Influence of contact solid angle on anode spot formation threshold current in vacuum circuit breakers

The objective of this paper is to experimentally understand a relationship between anode spot formation threshold current I_th and a contact solid angle in vacuum arcs subjected to an axial magnetic field (AMF). Experiments were performed in a demountable vacuum chamber, in which an axial magnetic field coil was installed coaxially with a pair of butt contacts. Solid angle Ω is subtended by the anode of the cathode center In the experiment, butt type contacts with contact material CuCr25 and contact diameter 12 mm, 25 mm, 40mm, 60mm and 80 mm were tested. At the end of arcing, the contact separation was about 12~24 mm determined by an opening velocity with arcing time about 10 ms. The AMF applied to the pair of butt type contact was adjusted from 0 to 122 mT. Arc current was tested up to 30 kA at 50 Hz. The results showed that anode spot formation threshold current I_th was linearly proportional to not only contact solid angle but also the AMF applied. Besides, the influence of contact separation length on I_th was not as significant as that of contact diameter. With increasing of AMF, the anode surface temperature at current zero in the condition of I_th declined and also the temperature distribution tended to shrink.