Influence of the Copper Electrode Surface on Initial Arc Movement

To investigate the influence of the copper electrode surface condition on the initial arc movement and the immobility time respectively, a special experimental device was developed. It permits to keep constant, or vary those parameters independently, which influence the initial movement, i.e., arc current (170 A), magnetic flux density (10-1000 × 10-4 T), electrode distance (1 mm), and surface condition (oxide-film thickness, roughness). The magnetic field can be applied at any moment and can be varied independently of the arc current. The arc is ignited by means of a dielectrical breakdown in order to eliminate the ignition effects, such as liquid and vaporizing electrode material. The most striking result of the experiments was that only arc movements of various velocities, but no absolute immobility was measured. This observation is due to the elimination of ignition effects as well as to the use of an adequate sensitive measuring technique. Impeded arc motion characterized by shorter or longer lasting creeping (υ≲3 m/s) occurs, if certain magnetic flux density or copper surface conditions do not permit velocities υ>10m/s or if the electrode surface is thermally prestressed by a deliberately standing arc.