Ok more info, first off I make my living with "high voltage" (120, 208, 240, 277,480v ) and am aware of what can go wrong. I am looking for advice on factors that affect how accurate and reliable the use are, not if it can kill you.
The project is monitoring (internet) of backup generators.
Can you tell us why you CAN NOT use a isolation transformer ( a cheap take-apart AC Wall adapter ) And why you want a DIRECT way ? So what is the "issue" here ? My opinion is : Risk to damage the Arduino <--- BIG TIME !!!
I never said I "CAN NOT", I said I would prefer not to use them
. When finished this project will be a potted PCB in an aluminum enclosure, so the smaller I can keep the components the better. It would also require six transformers to monitor each unit so the size of even "small" transformers adds up fast. Transformers also add thermal load.
Why is a PC transformer an option but a wall wart isn't?
The finished product will be a one piece unit that is hardwired into the transfer switch.
Are you trying to sense supply anomalies (interruptions, brownouts, spikes, RFI, etc.)?
For this part only voltage.
Your main problem of using a simple resistor divider network is that your circuit will be either at near ground or near mains voltage, depending upon the "polarity" of your input wiring. Hook it up wrong and your whole arduino circuit is potentially lethal.
Yep. if it is connected wrong bad things will happen