A model is developed to calculate the threshold welding current for the two designs of large area vacuum interrupter contacts: the transverse magnetic field (TMF) contact and the axial magnetic field (AMF) contact. The model considers: (1) that large area, closed, vacuum interrupter contacts have more than one region, i.e. 'n', regions of contact; (2) the total contact resistance of the closed contacts by including the expected constriction resistance as well as the resistance resulting from the high temperature of the contact regions; (3) the effect of the 'blow-off' force from the passage of the current through the contact regions; (3) the attractive force generated by the magnetic field of the AMF contacts; and (4) the reduction of the hardness of the contact metal in the vicinity of the high temperature at the contact regions. A discussion of the model shows how it can be used as a design tool to analyze not only welded contact structures, but also to determine the contact force required to minimize the effect of welding during the passage of overload and short-time withstand currents.
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