Some of you may recall my previous post about a ~20V potential between my electrical ground and my concrete slab. That’s still not resolved - it’s currently sitting just under 10V.
Today I have a new mystery - to me anyway…
I’m sitting at my desk and notice that I got a tingle from the outer shield/shell of a USB-C cable. I got my multi-meter and measured 65V from the cable to me with my bare feet on the slab! It drops to about 16V if I lift my feet off the floor. I immediately assumed the charging brick it’s plugged into was faulty, but just in case I took a more measurements and found that the another similar charger has a similar offset, the “ground” part of a TRS cable plugged into an amplifier is similar, the accessible metal shield part of a USB-A port on an ASUS ChromeBox is similar. I assume that’s not normal?
This is a new slab on grade build. Ground and neutral are properly bonded - I checked a few outlets and ground to neutral is ~0.3V.
Edit - I don’t think there is any safety risk - I measured 0.3μA current.
I have a Generac backup generator so the first disconnect, and neutral bonding, is in the generator transfer switch which is mounted on an outside wall of my house. The ground rods are now buried under the backfill around my slab so it’s not easy (near impossible with the frozen ground) to check them. I backfilled by hand with a shovel so I doubt very much they were damaged. There is another ground wire coming out of the generator transfer switch that connects to the propane line coming into my house - this is accessible and strange - I measure 5V between this ground wire and the actual ground (gravel) just a few inches away from it.
Pretty normal to ground the gas pipe to prevent accidental sparking. So the potential is positive towards the ground wire and reverses when you measure the other direction?
You might need to go around and test the voltage from ground to hot and neutral to hot everywhere and see where this is leaking. Because it sure seems like you have something leaking. I wonder if you could locate it with a CT style ammeter and test all your branch circuits with everything turned off.
When I first discovered the 20VAC potential between electrical ground and my slab I turned off the main breaker in my panel and it was still there. I have not tried turning off the main disconnect in the generator transfer switch. But, turning off the main breaker means the leak isn’t inside the house - doesn’t it?