Electronic – Why are USB power and ground connected at the same time

I'm looking at and interpretting the schematics and here's my take.

1. If V_IN (external power minus a protection diode drop of probably about 0.7V) is above 6V, the USB power is cut-off from the 5V rail through a P-channel MOSFET.
2. If V_IN is below 6V then USB power will be connected to +5V through the MOSFET. V_IN will still energize the on-board 5V regulator, but since USB power and the regulator are at the same potential, they shouldn't fight each other , and you should be OK.
3. Looking at GND in the schematics, USB GND is tied to the 5V GND reference on the regulator, so again, you should be all set in terms of USB power not fighting your on-board regulated power.
4. In principle, having both USB and external power connected should allow you to source more draw a higher max current (the sum of the USB and your external power source).
5. Plugging in external power after USB power is already connected shouldn't do any damage, but you might get a transient dipping in +5V while the Op-Amps and MOSFETs settle, which might cause your Arduino to reset.

Anyway, my opinion is that it should be harmless. Not that I'm an "official source" :).

You have a bench supply and USB supply with AC mains feed.
These may have two wire or 3 wire (phase + neutral + ground) leads.
The outlets used may or may not have grounds.
If both outlets and leads do not provide grounding so that both power supplies have third wire grounded input grounds then the following is likely (based on what you describe and my experiences):

- You are very probably getting coupling from mains via a Y capacitor in each lead of mains to input ground.

• If you measure the 0.4 VAC with no load and a decent AC meter you will probably see about Vmains/2 or 55 VAC or so in USA.

  - This capacitor coupled mains voltage can destroy things.

• Y caps may be 0.001 or 0.01 uF.

"More data needed" including certainly whether PSU ground is connected to mains ground, what a "Bus Pirate" is (Gargoyle will for sure know but I/we ought not need to ask it) etc.

BUT some supplies have isolated in and out DC wise and Y capacitors from phase & neutral connected to input ground. If you use a 2 wire power feed and connect in-gnd to out-gnd you get mains/2 capacitively coupled. If you are getting bitten you have >>> 0.4 VAC and probably half mains via Y caps.