Getting shocked--by touching DC5V??

1

1pet2_9

Well-known member
I'm stumped by this one, and it's a safety issue:

Every time I touch my GND or VDD line from my 60A 5V supply, I get shocked--as if I was touching an AC line. Every single power injection point--shocked. Yet nothing is fried, no fuses blown, nothing. A few melted connection joints where the wire got a little thin, and that's it. When I probe with my voltmeter across the GND/VDD leads, I get 5V. The only way I can identify a hot at all is by touching it (and getting shocked). So I disconnect everything and move my (working??) controller back to the lab, and now I can touch both 5V leads all day. I'm not so sure what will happen if I turn around and move it back outside, though.

How in the world does that happen? Without frying anything?
 
Assuming that you are using 2 different power supplies in this scenario, the one outside needs to be replaced............
 
remember that voltage is the potential between two points. When you measure between DC ground and +5V, you are getting 5V. Of course that makes sense. But what is the ground point when your finger is on the +5v wire? Unless you're touching something else, it's the earth. I'd bet if you measured between that 5v point and earth ground you get a value rather different than 5v. Not all PSUs are earth grounded. Some are, some aren't.
 
If half my circuit was 3-prong, and half 2-prong, would that do it? I do have 2 power supplies, and they share a common ground but it straddles the 2-prong/3-prong outlets. Controller's 5V but the power supply has to be 12V. And I cheated the 3-prong 12V supply with a 2-prong adapter into a 2-prong extension cord. I just reassembled everything outside and everything decided it was going to work this time, but I really need to root cause that one--that one's bad. It boggles my mind that nothing's fried.

I guess I've got a new rule of thumb now: when using at least one grounded power supply, you need to share a common AC ground among them all--not just DC ground. This must be a situation similar to back when surge protectors used to catch fire on cruise ships.


Thanks all, by the way.
 
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Some un-isolated devices/design tie the circuit ground to neutral, if your outlet is wired reversed (hot and neutral swapped) or you plugged in reversed using the 2-prong plug(good 2-prong plug have a large and small blade, small is hot), your DC ground will be hot (120v) to earth ground, you will get shocked. The earth ground(also same level as neutral) on the power supply case is to protect any loose hot wire touching the case to pop your breaker before you get shocked. If you measure your GND/VDD with AC setting to earth ground (ie: metal water pipe in your home), you are likely see 120V when you got shocked, reverse the plug will fix your issue, but using 3-prong is the best because it's keyed and you will not shock yourself when something is wrong with case grounded as well. With that said, you will need to be careful with most of the Chinese none UL rated stuff, mainly 2-prong, I see a lot of them wired backward and case was hot.
 
I am impressed. I'm pretty sure you answered my question completely. I migrated the 2-prong circuitry to 3-prong, and everything works beautifully now. DC PSU's need to share the same AC ground from now on, and that needs to go to earth.
 
The tingle you got could be to do with the suppression capacitors that go from active to chassis and neutral to chassis.

Normally this leakage current is shunted to the earth (ground) connection, but if that's not there, it will go via you.
 
1pet2_9 said:
I am impressed. I'm pretty sure you answered my question completely. I migrated the 2-prong circuitry to 3-prong, and everything works beautifully now. DC PSU's need to share the same AC ground from now on, and that needs to go to earth.
Click to expand...

It doesn't necessarily need to share the AC ground, but it should go to earth. You can earth it right at the controller location rather than using a 3 prong extension cord. For this purpose, a less than ideal earth ground should be adequate. So long as the conduction path is better than your body would provide, you should be fine. I ground my boxes to the steel stake that it's mounted on, which is driven into the ground. It's not a 6' copper rod, but it's better than my body.
 
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