I've made this simple circuit to read the line voltage (the 1 pin is conected in a uC by a resistor):
In the image NEUTRO is Mains Neutral AC, FASE is Mains Line AC and TENSÃO is the signal to the uC (microcontroller).
The circuit worked very wel until a connecting with the PC's serial connector has been made with de eqp. This connection had increased the signal readed (1 pin), making a measure error by 30Vrms in the uC.
When i was probing and investigating what would be the problem, i noticed that in the PC everything is connected with earth! USB shield and GND pin, GND of audio conector, GND signal tracks in the motherboard, shield and GND of the serial conector, everything. I've tested 3 PCs and all of them is the same. Searching in the internet i have found conflicted opinions about the union between Signal GND and Earth.
What is better to stop the circuit functioning variation? Make the union once of all?
If this represent's users risks why in the PC everithing is Earthed?
And finally what kind of problem this connection could cause? Could expose the circuit to line noise for example?
Any other thing to concern?