We usually have 3 wires in the outlet: L (line), N (neutral) and PE (protection earth). It is pretty common that the N is tight to PE in the distribution board or power plant.
If the device have conductive case, this case should be connected to PE to make the device safe for a human. However I still have a question: should be signal ground network connected to PE or not? Or it should be connected through let's say 10meg 1W resistor?
If the device don't have galvanic isolation there should not be any connection between SGND and PE. Right?
If there is a galvanic isolation the question become tricky for me. For example - if it is a RS-485 transsiever - I should have a return path for diferential pair. This means that the isolated signal grounds of the transsievers should be connected to each other. This connection can be done through a third conductor in the cable or - if it is absent - it can be a PE, right? So in this case I should directly connect the SGND to PE.
I suppose that in some cases SGND should be connected to PE through a 'big' resistor (for example 1MOhm / 1W) to prevent the schematic from float (to drain the static or any charge).
I examained some similar question on Stack I was able to find (one, two, three). They have some useful information which I included in this question, but they actually asked for some particular cases (say RS-485 usage in the first question) and don't have general information on the subject.