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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