Over a year ago, I installed this wooden cabinet on the wall that I can put equipment in. Recently I added in a DC power supply that is used to run 19 Volts DC around my house. It's fused and stuff and meets code from what I understand here in the US.
In the wooden cabinet I put a small steel grounding strip. I ran a 14 awg wire over to the edge of my house(through the attic, etc.) but forgot to ground it. So my grounding strip is just a "pretend ground", I will call it that from now on. It's got a whole bunch of surge suppressors tied to it but it is not tied to the mains ground.
I was troubleshooting some unrelated issues today and put a multimeter between the mains ground and this "pretend ground". It read 26 VAC. So I found a AC current meter and it showed 200 ma between the same points. At this point I figured all my meters were bad. So I rigged up this.
It is exactly what it looks like. An LED between mains ground and "pretend ground". It has a 100 ohm resistor as well. It lights up, so there is real current here. Remember that the "pretend ground" is 100% isolated from earth.
I eventually discovered unplugging my 19 VDC power supply would "fix" this current. I can't comprehend why. This power supply runs over a 3 wire cord about 20 ft long. Two wires being the isolated DC output and another being the "pretend ground". The "pretend ground" is connected to a cabinet mounted on a wooden wall in my garage. So it's sort of earthed but 2 x 4s aren't really great conductors. So there are 3 states I found
1. Power supply unplugged - no current, LED not lit
2. Power supply plugged into multi outlet strip that is off - very little current, LED lit dimly
3. Power supply plugged into multi outlet strip & turned on - 200+ma current indicated, LED lit brighter
Next I dug out the scope and hooked that across the LED + resistor. I captured the following traces from the cases above
Case 1:
Case 2:
Case 3:
So in case 1 it looks like some switch mode power supply is coupling into the long unterminated "pretend ground" wire. It shows approx 1 volt amplitude. Case 2 is just weird, I don't understand that. But case 3 has 30 volts of amplitude and some sort of 60 Hz component. So mains is getting coupled in here! Enough to light the LED.
I tried a transformer based linear regulated power supply and did not see anything on the scope nor did the LED light.
I tried a 24 VDC wall wart (300 mA) rating that came with some Harbor Freight drill. The scope shows this waveform and the LED is really dim. Still seems to have a 60 Hz component.
Can someone give me an idea what is going on here? I've shorted the grounding strip to mains ground for now. But I seem to have 200 mA of mystery current floating around that I just don't understand at all. I don't think you can get this kind of capacitive coupling between stuff at 60Hz. I have tested the outlets in my house and they all appear to be wired properly. The electrical system of my house is grounded with 4 ft long copper clad steel rod connected to the main breaker panel on the outside of the house