The motion of vacuum arcs between spiral contacts with a fixed-polarity arc shield [in vacuum interrupters]

Abstract

A framing camera was used to study drawn vacuum arcs between spiral-type contacts. This design imposes a transverse (radially directed) magnetic field on the arc column. Vie fixed electrode was brazed to a cylindrical arc shield. With the fwed electrode and shield as cathode, cathode spots were observed to be produced on the shield by two mechanisms. One of these occurred only for the intense columnar arc modes which had active erosion at both the anode and cathode arc roots (IARc > -25 kA peak). In this range, the arc column interacted with the shield for brief intervals as it was driven along the edge of the contacts. With the fwed electrode and shield as anode, there was less interaction of the arc column with the shield. In this case, there was no formation of an anodic arc root on the shield at least up to -38 kA peak. INTRODUCTIClNIn vacuum interrupters (VIS) for breaking ac currents above several kilosunperes rms, a transverse or axial magnetic field is imposed in the contact gap to control the vacuum arc and promote its transition to the diffuse mode as the current goes to zero. An overview of transverse and axial magnetic field contacts for VIS is given in 1:1]. Vacuum arcs can assume a variety of modes in VIS. For spiral contacts, in order of increasing current, these modes are the fully diffuse arc, diffuse column, constricted column, or (if both arc roots are molten) jet column [ 2 ] .A VI typically has a cylindrical metal arc shield which surrounds the contact gap. In some designs, this shield is electrically isolated from both electrodes. Alternatively, it can be connected to one electrode -usually to the fixed electrode through an endplate. In past studies, a vacuum chamber apparatus and a high-speed camera were used to determine arc appearance diagrams for spiral contacts of diameter I$ = 62 nim, using isolated arc shields with I$(shield)/I$(contact) equal to 3.4 [3] and 1.3 [4]. 'The present work extends those results to a case in which a Cu-alloy shield wais electrically connected to the fixed electrode through a stainless steel plate. The effects of'the fixed shield on the development of the vacuum arc modes, including the arc motion and the transition to a diffuse arc near current zero, were determined for both polarities as a function of instantaneous arc current I and gap d (up to -38 kA peak with al full contact stroke of 8.3 mm). The arc appearance diagrams were constructed from the arc movies and the curves of I(?), d(?) and V(9. These are compared here to results with a floating shield of the same diameter [4]. Special attention is given to describing the conditions which produced cathode spots on the shield, and how the vacuum arc modes interacted with the shield. EXPERIMENTThe vacuum system ( 1 O-' Pa-1 O4 Pa), the 420-V electrolytic capacitor bank, and the framing camera (7000 framedsec) are described in [3]. The single-polarity current pulse had a 60-Hz half-cycle shape up to 7 ms, followed by a crowbar which forced the current to zero at a somewhat slower rate. Before the high-curre...