A Model to Determine the Current Level to Weld Closed Contacts

A model is developed to determine the peak threshold current required to weld closed contacts of a circuit breaker or switch when passing fault currents. The model considers: 1) the total contact resistance of the closed contacts by including the expected constriction resistance as well as the resistance resulting from the high temperature at the contact region; 2) the effect of the “blow-off” force from the passage of the current through the region of contact; 3) the reduction of the hardness of the contact metal in the vicinity of the high temperature at the contact region; 4) the effect of forces on the contacts from the current path through the circuit breaker or switch design; 5) the effect of the contact design; 6) the short term passage of current; 7) the effect of multiple regions of contact; and 8) the effect of the passage of current from 1 to 4 s. Published experimental data of threshold welding current are compared to the analytical model.