i have an enclosed device which incorporates some 5V custom PCB circuits that control some relays and LEDs and also an IntelNUC mini pc that runs off a 230V. The whole device case is grounded to safety ground and the custom PCB runs on a isolated 5V DC converter and also the PCB is isolated from chassy. I had some problems whit mosfets on PCB not opening and closing as they should, because there was one bolt touching the case (IntelNUC has signal ground tied to the mini pc case chassy), so any noise on chassy(safety ground) was changing signal ground levels, changing gate->source voltage on mosfet, causing them to not open or being stuck open. Now i removed that bolt, all works fine, but someone adviced me to put a capacitor beetwen my signal ground and chassy ground, to eliminate noise from ground coming to my signal ground, but still keeping them at same ground potentional. But i dont think this is important, since my 5V/12V pcb runs indepentend from the 230V device and only conenction to other stuff is via USB to intel nuc, which i also isoalted from chassy ground, since it uses signal groudn to protect itself from EMI and other stuff?