Contact Blow-Apart Forces: Experience in Molded Case Circuit Breaker Contact Systems

The blow-apart forces due to current constriction cause popping in the contacts of Molded Case Circuit Breakers (MCCBs). The current level at which popping occurs is a critical design parameter. While there are many studies available regarding contact resistance, comparatively few are available regarding blow-apart forces. One reason may be that in the case of MCCBs it is necessary to calculate the magnetic influence of the overall structure in order to separate this from the forces due to current constriction. Usually FEA is needed to accomplish this. The authors have the remarkable situation of not only having completed magnetic FEA simulations on a wide variety of MCCB contact systems, but they also have access to spring force measurements and high power test results on these same devices. Correlation was made between test results and predicted values from a commonly accepted formula. In general, the accepted formula predicts popping currents that are significantly lower than actual experience. Possible reasons are explored to explain the observations.