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

Contact erosion depends on both duration and efficiency of interaction between arc and contacts. Arc motion in a magnetic field was recorded and the material loss caused by arc erosion was determined by weighing. The material loss related to 1 ms of arc interaction-named 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 for partly eroded contacts prestressed during life tests under AC 3 and AC 4 conditions according to IEC Test Specification 158-1, AC 4 erosion was caused by break arcs mainly, the effect of make arcs being negligible, whereas AC 3 erosion was due to both break and make arcs; the latter was more effective in Ag/SnO₂ contacts. After AC 3 life tests the erodibility values determining the erosion of Ag/SnO₂ 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