a three - phase vacuum interrupter switch assembly for power distribution systems comprises an outer case having at least one window , a plurality of internal disconnect switch assemblies , and a plurality of vacuum interrupter bottle switch assemblies within the case . each vacuum interrupter bottle switch is coupled in electrical series with a corresponding internal disconnect switch assembly . because the open / closed state of a bottle switch is not directly observable owing to its sealed interior , a direct visible indication of the state of the three - phase vacuum interrupter switch assembly is provided by a visually detectable contact rod for each internal disconnect switch that is visible through the case window . to prevent potentially serious damage caused by arcing between the contacts of the internal disconnect switch , the internal disconnect switch is prevented from opening or closing when the bottle switches are closed . the case interior is preferably free of oil and / or sf 6 gas .