I installed a double 13A socket to BS1363-2 with USB outlets to power an RPI W with some one wire temperature sensors attached, these are used to measure flow, return & hot water on a central heating boiler. Also using a https://thepihut.com/products/adafruit-veml7700-lux-sensor-i2c-light-sensor-ada4162 to sense the dim boiler on led after failing to get a generic photo transistor to work.
The one wire sensors are metal cased, some with a connected earth wire.
I had it breadboarded & tested working, but each time I put the sensors onto the (Earthed) boiler pipework, the RPI crashed.
I then discovered that there was 80V AC between the RPI negative & the pipework earth, all be it at extremely low current, not enough to feel by touch, I think this was overriding the RPI inputs static protection.
Solved by sleeving the probes with heatshrink.
I was under the impression that the socket USB outlets would be connected to mains earth, further reading of the standards tells me they are floating with stray capacitance responsible for the 80 ish Volts.
Thinking about it, most wall wart usb chargers have a plastic earth pin, same problem,
Updated the RPI to an ESP32 running tweaked tasmota as the RPI MQTT will not reconnect after a wifi outage.
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