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I'm putting on my desktop computer on steel rack as usual. A while ago, when I touch the steel rack for seconds, practically I feel a tingling sensation, sort of electrical shock in my finders. After that I measured how voltage the steel rack has, the multi-tester meter indicated almost 50V by AC mode. Is this safe to touch or keep the PC on the position, or I should get away the PC somewhere electricity-safe? I'm not sure this site is suit for asking this kind of question, but I'm very worrying whether this is safe or not since this experience is my very first time. Thank you.

丶 Limeー来夢 丶
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  • So it has 50V AC, but in reference to what, was the multimeter leads between rack metal and earth/ground somewhere? Or between rack and computer? Is the computer plugged to earthed/grounded mains outlet with proper 3-pole power cord? Are there ungrounded/unearthed devices in steel rack? – Justme Jan 30 '20 at 05:45
  • are you saying that the rack is connected to an electrical outlet? – jsotola Jan 30 '20 at 05:46
  • @Justme The cable for the computer has no grounding cable, so I didn't do grounding for the PC. Maybe since I'm not living in US but Japan, I guess. – 丶 Limeー来夢 丶 Jan 30 '20 at 05:50
  • @jsotola I can't say so for sure, but The rack is connected with PC, the PC connected with power, the power connected with the outlet so I could say those are connected indirectly. sorry for my poor knowledge. – 丶 Limeー来夢 丶 Jan 30 '20 at 05:53
  • it could be that the shelf is connected to the ground wire, but the ground wire is not connected to ground – jsotola Jan 30 '20 at 05:57
  • @igar so what kind of PC (desktop or laptop or mini) it is and what kind of mains power input it has (direct mains input to PC, external power brick with mains in and e.g. 12V out)? So is there anywhere a 3-pole earthed mains input? What you describe sounds like the PC power supply would require 3-pole grounded mains if it has such an input, or, if it is really a 2 pole input, it is designed to work without grounding, and the tingling is a feature from capacitive coupling from internal EMI filtering. – Justme Jan 30 '20 at 06:04
  • @Justme sorry but I don't know voltage but only know the PC has 700w power supply. The one is desktop PC. In Japan almost all outlets have no third pole for grounding, only two poles they have. "it is designed to work without grounding", so, I don't have to worry too much about touching that? – 丶 Limeー来夢 丶 Jan 30 '20 at 06:18
  • @igar no what I meant is that if the PC power supply is a standard ATX power supply with standard 3 pole IEC input for mains, it must be connected to earthed outlet, so in this case the power supply is not designed to work without ground. EMI filter capacitors inside the power supply will make the PC metal parts float at half mains AC voltage. – Justme Jan 30 '20 at 08:00
  • @Justme sorry for my poor English but what that means "the PC metal parts float"? means the PC parts could be damaged? I understand I'd better to make the PC ground. – 丶 Limeー来夢 丶 Jan 30 '20 at 08:39
  • To be clear OP, your case is *live* at half mains voltage. – Keeley Hoek Jan 30 '20 at 16:31

3 Answers3

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A comment by the OP:

In Japan almost all outlets have no third pole for grounding, only two poles they have.

Because modern SMPS wall warts (or equivalent) don't tend to use the ground wire in the socket because many times it's just not available, they have to reliably reduce emissions on the DC output wires by connecting a capacitor from output to rectified line voltage as per this answer: -

enter image description here

Now, the impact of adding the capacitor in the red box is that it can pass a small (safe) amount of AC current through to the secondary and, if the secondary isn't earthed via the laptop or PC then you will feel a (harmless) tingling sensation if you make contact with it.

Andy aka
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    That applies to 2-prong power supplies, but not to 3-prong power supplies. 3-prong power supplies such as ones commonly used in desktop computers have a different mechanism. The mains input has Y caps from Live and Neutral to ground for EMI filtering, and the ground is connected to metal chassis. So when such a device is connected to ungrounded outlet, the Y caps form a capacitive voltage divider and makes the chassis live with half mains voltage. This is why 3 prong devices need ground, and the ground connection should never be defeated on purpose. – Justme Jan 30 '20 at 11:34
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    @Justme And as [Japan uses a 100V mains supply](https://www.japan-guide.com/e/e2225.html), then "_makes the chassis live with half mains voltage_" fits with the OP's measurement of 50V . – TripeHound Jan 30 '20 at 14:09
  • this is a startling thing to learn... why aren't people getting killed all the time there? is it because neither AC leg is a grounding leg, so ground ref is meaningless, both sides are insulated? – Grady Player Jan 30 '20 at 16:59
  • @GradyPlayer, in normal circumstances, the amount of current is not enough to harm. However I’d be concerned about using a desktop power supply that must have an earth and not earthing it in the correct way. The op appears to be not doing what might be required. – Andy aka Jan 30 '20 at 17:18
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    @GradyPlayer no, it's because the capacitor is only 2.2nF. Given 120V 60Hz at the input, it will never pass more than about 22 microamp, because it can only accumulate a tiny tiny amount of charge per cycle of the AC. – hobbs Jul 02 '20 at 15:57
  • @Andyaka It seems the OP has no other choice as "In Japan almost all outlets have no third pole for grounding"... – Déjà vu May 04 '22 at 03:28
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Any device such as a computer that has a mains input with ground pole really should be connected to a mains socket that provides ground. And with a supply cord that connects the grounds. A standard ATX power supply connects mains ground to metal case. If it is not grounded, the EMI filter capacitors from Live to ground and Neutral to ground will act as a capacitive voltage divider and thus PC metal case measures half of mains AC voltage. Not hazardous itself, but connecting other mains powered devices should be made while mains cords are unplugged to prevent discharge currents from destroying devices. The slight shock can also surprise people and their reflexes can cause movements that might hit something.

Justme
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3

The case of the computer should be grounded. If in doubt, the steel rack can be grounded as well. A stray 50V suggests that there is no ground, and you are feeling some small current leaking to the rack, perhaps via a power supply capacitor (see the answer by Andy aka).

Check your wiring - it may be in a dangerous state.

Simon B
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