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G’ day, I am working on designing protection devices for my research work and I am struggling to model the thermal part for the low voltage circuit breakers based on IEC standards. Based on the standards, the time current curve for types B, C and D are as mentioned on the attached figure. I could not find the proper formula which represents the thermal characteristic for these types (the curve inside the red circle). Please, assist on this matter. Thank you for your cooperation in advance.

Time-current curve

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  • \$\begingroup\$ Do you mean sensitivity to temperature, or the details of how the circuit breaker's innards heat up under load? Keep in mind that the sort of graph you posted isn't supposed to be what any breaker actually does -- it's supposed to be a specification for what a given breaker should do. Actual breakers just need characteristics that stay inside the shaded areas to pass. \$\endgroup\$ – TimWescott May 1 at 1:18
  • \$\begingroup\$ I mean the thermal part of the mentioned figure, which is the curve that starts from 7200 sec. up to 0.5 sec. for type c for example. \$\endgroup\$ – Ibrahim Ibrahim May 1 at 2:08

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