The vacuum switch. Part 2: Extinction of an a.c. vacuum arc

The paper, using as a basis data already described, analyses the process of arc extinction at current zero in vacuum switches. The interruption of direct current is not considered, since it is possible over only a limited range of current. It is shown that there are three sources of neutral atoms and ions in `ideal¿ vacuum switches, and equations are developed describing the rates of fall of metal-vapour and ion densities between the contacts, and also the form of the post-zero current under the influence of a given restriking voltage. These results are used to predict the order of magnitude of the interrupting ability of a vacuum switch in terms of current, rate of fall of current and rate of rise of restriking voltage. While technical difficulties related to vacuum processing have prevented the full potentialities of the `ideal¿ vacuum switch from being realized, the experimental results obtained suggest that the theory is basically correct in its prediction that the vacuum switch has, at least on a basis of commutating factor, an interrupting ability very much greater than that of any switch employing high-pressure gas discharges.