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According to well-known standards, e.g., IEC 62271-100, vacuum circuit breakers and particularly vacuum interrupters (VIs) have to fulfill miscellaneous forms of dielectric strength requirements depending on the rated voltage. Besides conventional conditioning methods, e.g., with alternating current, further techniques have to be investigated in the case of higher voltage levels. This paper describes the high-frequency (HF) current conditioning method with the objective to increase the dielectric strength of modern and very compact VIs for medium voltage applications. A test setup for conditioning of VIs in kilohertz range is presented. The conditioning current has peak values up to 10 kA, with durations of microseconds and voltages up to 135 kV (peak). The dielectric properties of the investigated VIs are described with emphasis on the obtainable lightning impulse withstand voltage level after one or more HF sequences. In addition, the surface microtopography and roughness of contact surfaces are analyzed by means of scanning electron and atomic force microscopy before and after applying HF conditioning.