asuratman wrote:If you use switching power supply, on the output there is earth, V-, and V+. So it is not common ground.
The -V is the DC 'ground' in this example. The earth ground is for the AC input -- to handle faults only.
Delco wrote:Are the 5v and 24v grounds not common grounded anyway due to them both being on the 240v ground circuit , or are power supplies normally isolated between the dc and ac ground ?
The ground on the AC is called 'earth' or 'safety' ground. AC supplies don't use ground in normal operation, it's there for faults.
That 'ground' has nothing to do with the ground on a DC ciruit. Often referred to as -V or GND instead of as 'ground'. The switcher produces its own 'ground'. Wall-Wart power supplies produce their own, too. They are isolated via a transformer which is also used to step down the AC.
You must tie the DC circuit GNDs together. For example if you connect a UC100 to an Arduino, their GNDs must be tied together. Otherwise, each PCB has a different opinion of what +5 looks like. The GND is the baseline, or reference.
A good example from the past is the old LPT printer cable. The printer had to connect its GND to the PC's GND. Otherwise, it would not have been able to recognize the +5 (high) and +0 (low) coming from the parallel port. It would've had no reference to compare them with.