</sub> are 1.59, 1.69, and 1.53, respectively, in dielectric recovery strength. Depending on the external magnetic field, the short-circuit characteristics decreased by 8.56% only in the t<sub>21</sub> period. The dielectric recovery strength decreases by 4.92% in the initial section (t<sub>i</sub>) and 14.45% in the later section (t<sub>l</sub>), respectively. It has been confirmed that the external magnetic field interferes with the emission of hot gas.
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