Anode Surface State and Anode Temperature Distribution After Current Zero for Different AMF-Contact Systems

The aim of this paper is to compare the state of anode surface of different AMF contact systems during arcing and after arc extinction. Arcs were ignited by opening of electrodes and were fed by sine half-wave current pulses with durations of 10 ms. CuCr electrodes were used. We performed investigations in the range of arc currents from successfully interrupted ones up to the levels of guaranteed failure (limiting interrupted current) if the transient recovery voltage was applied. The measurements were implemented by a high-speed Phantom M310 video camera equipped with a Carl Zeiss 100/2 macro lens. We studied the anode surface state during the arcing using the set of neutral density filters. We found that at currents close to the limiting interrupted current, most of the anode surface melts. Liquid metal is squeezed out of the axial area and an annular crest is formed. The crest splashes out and breaks into separate jets. The jets are broken up into fragments, from which droplets separate. The anode surface temperature distribution just after current zero was obtained by comparison of the results of anode surface filming and the results of calibrated band-lamp filming in identical conditions. We carried out the filming using an interference filter (775 nm). The effect of reflection from electrodes was evaluated using mathematical simulation. The temperature distribution of the anode surface is highly non-uniform. The temperature of the most heated fragments of the anode surface increases with increasing current and reaches 2000 K with the limiting interrupted current. However, the size of these fragments is small, substantially less than the length of the interelectrode gap. They cannot create the vapor concentration in the entire gap up to the opposite electrode sufficient to provide the Townsend breakdown of the gap when the transient recovery voltage is applied. The analysis of the obtained results suggests that droplets, and hence, the phenomena on the surface that lead to their appearance play an important role in the breakdown process in the investigated contact systems at extreme currents.