If you do tie your circuit ground to the case, make sure it is only at a 
single point, otherwise you can get ground loops and in some cases that can 
cause noise problems even in digital circuits. Many years ago my home-built 
computer had such a horrible mess of ground loops that I could hear radio 
interference on a pocket radio several houses away. Once I removed the 
ground loops, there was no detectable interference outside the metal 

If my project has a metal (conductive) case, I connect it directly to the 
ground terminal of the line cord with high-current wiring (no connectors, 
splices, etc). As far as I know, the reason consumer and industrial 
equipment do the same is to provide user protection in case the device gets 
damaged and the AC line contacts the exposed conductive areas. If that 
happens, the ground protects you until the circuit-breaker/GFCI trips. But, 
I dont tie the circuit gnd directly to the case; instead I use about 1 meg 
of resistance to bleed-off ESD and provide a DC path to GND. Circuit-wise, 
if you are using an isolated supply there's really no need for your circuit 
GND to be tied to the earth.

Now for high-voltage circuits that have an isolated supply, if your circuit 
GND is tied directly to the earth, you probably will get shocked if you 
touch the high-voltage supply, depending upon your body resistance (varies 
wildly from 500 ohms to about 100K), and possibly additional protection 
from clothing. However, if your circuit GND has a 1Meg path to earth 
ground, you may not get shocked at all; even if you touch a 500V supply, 
the current will be less than 500uA which I think is below the threshold of 

Where things get tricky is if you are going to connect the device you built 
to another device, such as a USB port on a computer. If both devices have a 
low-impedance path from their respective circuit-gnd to the AC power-cord 
(earth) ground, you will get a ground loop.

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