Anodic to cathodic arc transition according to break arc lengthening

Material transfer in switches and relays has been described in several reviews. Different transfer directions have been attributed to the two well known arcs type: anodic and cathodic, but no further investigations have been performed. In this paper, we use a breaking apparatus (14 V DC, 20–80 A, 0–50 mH) equipped with an arc length control device in the range of 1 μm to few mm. This device allows us to examine mass transfer with arc lengthening for pure metals (Ag, Cu), silver alloys (AgCu, AgNi) and silver metal oxides (AgCdO, AgSnO2). The main result is that arcs occur first in an anodic stage and then in a cathodic stage, and that this transition takes place at a critical length which is not affected by current and contact materials. This length has been measured to be 15–30 μm, and corresponds, for physical reasons, to a few times the electron mean free path length. In addition, the cathodic arc is divided in two stages with arc lengthening: The compensation stage where anodic loss and cathodic gain are still measured because of the mass accumulated in the earlier anodic stage; The pure cathodic stage where cathodic loss and anodic gain are measured. This last transition takes place at the net zero erosion point and depends on arc energy and contact materials. Finally, we observed different erosion rates according to these successive stages and different contact materials. For example, we measured less erosion for AgSnO2 than for AgCdO before the net zero erosion point and reversed material performance in pure cathodic stage. This shows us that material evaluation must take into account arc length/duration, to operate in a well defined erosion stage.