Substation Bus Supports: When are Static Eliminator springs used?

[RSChinn]

When going thru the Travis-Pattern Bus support catalog, some slip fitting supports will include the Static Eliminator springs if an "SE" is suffixed on the part number. They are supposed to push the bus up against the walls of the ring type support to eliminate chatter/vibrations and minimize corona effects, correct? So when do we need them and when do we not? I suspect the length of the bus between supports may be the determining factor, but I need help on their usage philosophy.