Electronics > Power/Renewable Energy/EV's

Strange ground loop or something similar in my home

(1/3) > >>

hydrogen18:
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

wraper:
Sounds like Gateway PSU is faulty. Probably a short between primary and secondary within a transformer. That's why you need a real ground when using devices which have ground connection. Devices which require ground usually have weaker insulation compared to those which do not (reinforced/double insulated). Also that's why having GFCI is important. What you described is a life-threatening issue. Of course there is possibility that PSU injects high frequency noise into earth connection and your meter just goes nuts, no real 200 mA present there. Small leakage current like 1 mA is acceptable and is completely enough to light a led. Also it would be way more informative if you measured waveform just across resistor, without LED present. So you could see a real current flowing through it.

BTW, you should learn how to use/not use autoset. 500 ns/div and 2.5 us/div timebase is useless for displaying this issue. Other are too low as well but not that bad to be completely useless.

T3sl4co1l:
SMPS normally have line-to-ground caps for filtering.

You shouldn't see much waveform though, on account of the filtering, so it is quite possible something has gone bad.

Tim

f4eru:
You're measuring normal EMC with the scope. That's normal when you have a gigantic antenna connected to your scope, and a SMPS nearby.

The 200mA is much too high, it should be much lower through the typical Y-caps -> it could be a crappy meter overreacting to the EMC, or the PSU being faulty.

Your wiring is dangerous. Fix the Ground connection before somebody gets hurt or dies.

hydrogen18:
When you say EMC do you mean Electromagnetic compatibility? This is a new concept to me.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod