Generally it is desirable to have all the power supply commons at ground potential and if the power supplies are all initially isolated it is considered safe to make them common ground, a test to see if commoning up will cause any significant problems is to do a test to see if doing so will cause a large leakage current to flow, measure the voltage both DC and AC from the supply common you wish to ground and the actual ground, you may see a voltage with a sensitive meter due to natural leakage, which is normal, while the meter is connected place a 100ohm resistor across the meter leads an see of any leakage voltage collapses, if it does, then you should be ok to common up.
Al
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.