This contribution describes the recent experimental results obtained at the high voltage (HV) Padova test facility (HVPTF), the laboratory able to carry out HV tests in vacuum up to 800 kVdc/1mA with a holding time of several hours per experimental session and aimed at developing the voltage holding capability of the Megavolt ITer Injector & Concept Advance (MITICA) accelerator, the prototype for the International Thermonuclear Experimental Reactor (ITER) neutral beam injector (NBI). The experiments described in this article refer to a double polarity symmetric configuration, a sphere-to-sphere electrode with an inter-electrode vacuum gap of 30 mm. The results showed that the onset of microdischarges can be ascribed to a mutual exchange of charged particles between specific accumulation points on electrode supports and the vacuum vessel rather than to a direct interaction between the spherical electrodes. The outcome of these experiments is reported and analyzed with both analytical and numerical models.