Here is a general case of protection for electronic systems connected to both networks : mains & transmission
Many devices use these connections and as they are connected to the cross point of two very different networks, it is very difficult to make a good protection of one side without disturbing the other side.
As each of the long lines coming to the equipment show a high capacitive coupling with the ground, they may be considered as a reference potential by the perturbed line with the effect that an incoming surge on one line induce a rise of the local ground potential which take the non-perturbed line as a discharge point. The high energy pulse coming through the line interface,opto-couplers,main board, power supply with or without transformer, can cause many destruction. So the drawing behind is a good compromise with minimal risk for the equipment.

Original file name: Surges