The evolution of break arc erosion from arc initiation to extinction on silver alloy electrical contacts in low voltage DC switches
This study concerns the evolution of electrical contact erosion in low voltage 42V DC (24A) applications. The aim is to describe the erosion mechanisms of a single arc discharge at both contact surfaces during operation. To this end the experiments conducted are designed to produce results that enable close examination of the surface profile changes, volumetric contact material movements, and relative quantities of metallic and gaseous species in the arc discharge. The experimental procedure is arranged so that the contacts undergo a low number of operation cycles in order to limit the cumulative effects of arc erosion. To obtain results portraying the evolution of arc erosion in a 42V system the apparatus allows for interruption of the arc at any desired voltage level. A series of tests are conducted at specified voltage steps from arc initiation to extinction. The samples collected are analysed using a non-contact laser profile apparatus giving 2D and 3D surface scans. During each arc discharge a record of spectral emissions is collected by a spectrometer and later analysed to show the constituents of the arc. The data is represented with reference to voltage level and shows how the erosion evolution from a low number of operation cycles is related to arc phenomena. In addition the voltage profiles of the arc are recorded and represented by amplitude modulation. It has been found that the erosion site dimensions are related to the arc voltage fluctuations. In these tests a marked change of voltage slope is seen but is not found to be related to the metallic to gaseous phase transition as often thought. It is determined that the anodic to cathodic arc transition is not due to a metallic to gaseous phase transition. Changes in surface profile at the cathode occurring during the cathodic arc phase, and before gaseous phase initiation, are affected by density per unit area of ions impacting the surface. The vaporization of the cathode requires a critical ion density. This determines the anodic to cathodic transition point. Conversely the anode is thought to endure intense electron sputtering throughout the arc lifetime and its volumetric loss is greater than that of the cathode.