The net zero erosion phenomena on opening switching contacts with AC loading

Studies have been conducted of silver metal oxide contacts on a purposed built apparatus to investigate the extent of erosion under opening (break) operations. The investigation focused on opening the contacts under AC loading of up to 30 A rms and opening these contacts at particular points in the current duty cycle (or Point-On-Wave, POW). A number of erosion and deposition mechanisms have been observed to dominate at particular current loading which were also dependent on the point-on-wave opening. These mechanisms were found to balance at particular current values and point-on-wave opening leading to a net zero mass transfer between the two contacts. This paper presents data on mass transfer against these two parameters of current and point-on-wave showing the net zero mass transfer curve. It was found that the curve was the same for both silver tin oxide and silver cadmium oxide. However, in the case of silver tin oxide the magnitude of erosion and deposition was larger than in the silver cadmium oxide case. The metal and gaseous ion phases of the arc discharge are presented as mechanisms behind this phenomena.