If cell assembly 20 is installed upside-down, then a special plate designated by the index numeral 54 will interfere with the closing of the door because ground spring 50 will attempt to push plate 54 in further than the ground spring can deflect. (..) Ground spring 50 also is preferably used to provide a solid electrical ground connection between the grounded doors and framework of electrostatic precipitator 10 and each of the cell assemblies 20 along their planar surface at the contact surface points 52.
http://tinyurl.com/5xcbkk

ground spring 22
ground spring 122
http://tinyurl.com/46kngv

W pdf-ie przy pozycji 22 (Rys. 1) i pozycji 122 (Rys. 9) widać symbol uziemienia ( http://je.pl/wmoc ).
http://www.freepatentsonline.com/5198612.pdf