Shield is an essential component of the vacuum interrupters design. It has multiple purposes:
- To capture metal vapor from the vacuum arc and thus maintain the internal insulation of the ceramic cylinders.
- To shape the voltage distribution inside and outside the VI and to maintain this distribution throughout the vacuum interrupter life.
- To protect the ceramic from the radiation and thermal energy produced by the vacuum arc. In extreme cases, to protect the ceramic from direct contact with the high current, columnar,vacuum arc.
Most manufacturers use stainless steel or oxygen-free Cu for the shield material. Stainless steel is a very good high voltage material, but is unforgiving if exposed to too much energy from the high current vacuum arc.Copper has very good heat sinking properties and resists the effects of the high current vacuum arc better than does stainless steel. Some manufacturers use cylinders of Cu–Cr as a shield material with stainless steel-end curls.TheCu–Cr material also resists the effects of the high current vacuum arc, but is expensive to manufacture.