Silver electrical contacts are separated at constant speed and break arcs are generated between them in a 200V-450VDC and 10A resistive circuit. The motion of the break arcs is restricted by some surrounding alumina plates. Transverse magnetic field of a permanent magnet is applied to the break arcs. Changing the supply voltage and the height of a wall located at the upper side of the break arcs, the arc lengthening time and motion of the break arcs are investigated. As a result, the higher supply voltage causes an increase of the arc lengthening time. The arc lengthening time increases significantly when the break arcs expand into the whole of the surrounding walls.

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.

Short gap arc V-I characteristics are essential to discussions on behavior of contact arc at opening and closing. From this point of view, some conventional arc V-I characteristics were reviewed and inconvenient points of them for practical use were pointed out: (1) not a few electrode-material-dependent constants needed in the equation of V-I relation, (2) difficulty in the prediction of real arc extinction phenomena. In order to overcome these inconveniences, the author measured short gap arc V-I characteristics originally, and tried to formulate them into a simple form on the assumption that the arc column V-I characteristics are little dependent on electrode materials but the unstable arc region is strongly dependent on electrode materials. Measured arc column voltage was directly proportional to the square root of gap length and inversely proportional to the cube root of arc current. Arc became unstable when arc current decreased near to the value generally known as the minimum arc current. It was not necessarily the case that the arc extinguished completely at the minimum arc current, but, depending on the circuit conditions, the arc often existed discontinuously below the so-called minimum arc current. Simple empirical V-I characteristics were proposed for practical use, together with the unstable arc region as the information for arc extinction phenomena.

At current interruption by electric contact, a repetition phenomenon of arc extinction and re-ignition is often observed before complete extinction of arc discharge, in some cases for a long time and in other cases for a short time. Occasionally, no re-ignition is observed. From a viewpoint of arc duration, the period of this repeating arc is also an important factor, if it is very long. However, the conventional explanation about the contact arc duration excludes this repeating arc phenomenon so that it fails to explain the reason and the duration of this phenomenon. For the purpose of investigating why this phenomenon occurs and how long it lasts, the arc extinction current and the arc re-ignition voltage have been measured for tungsten electrode, palladium electrode, copper electrode, and silver electrode. The circuit is, for simplicity, resistive with a capacitor source voltage and a fixed short arc gap of 0.5 mm. Taking newly into account the idea of unstable arc region by the measured results, the conventional explanation was improved. As a result, the reason and the duration of arc extinction and re-ignition phenomenon have become understandable. For simplification, the main description is based on the results by tungsten electrode. The results by other electrodes are summarized in appendix.

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.