Some power supplies may do this internally with a small resistor to pull the DC common up off the ground plane noise. Generally speaking you should not tie your DC common(s) to (earth) ground anywhere. That is the job of DC common, and your system may have more than one DC common which is not a big deal. 'Ground' SHOULD NEVER carry any current, it IS NOT a common return path for all circuits. This provides a path of least resistance to shunt the voltage away from important things like people in case something goes wrong. The incoming mains voltage is referenced to ground (through the 'ground' rod(s)) and so is your equipment. 'Ground' is this context refers to 'Earth Ground' which is a safety device. They do not mean the same thing, unfortunately we all throw around the term 'ground' very loosely. OK, 'grounding'.'(earth)ground IS NOT equal to DC common. I copied most of the following from another post of mine on the forum: You never want to have a situation where a signal wire can serve as a power return. You never want to setup a situation where you can have current flowing through a shield. Always provide a dedicated power or signal return wire, or you render the shield largely useless. NEVER use a shield as a power or signal return. So, for example, encoder cables should have the shield grounded at the end opposite the encoder, where the power supply is. For shielded cables, ground the shield at only one end, usually the power source end. This can be a buss-bar, heavy terminal strip, a metal chassis, or something on that order - something with essentially zero resistance between the many connections. You want to have a single ground for the electronics, and connect everything to this point. In the US, this is the GROUND terminal present on any self-respecting outlet. The body of the machine should be securely bonded, through a dedicated wire, to the power ground of your building. The most important thing is to HAVE a ground, and only one.