I had never worked with tasmoda before last night, so before I do a project at my parents house I wanted to make sure I had all the drivers sorted out and understood how to make tasmoda integrate with home assistant, so I purchased some Sonoff S31s to play with at home.

While reading about the s31 there is this warning on the Tasmota site:

*The GND connection of the S31 is connected to the live AC wire. Connecting serial with your PC will fry your PC and will electrocute you.

DO NOT CONNECT ANY SENSOR TO THESE DEVICES. Only use them as designed.*

I consider myself pretty knowledgeable about automotive and household electrical, I've even dabbled with three phase, but this warning confuses me and I would like some guidance and how I can verify that with my multimeter.

I did some more looking into it and it appears this is necessary for power usage monitoring. That makes sense, but I want to verify that problem doesn't exist within the relay box I'm flashing at my parents house.

To be clear, I practiced on an S31 which doesn't run an 8266, but the main event is a relay box that runs on an Ecoplug branded 8266. Flashing this chip is extremely well documented.

So, the Crux of my question, how do I verify with a multimeter that it is safe to add a sensor?

Thank you!

Edit: I tested with my multimeter, the ground connection pad and the VCC pad both have significant AC voltages when checked against the ground prong of the receptacle.

I'm still not convinced this is a real problem for the sensor, I think my next step is posting in r/askanelectrician or something.

Edit 2: cross posted: https://www.reddit.com/r/askanelectrician/comments/x33lib/i_have_a_33v_volt_dc_sensor_im_wiring_to_a_wifi