Flashover voltage of alumina ceramic contaminated by metal vapor in vacuum interrupters

We investigated the influence of metal vapor contamination of ceramic surfaces on flashover voltage (FOV) in vacuum. First, disk shape alumina (Al₂O₃) ceramics with surface resistivity (ρ) of 10²-10¹⁵ Ω were produced using deposition phenomena of metal vapor emitted from CuCr contacts. The impulse FOV for the ceramics decreased, as ρ reduced; FOV, the conditioning effect on FOV, and the scattering of FOV decreased when ρ was below 10¹² Ω. Therefore, the criterion value ρ, which maintains excellent flashover performances of ceramic surface, is 10¹² Ω. Second, experimental vacuum interrupters (VIs) were produced to measure breakdown voltage before and after forty short-circuit current switchings with 20-40 kA_rms and were disassembled to measure the ρ of their inner ceramic surface. In a VI, which has inside diameters at both ends of the main shield much larger than the contact diameter, ρ was reduced to 10⁴ Ω, further decreasing breakdown voltage between terminals.