Sulfide Layer Formation and Current Carrying Deterioration by the Joule's Heat of Contacts in SF₆ Gas

Abstract

The thermal deterioration of the contacts in SF₆ gas has not been generally understood. This paper describes the sulfide formation by the direct reaction of Ag and Cu to SF₆ gas with moisture at a relatively low temperature, and the current carrying deterioration of contacts by such formation. The results are summarized as follows :1) the thermodynamical approach shows that Ag and Cu react to SF₆ with moisture over 252°Cand 165°C, respectively, and then Ag₂S and Cu₂S are produced, 2)these reaction temperatures are supported by the experiments and much sulfide is produced by higher temperature and moisture and 3) the 160-280 A current carrying tests of the Ag-WC and Cu-W contacts with high contact resistance of about 1mΩ show that only the Cu-W contacts over the sulfide forming temperature were covered with a copper sulfide layer and therefore could not carry these currents after 55-130 hours. To examine the rise of contact resistance for the actual Ag-WC and Cu-W contacts by arc erosion, the switching tests of 200 A and 20, 000 operations were also carried out and the Ag-WC and Cu-W contact resistances at 0.01% occurrence probability were recognized to be 0.7 and 1.3 mΩ, respectively. Therefore it is concluded that the Ag-WC is sulfide reaction free but the Cu-W possibly produces copper sulfide under the above conditions.