Abstract
Significant number of medium voltage Vacuum Interrupters (VI) are currently operating at their lifetime limit of 20–30 years and beyond. Estimation of the pressure inside the VI and of the contact surface condition without demounting of the VI are of importance for Vacuum Circuit Breakers (VCB) operating at medium voltage range and for further development of the VI technology towards higher voltages. This paper reports on the measurements of the field emission current in a commercial vacuum interrupter 12 kV/2500 A/40 kA with the pressure controlled at the VI functional limit of 0.04 Pa. The Fowler-Nordheim equation was applied to indicate the vacuum pressure within the functional limit and to evaluate the influence of the contact surface condition on the microscopic surface projections. A relation between the microscopic processes and the field emission current was studied. The measurement results are presented for a small fixed contact gap of 0.3 mm allowing relatively low DC test voltage level up to 40 kV.
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