I am trying to understand the mechanism and characteristics of Weidmuller surge arresters for control and measurement signals.Wiemuller has presented two types of surge arresters for such signals: VSSC4 and VSSC6. But there's something kinda basic about these surge arresters that makes me confused.
I can't get what is the third line in VSS6 for? Here is the data sheets and photos of these 2 arresters. http://catalog.weidmueller.com/procat/Product.jsp;jsessionid=5CE47B92400AB638C2D6EBC242B522DE?productId=(%5b1063730000%5d)&page=Product