Difference of vacuum arc appearance between CuBi and CuCr electrodes

It is well known that axial magnetic field electrodes can interrupt higher current than other types of electrode. The arcs in the electrode spread uniformly in the inter-electrode space and, thus, the electrodes are hardly damaged by the arcs. As interruption tests of the axial magnetic electrodes indicate, CuCr has higher interruption capability than CuBi. Some differences of arc appearances between CuBi and CuCr vacuum arcs might he expected. To clarify these differences, we conducted arc observation tests of axial magnetic field electrodes with CuBi or CuCr contact material using a de-mountable chamber. We used digital high speed CCD video-cameras with and without infrared sensitivity, and set them to observe the vacuum arcs obliquely toward anode or cathode electrodes. The diameter of these electrodes was 52 mm. If we use these electrodes, interruption of 30-35kA of crest value is expected for CuCr electrode and much less is expected for CuBi electrode. Following phenomena were observed. 1) Many bright spots with visible size were observed during arcing period in CuBi electrodes, not in CuCr electrodes. 2) Big difference was not observed in arc distribution between CuBi and CuCr electrodes. 3) Uneven distributions of infra-red images on the electrode surface were observed in high arc current periods. These are suggesting the reasons of CuCr´s higher capability of current interruption.