Arc occurrence rates are measured for Ag and Pd contacts operated in a DC resistive load circuit. Based on the obtained results, it is confirmed that arc certainly occurs at current levels lower than the conventional minimum arc current value for each of the tested contact materials. Arc occurrence rate in general comes close to 100% at the conventional minimum arc current level. Accordingly, careful attention should be paid to use of the term "minimum arc current" in order to avoid misinterpretation thereof.

Arc and contact resistance characteristics of Ag and Pd contacts were determined in several kinds of dielectric liquids, such as distilled water, methanol and n-hexane, under the inductive load condition. The experimental results showed that arc discharge types are dependent on dielectric liquids. A steady arc develops in air under this test condition. However, it was found that not the steady arc but the showering arc occurs in distilled water and methanol at a low load current. It was demonstrated that this phenomenon is caused by the high capacitance generated by the ambient dielectric liquid. Also, in almost all cases, the contact resistance behavior in dielectric liquids is satisfactory because metal spots remain on the contact surface. However, in n-hexane, the contact resistance tends to deteriorate, particularly for the Ag contact, with increasing load current. It seems that the deterioration of contact resistance is caused by carbon included in n-hexane.

In a DC 50 V/5 A circuit, the relationship between the number of breaking arcs and the spatial distribution of the spectral intensity of breaking arcs of long duration near the cathode in palladium contact were examined through substitution of the contact surfaces of three different shapes: flat and spherical (1 mm radius and 2 mm radius). Findings show the distribution of spectral intensity in Pd arcs to be influenced remarkably by the shape of contact surface and the number of breaking arcs. However, the temperature of Pd arcs was affected neither by the shape of contact surface nor by the number of breaking arcs. The metal-vapor quantity present differed for flat and spherical surface contacts; however, it was not affected by the radius of the curved contact surfaces or by the number of breaking arcs. Additionally, the longer the duration of the breaking arc, the more metal-vapor was presented in the beginning of the arc. Furthermore, arc tracks on contact surfaces were observed with microscopes, clarifying that the relationship between the area of the clouded white metal on the cathode and the shape of contact surface is the same as the relationship between the existent area of measured spectra and the shape of the contact surface.

The former experimental results have already shown that it is oxide films formed on contact surface causing the contact resistance to degrade in dc. breaking arcs for Ag and Pd materials. In order to understand the detailed information about it, the experiments are performed to break dc. inductive load at 20 V, 0.5 A and 1.0 A in nitrogen gas with different oxygen concentrations. The contact surface morphology and surface contamination are evaluated by SEM and AES, respectively. The tested results demonstrate that, for Ag contact, the severe oxidation occurs with increasing oxygen concentration, and the critical value of oxygen concentration is found to be about 10% and 5% in 0.5 A and 1.0 A, respectively, above those values the contact resistance degrades due to the oxide films formed on the contact surface, especially on the anode surface. While, for Pd contacts, a remarkable contact resistance degradation is not found even at 1.0 A in oxigen. Evidence shows that the arc duration, in particular the gaseous phase arc duration affects the anode oxidation, which in turn causes the significant fluctuation of contact resistance.