The comparison of the characteristics of an electric arc breakdown in vacuum between two silver or silver-nickel alloy convex electrodes, enabled us to show experimentally that the process initiating discharge in vacuum is the electronic emission at the micrometer scale. Increased content of nickel in the alloy improve the insulation and achieves high resistance to erosion for low power arcs. Heating in vacuum demonstrates the multiple layer structure of study alloys. Secondary emission microscopy and energy dispersing spectroscopy of the electrode surface after breakdown, as well as the measurements of fall and delay times, enabled us to evidence the origin of the electric arc.