ABSTRACT
If the current at the instant of contact separation approaches approximately 15 kA, the bridge rupture leads to formation of a single, high-vapor-pressure arc column. This arc is a high pressure arc and is similar to the arc described in Sect. 8.5 (see Fig. 8.38). The appearance of the high-current vacuum arc has been extensively studied (46,65,66). It has been shown that, depending on the current level and contact spacing, the diffuse arc can also form an anode spot and a columnar arc before going diffuse again close to current zero. It is possible, however, to form a columnar column (Fig. 8.39) from the initial bridge arc which will stay columnar until just before current zero. At the highest currents, the electrode regions of this columnar column exhibit intense activity, with jets of material being ejected from the contact faces (46). In spite of this severe contact activity, even this arc mode can return to the diffuse mode just before current zero. Figure 8.40 shows an example of an arc appearance diagram developed by Heberlein and Gorman (46).
