The Effect of Glass Debris on Electron Emission and Electrical Breakdown of Vacuum Interrupters

Pre-breakdown electron emission and breakdown voltages have been studied for three experimental vacuum interrupters, two of which contained numerous glass particles the largest of which were typically 50 μm in their longest dimension. The contaminated interrupter differed from the uncontaminated interrupter in the following ways. 1) Continuously recorded Fowler-Nordheim plots had lower slopes and showed several stepwise changes in emission as the applied high voltage was varied. 2) 60 Hz breakdown voltages were lower and occurred randomly for sustained ac voltage at constant peak amplitude. 3) Electron emission current with dc high voltage applied was markedly sensitive to mechanical shock. Changes in emission occurred at statistically varying time intervals which may be several tens of milliseconds from the imposed shock. We conclude that the presence of glass particles within the interrupter strongly alters the emission properties of highly stressed internal surfaces and significantly degrades dielectric capability.