This investigation presents new practical-based achievements for velocity of dielectric recovery process lead to successful quenching arc inside a vacuum interrupter (VI) chamber related to arcing interval duration with utilization of thermionic emission theory. A high voltage impulse is applied to the contacts of a vacuum interrupter to cause insulation failure during quenching arc interval with utilization of new high-speed spark gap switch concerning 4 mm contact distance. Regarding to left branch of paschen's curve the achieved results are admissible. But for some data with equal HV pulse time, it was observed that the breakdown voltage level, for similar quenching time, may differ regarding to arcing time duration. Longer the arcing time results stiffer insulation recovery process. In addition, more arcing time results to more corrosion in contacts. These truths demonstrate the requisition of designing a synchronous observer system to control the moment of mechanical opening process regarding to quenching sinusoidal current waveform. This system should continuously analyze and predict the next zero-crossing of the current waveform to trigger the mechanical opening mechanism after last peak of the flowing current.