Voltage steps in atmospheric low current arcs between opening silver metal oxide contacts

When breaking a low current dc circuit (typically <10 A), the arc voltage exhibits a stepped rise as the contacts move apart. As the current increases, these steps gradually disappear, and the voltage rises smoothly. The paper presents experimental results for atmospheric arcs between silver metal oxide contacts. Up to 5 steps can be observed for currents less than 0.5 A. The probability of occurrence of the steps decreases as their voltage level increases. The number of steps also decreases with increasing current. At currents above 7 A the steps disappear and the voltage rises smoothly. The contact gaps at which the steps occur were found to increase as contact opening velocity increases. The paper discusses the different theories proposed in the literature to explain the phenomena, in the light of the presented experimental results.