In order to study the influences of contact opening speeds on arc extinction gap length characteristics, Ag contacts were operated to break DC inductive load currents from 0.1 A to 2.0 A at 14 V with contact opening speeds of 0.5 mm/s, 1 mm/s, 2 mm/s, 5 mm/s and 10 mm/s in a switching mechanism employing a stepping motor, and arc voltage waveforms were observed at each opening of the contacts. From the results, the average arc durations were determined at each current level under the respective contact opening speeds, and the average arc extinction gap lengths were calculated by multiplying the average arc duration value and the contact opening speed value. It was found that average arc durations showed no significant differences with increasing contact opening speeds. Thus, arc extinction gaps became larger at faster opening speeds in the inductive load conditions of this study.

Arc occurrence rates were measured for Ag contacts in DC resistive and inductive load circuits. In inductive load conditions, arc occurrence rates in general came close to 100% at around 0.4 A, the conventional minimum arc current level of Ag. In resistive load conditions, the similar results were obtained with 30 V, while arc occurrence rates close to 50% were still obtained at around 0.4 A with 14 V and 10 V. Careful attention should be paid to the term "minimum arc current" to avoid misinterpretation thereof.

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 duration of silver contacts was measured with fixed short gap under the condition of different constant gaps and different constant currents, which are close to the minimum arc current of the contact material. The results indicated that the arc occurred even at the current less than the minimum arc current. The arc duration varies in different operations at the same testing condition and it distributes exponentially in the time range up to 100 microseconds. The time constant τ and the extinction rate λ of the distribution were calculated, and related to the arc current. It was assured by silver electrode arc that, even below the so-called minimum arc current, arc ignition may occur but, in this case, the arc duration is momentary, and, as the arc current decreases, the arc extinction rate becomes large.

Arc discharge between electrodes of relays and switches often causes contact surface damage through material transfer and arc erosion. Especially, material transfer sometimes occurs and brings serious failure even under lower load that is quite smaller than the minimum arc current value of contact material. In this paper, contact surface configuration, material transfer, and arc erosion characteristics of Ag and AgPd 60 contacts were experimentally studied after 0.5 or 1 million switching operations at various load levels. The followings can be made clear. Firstly, it was confirmed that the arcs and material transfer occurred even under such current that was lower than the minimum arc current. Therefore, the definition of the arc occurrence boundary current was newly determined. Secondly, the relation between load conditions (current and power supply voltage) and contact surface configuration (craters and pips) caused by material transfer was studied. The arc erosion behaviors of tested samples could be classified into two types: material transfer type and wear-out type. As one of the primary factors of transition from the former type to the latter one, contact activation was considered. The influences of load conditions and organic gas emitted from relay structure on arc characteristics was experimentally examined. The results indicated that load current greatly influenced the amount of material transfer and that power supply voltage affected the occurrence of the wear-out type significantly. The activation behavior of the contact surface could be found through observing the bridge voltage waveform.