Vacuum switching tube for low-voltage and medium-voltage switches, particularly for vacuum contactors

In a vacuum switching tube for low-voltage and medium-voltage switches, including,

• a) a first contact piece disposed in a fixed position,
• b) a movable current conducting rod having a second contact piece for making an electrical contact with the first contact piece, and
• c) a switching chamber, consisting of a metallic cap surrounding the contact pieces, a metal bellows connected to the metallic cap and an isolator to provide a movable vacuum seal, the movable current conducting rod being connected via a metallic flange to an end of the isolator by a high vacuum sealing technique, wherein said isolator is a ring-shaped insulator body having a vacuum side and at least one end surface at least partially on the vacuum side, the improvement comprising:
• said end surface of the ring-shaped insulator body is at least partially free of metallization and faces away from metal vapor formed during switching, whereby said end surface is protected against metal vapor condensation;
• said partially unmetallized end surfaces has a radial expanse on the vacuum side which is sufficient to maintain an insulating function in the vacuum switching tube;
• and the metallic flange connected to said end of the ring-shaped insulator body forming a gap with said insulator end surface, said gap having a predetermined length in the radial expanse and height in the axial expanse, respectively.