The influence of switching arcs on contact resistance of Ag/SnO2 materials
Silver tin oxide (Ag/SnO2) contact materials are widely used for relays and contactor applications. Device approbation in accordance to standards requires several types of temperature rise test in new and switched conditions. Significant differences in temperature rise after the various switching sequences are experienced. In addition, differences between individual phases of one device - especially in switched condition - are known from test. In this work, the contact resistance evaluation under different load conditions has been studied. Therefore the arcing events at make and break operation during different endurance tests (AC-3 and AC-4) have been analyzed and correlated to the contact resistances at rated current level during temperature rise test. Surface layer changes of contact materials after load are characterized by metallurgical methods. The paper shows the formation of different surface layers as a function of arcing energies during make and break operation. The results demonstrate that make and break arc together are creating the surface layers defining the final contact resistance.
Effects of different loads on the surface of silver metal oxide contacts for general-purpose relays
Silver metal oxides have become very popular as contact materials in electromechanical switching devices due to their high resistance to erosion and contact welding. Initially, these materials were developed for contactors and circuit breakers. Investigations on silver metal oxide contacts for general-purpose relays were conducted in order to gain an insight into the effects and failure mechanism of such contacts under specific loads, such as resistive and inrush current loads. It will be shown that contact failures may be influenced by very thin silver layers on the surface of silver metal oxide materials and the behavior of these layers at thermal stress. The silver layers are a result of the mechanical treatment in the production of rivets and the following riveting process. The results support the development of new silver metal oxide materials for general-purpose relay applications.
Physical and chemical properties of metal oxide additions to Ag-SnO/sub 2/ contact materials and predictions of electrical performance
The new contact material AgSnO/sub 2/, which is replacing AgCdO in many applications, requires the addition of doping agents to improve electrical contact performance. These additions effect thermodynamic stability, the wettability of the SnO/sub 2/ surface by liquid silver, as well as the microstructure of the area melted by the electric arc. This work considers the frequently used dopants WO/sub 3/, MoO/sub 3/, Bi/sub 2/O/sub 3/, CuO and RuO/sub 2/, as well as others: ZnO, Ta/sub 2/O/sub 5/, HgO, RuO/sub 2/, Sb/sub 2/O/sub 3/, TeO/sub 2/ and TaC. DTA and TGA analyses are performed on mixtures of these metal oxides with SnO/sub 2/, Ag, and AgSnO/sub 2/. Reactions products, if present, are characterized by x-ray diffraction. The wetting angles of molten silver on surfaces of these mixtures are established. The results are used to interpret the microstructures found in the molten regions of AgSnO/sub 2/ materials doped with these substances. These structures are interpreted and related to performance in arcing contact applications. >