The Net zero mass loss phenomenon on opening switching contacts with AC loading

Studies are conducted on silver metal oxide contacts on a purposed built apparatus to investigate the extent of contact erosion under opening (break) operations. The contacts are opened between 0.1 and 0.8 ms/sup -1/ at particular point-on-wave, t/sub POW/, of the ac waveform. Current loading of up to 30 A rms, I/sub rms/ is investigated. Contact material deposition, as well as mass loss, is observed and is found to balance at particular current values and point-on-wave opening leading to a net zero mass loss on the contacts. Results are presented on mass change for each contact against the two parameters of current and point-on-wave. An arc duration characteristic, /spl zeta/, is plotted against current, I/sub rms/ which has a net zero mass loss. The characteristic is shown to be the same for both silver tin oxide and silver cadmium oxide but is influenced by the opening velocity. The metallic and gaseous ion transfer mechanisms of the arc discharge are presented as the main processes behind this phenomenon.