Bridge Erosion in Electrical Contacts and Its Prevention

Careful examination of contacts operated under conditions in which bridge erosion predominates shows that, despite a gross transfer of material from anode to cathode, the explosion of a molten bridge leaves a small crater in each contact. The diameters of such craters are believed to be those of the molten bridge just before rupture and they vary with current and with resistivity of the contact material in the expected manner. Although for some materials the bridge has equal diameters in the two contacts, in other cases it is asymmetric, that is, the diameters in the two contacts may differ considerably. Results of bridge erosion tests suggest that a transfer of material in the direction of the smaller end of the bridge is associated with such asymmetry. By pairing two materials which differ in natural bridge diameter, an artificial asymmetry is created. The transfer tendency of such a combination is called here asymmetry transfer and is shown to be self-limiting, because the act of transfer causes the gaining electrode to become similar to the losing electrode, thereby removing the driving force which causes the action. The concept of self-limiting transfer is developed, whereby it is shown that a condition of zero net transfer will result if contacts are operated under conditions so that, (a) asymmetry transfer is greater than the resultant of other transfer forces acting on the contacts, and (b) is opposite in direction to that resultant.