Erosion behaviour and `erodibility´ of Ag/CdO and Ag/SnO2 contacts under AC 3 and AC 4 test conditions

Contact erosion depends on both the duration and efficiency of interaction between arc and contacts. The arc motion in a magnetic field was recorded, and the material loss caused by arc erosion was weighed. The material loss related to 1 ms of arc interaction, called erodibility, was evaluated for the contact materials Ag/CdO and Ag/SnO ₂. All tests were performed in one type of commercial contactor (rated current 110 A). The erodibility was determined for new and partly eroded contacted prestressed during life tests under AC 3 and AC 4 conditions according to IEC test specification 158-1. AC 4 erosion was mainly caused by break arcs, but the effect of make arcs was negligible, whereas AC 3 erosion was due to both break and make arcs. The latter were more effective in Ag/SnO₂ contacts. After AC 3 life tests the erodibility values determining the erosion of Ag/SnO ₂ contacts at the site of arc formation were higher than those of Ag/CdO; after AC 4 life tests, however, there was hardly any difference between the erodibilities of the two materials