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.