Formation of a 40 A DC Current Arc During the Opening of Silver Contacts

The erosion of silver contacts due to break arcs with length proportional to time and of variable duration has been measured by weighing the contacts following 5000 openings at a constant current equal to 40 A. The experimental results show that, for arc durations shorter than 60 µs, the transfer of metal from the anode to the cathode occurs, but after passing this stage, when the two electrodes are separated by greater distances, each will display erosion. This is the result of the diffusion of material outside the space between the two electrodes. In order to interpret these results, we have applied a classical model of the physical phenomena occurring at the root of the arc. Analysis of the experimental results shows that for an arc duration of less than 15 µs, no distinct cathode root is seen to exist, but beyond this, several spots appear gradually on the cathode for arc duration up to 50 µs, after which they merge into a single spot. The comparison between experiment results and theoretical interpretation is reasonable up to 60 µs.