Has anyone seen this type of protection before

Came across an exd junction box for a motor winding rtd. It’s really old and been there for years and has finally given up. when opening it up I found all the spare terminals and one lef of each side of the rtd tied down to these metal plates. the plates are insulated from the jb, but obviously serve some purpose. On the drawing the only thing it says is overvoltage protection jb, is this some sort of very rudimentary over voltage protection, ie ithe insulation breaks down at a certain voltage, putting the terminals down to earth?