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.
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I have found something similar at work several years ago. I got a tingly sensation from touching metal case of a device. I checked out with multimeter and consistently measured ~50V from case to ground. Never found out what was going on, even with help of company’s engineers.
Have you done a good look over on the breaker box(es)? What’s the configuration of the breaker system? Often in rural areas, you’ll have a main breaker box out at the road or a pole in the yard, and that feeds the main box in the house and any boxes around the yard. That should be where the neutral bonding is done as well, not at the sub-breaker boxes. You can see if you have potentials on the boxes themselves to ground, even pounding in a ground rod to test against (though if it’s still fozen like it is in my neck of the woods, good luck).
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?
What is the grounding situation at the panel? Seems like your electrical ground has no path to earth ground. Maybe the lug on the grounding rod loosened off after inspection.
This needs to get fixed quick. Even 10mA can cause serious problems. Without a safe ground your whole house is a minefield.
I have two grounding rods and I believe they are good - they are now buried under foam insulation and gravel so I can’t (easily) check them, but the electrician that wired my house assured me it was OK to bury them. I was a bit paranoid about burying them and checked and double checked that they were both firmly connected before backfilling gently with a shovel. Is it possible my earth ground (gravel fill) just isn’t conductive enough? As I said to someone below - I measure 5V from the electrical ground to earth ground (literally just sticking my meter probe in the earth) right near where the grounding rods are installed.
That sounds like a fine setup. Passable grounding could give 5v if somewhere in the ballpark of 200mA are passing through it. Reading through maybe the generator has a short to ground.
Edit: the only place ground and neutral are connected is in the main breaker panel, correct?
No idea. Commenting to boost engagement as this sounds potentially dangerous
When you say “charging brick”, is this a 2-prong or 3-prong USB charger? I’ve only ever seen 2-prong units in the USA, which would make this deeply puzzling since that means the charger has no ground conductor (which is fine) yet somehow has a consistent potential w.r.t. the floor you’re standing on.
Assuming it is indeed a North American 2-prong wall adapter, those are usually non-polarized, meaning that they don’t rely on line and neutral to be in a particular orientation. So still, I can’t see how the charger itself could be passing through a current – however slightly – across a difference of 16-65 VDC.
To be clear, you don’t live near any large magnetic fields, right? Does a magnetic compass work properly in or around your home?
Anything electrical in your area that might be emitting strong EM fields?
I have a metal roof and I’m maybe 80’ from the high(er) voltage residential supply lines that go into the distribution transformer that feeds my house.
Not that I can think of. I’m in the middle of nowhere - nearest neighbours are about a kilometre away, no industry. The only power lines in the area are the ones that feed my house.
