SELF

42

and S.B. Karavashkin

Proceeding from the said, most general structure of showering arcs will be as shown in Fig. 25.

Fig. 25. Reconstruction of most general structure of showering arcs

In Fig. 25 we can select six typical stages.

1. The stage of "short" bridges preceding the long arc. The first bridge well studied and described in literature belongs to them, too. It bursts at 1,5- 2 V. The long arc cannot be produced immediately after it, as it is in action at 12- 18 V. Before the instant at which long arc arose, there can form several bridges, until the voltage at the contacts achieved a necessary value. Slade [12] also expected the possibility of these bridges.

2. The stage of long arc. During its arching, mainly the metal evaporates from contacts. The long arc is arcing all time, while the amplified evaporation from contacting surfaces impedes to form the bridge.

3. The stage of "mean" bridges that are formed after the long arc. At this stage the current of inductance I₀ is sufficient to form the short arcs with a small resistance, so the voltage relaxations have negative ejections. If the bridge formation finished at this stage, after this there arises the transient (4th stage), as the dotted line shows in Fig. 25. But if the bridge formation goes on but the current of load abruptly falls, there occurs the transition to the next stage.

4. The stage of long bridges. Relaxations in this case have a shape of growing crests. As the current in this section is small, bridges can be not bursting but breaking under affection of tension forces. If the bridge formation finished at this stage and the voltage, to which the capacitance discharges, was less than 300 V, the process is finished by the transient (6th stage), as shown by the dotted line in Fig. 25. But if the voltage of capacitance discharge was more than 300 V, there can occur the transition to the next stage.

5. The stage of glowing discharge. Until the glowing discharge lasts, bridges cannot produce, as the voltage applied to contacts is too low (300- 400 V) and the distance between them is considerable (dozens of micron). As the inductance current falls, the glowing discharge extinguishes. It can be finished by a transient (6th stage), as shown in Fig. 25. But if the contact surfaces were still not enough heated, and the voltage after glowing discharge extinguish is high, there can again form long metallic bridges. Just such case takes place in the oscillogram 25e.

6. The stage of transient in the circuit, when the contacts were fully open. The energy remained in the circuit dissipates at its active elements.

In particular cases not all considered stages take place at once. Some of stages can be absent. This depends on parameters of load of switched source, as well as on parameters of contact element.

Such pattern of contact opening of inductive loads allows to explain the Slade erosion curve shown in Fig. 26).

Fig. 26. Contact erosion G against inductive load L [12]

- Rapid growth of transfer to cathode in the first section is caused by the growing number of bridges as the inductance grows.

- Further growth of inductance in the second section creates the conditions for producing the long arc whose lifetime grows with the inductance growth. During the long arc arcing, the cathode dispersion is the more the longer arc is. Accordingly, the transfer to cathode lessens.

- In the third section, the inductance growth provides longer current of its discharge. This promotes the mean and long bridges formation. The number of such bridges grows with the inductance growth. At the same time, the duration of long arc increases. Joint action of these two factors provides slower than in the first section growth of metal transfer to cathode.

- The further elongation of long arc arching in the fourth section is accompanied by less number of bridges. The metal evaporation prevails upon the bridge transfer. Due to this, total transfer to cathode lessens.

Finally, on the basis of advanced statements, let us substantiate Mills' curves (see Fig. 1).

Growing speed of contacts divergence (see Fig. 1a) causes the growth of showering arc periods and relaxation amplitudes for the same instants of time, as well as for maximal bridges. Furthermore, under these conditions the duration of the whole relaxation process shortens.

Growing periods and amplitudes of relaxations can be explained so that with larger speed of contacts partition during the capacitance charging, the bridges are pulled longer, since during the time between the bridge formation and break, contacts diverge to a larger distance. Their resistance, as well as maximal value of voltage fall at them, becomes larger, and this causes the showering arcs amplitude growth well seen in Fig. 1 a.

Growing load I₀L (see Fig. 1b) causes the growth of relaxation frequency. But total time of relaxation grows slightly. The relaxation frequency grows in this case because of growing switched current  I₀ , as this provides faster charging of capacitance. Some longer time of relaxation with growing load can be explained by growing size of maximal bridge because of larger melting under larger switched current.

Finally, longer cable (see Fig. 1c) causes less frequency of relaxations, duration of relaxation process and oscillation amplitude.

Longer cable increases the parasitic capacitance. The growth of this last requires longer charging, which promotes longer bridges and better conditions of metal cooling. This last can be the cause of shorter maximal bridges and shorter time of relaxation.

By way of capacitance increase, we can achieve fully absent relaxations, as this is used in RC-circuits to protect the contacts against erosion.