Change of dielectric strength of vacuum interrupters in process of life-time degradation

60 kV vacuum interrupters (VI) for 110 kV circuit-breakers are investigated. Reduction of dielectric strength of VI in process of expiration of an electric resource is revealed at lightning pulse test. It occurs owing to reduction of dielectric strength of system “case-shield” owing to metallization of a part of an internal insulation surface of VI case with products of contact erosion. Metallization increases with reduction of switching resource. Dielectric strength of an inter-contact gap with its sufficient length does not provide dielectric strength of VI. The original shielding system providing preservation of dielectric strength of VI up to expiration of an electric resource is developed and offered.