An experimental study of the minimum arc current of silver contacts with different opening speeds

In order to study the influence of contact opening speed on occurrence rates of break arcs, Ag contacts were operated to break inductive DC load currents from 0.1 A to 0.6 A at 14 V with contact opening speeds of 0.5 mm/s to 10 mm/s in a switching mechanism employing a stepping motor. Occurrence rates of break arcs and their average durations were determined at each current level under the respective contact opening speeds. From the results, irrespective of the opening speed values, the arc occurrence rate was almost 100% at 0.4 A. For all of the contact opening speeds tested, the conventional minimum arc current level can be considered, not as the lower limit for arc ignition, but rather as the threshold level for arc discharges of certain duration to occur at the rate of almost 100%. Moreover, in the current range from 0.2 to 0.4 A, slight differences in the arc occurrence rates were seen with differences in the contact opening speeds. The average arc durations were around 50 mus at the conventional minimum arc current level of 0.4 A, and not influenced significantly by changes in the tested range of contact opening speeds.