And regarding the RS-485 serial line for the Modbus communication (a balanced line by the way), I would ground both ends too, as the setup is connected on the same local installation and the same ground:
– Source: Omron MX2 User’s Manual
An example for a ground loop is if your cable TV provider grounds their cable in their distributor board out there a few miles down the road and then it is grounded again in your house at your cable TV wall outlet or in the receiver. This can produce a lot of hum that deteriorates the signal. This is because in an unbalanced line, ground participates in signal flow (=ground is a circuit conductor that completes the circuit back to the source here). The solution here would be galvanic isolation. I have an isolation transformer as a sheath current filter in front of my cable receiver, and I have much better signal quality since then.
Note that what is more important than the shield on balanced serial lines, is the proper termination: