We are designing a product which has to meet EMC directives. It consists of metal backplate folded into a chasis (the front is open). on the backcover there is a SMPS mounted. and on top of this we have our main PCB which has a signal ground plane on top of the PCB. the whole thing will be surrounded by a plastic cover with some space for user interface on the PCB. See image for clarification.
All SMD components will be on the back of the PCB, the front has the GND plane. The product will interface in a master-slave configuration via RS-485. because the units can be far away from eachother and be on different mains circuits, there can be big voltage differences between individual grounds so we also connect signal ground with the RS-485 connection.
My question is: (how) do we need to connect signal ground to safety ground (metal backcover)? I understand if we connect the signal ground to the safety ground, and also connect the individual signal grounds between units, we create a ground loop.