SELF

24

and S.B. Karavashkin

1. Introduction

The issues of higher reliability and erosion immunity of contact elements are intimately interweaved with the issues of reliability and durability of radio and electronic instruments and electric devices. Especially hard for contacts are conditions of commutation of inductive load fed by direct current, because of inductance ability to maintain the current in a broken circuit. Due to this, even at the first stage of break, the inductive current, at the increasing transient resistance of contacts, releases an additional energy that is proportional to the value of inductive load, and after full break charges the circuit capacitances up to breakdown voltage. This provides a considerable heating of contact groups and stimulates intensive erosion of contacting surfaces, due to intensive discharge phenomena - so-called short arcs and unstable discharges.

Unstable discharges called "showering arcs" [2] because of their properties were first discovered by Curtis [1] in 1940. Most completely they were studied by Mills [2]. Germer [3], Atalla [4] and Wagar [5]. Typical oscillograms obtained by researchers are shown in Fig. 1..

Fig. 1.1. Typical appearance of showering arcs at opening contacts against the speed of contact partition [2]

Fig. 1.2. Typical appearance of showering arcs at opening contacts against the load current [2]

Fig. 1.3. Typical appearance of showering arcs at opening contacts against the load inductance [2]

In these diagrams Mills showed, how the parameters of showering arcs (duration and frequency of separate relaxations, duration of the whole process etc.) change with respect to speed of contacts partition (Fig. 1.1), of product of switching current by the load inductance (Fig. 1.2) and of length of cable connected to contacts (Fig. 1.3).