Scaling of high current GIS contact thermal stability

When two pieces of metal are pushed together with moderate force as in a sliding high current contact, they typically touch at 10 to 50 spots which must carry all the current passing through the contact. The total contact area is determined by the force and hardness of the contact material; however, the number of contact spots and average area of each can vary with the nature of the contact surfaces. Through the use of transient nonlinear finite element analysis, we have solved for the temperature as a function of time in the contact spot to determine the conditions under which a spot is thermally stable. Based on this investigation, we have developed physically based scaling rules for thermal time constants and the maximum current for which the spot will be thermally stable. These scaling rules imply that the total current for stable contact operation goes as the square root of the number of contact spots, recognizing that the total contact area is fixed.