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I was watching a YouTube video (I've provided link in the comments) where the user claims that sometimes when we don't have earthing done in our homes or you are using two pin plug in the socket, the PC cabinet gives you mild shock.

He said it's not lethal (it's like the small DC shocks that you get with metal laptops while charging) but it happens because earthing is not working.

Then he explains why:

He said screws of PSU are internally connected to the earthing pin of itself. He also showed it using some testing device as shown in the photo below.

enter image description here

He also said this also true with motherboard screws.


Now I want to know if these are some specific old generation PSUs/Motherboards or it happens in all modern PSUs/Motherboards too?

If yes (the logic given by him is correct), I wonder why would a manufacturer do that because a lot of homes in India literally don't have any earthing grounding done in their homes. Including mine.

In fact, most homes in rural or small towns don't have any earthing. It doesn't matter you use a 3 pin plug or 2 pin plug. All will work same.

So I wonder if that indeed is true, I think it means when it is plugged in and you touch the PC cabinet, it will give you a strong AC shock. Which would be lethal. Because the screws are always connected to metal PC cabinet. Even if the PSU is not faulty and all connections are done properly.

And I somewhat doubt what he said especially when he said it may be mild shock and not lethal (he said this in Hindi). I might have not understood it correctly.

Please correct me if I'm wrong or misunderstood it.

Given all that, do all modern PSU screws have this kind of intentional earthing connection with PSU earthing pin?

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  • Link to the video: youtu.be/gm44eo-V19E?t=105
    – Vikas
    May 15 at 8:00
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    That is literally the point of the earth pin.
    – Aron
    May 16 at 8:22
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    no. I mean, literally, the point of the earth pin is to make the default failure mode, what we call an Earth/Ground fault. By connecting the entire shielding to earth, it is very hard for mains voltage to connect to the user. In fact, the lack of an earth for this setup is extremely dangerous. What would normally be a ground fault, would in fact energize the whole case to mains voltage...
    – Aron
    May 16 at 10:56
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    For what's it's worth, what you describe for India is very much not the case in much of the rest of the world. Building a home with no earthing system would be illegal in the U.S., for example (unless maybe it had no electricity at all, but, in practice, that's pretty much never the case.) I think the same would be true in most or all of Europe, though I'm not as familiar with their electrical code requirements.
    – reirab
    May 16 at 20:53
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    @Vikas. Example. The PSU overheated. Insulation failed on the live wire. The PC continues to work exactly as expected. You touch the PC, you die.
    – Aron
    May 17 at 5:51

2 Answers 2

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Yes it is true.

The mains inlet connector earth/ground pin is connected to the ATX power supply metal case inside the power supply.

This will also connect the entire PC metal case to earth/ground when the power supply is mounted correctly in place with screws.

Also the ATX power supply output voltage is earth referenced, as the black wires 0V potential is also connected to metal case in the power supply.

The problem of getting shocks happens when a device which has a mains plug with earth/ground is not connected to earthed/grounded mains socket.

Devices with earth/ground pins usually have a mains input filter to suppress electromagnetic interference. One part of the filter consists of two capacitors, one capacitor from Live to Earth, and another capacitor from Neutral to Earth. Earth connection is needed for the input filter to operate as intended.

Problems happen when the earth/ground of the ATX power supply is left unconnected. The capacitors form a capacitive voltage divider, which means that for a 230 volts AC mains input, the metal case of the PC floats capacitively at 115 volts AC.

While that may sound dangerous, it really isn't, as the capacitors are quite small in capacitance, let's assume they are 2.2 nanofarads. At 50 Hz or 60 Hz mains AC frequency, the impedance of 2.2 nF is quite high, approximately 1.2 megaohms.

So touching the PC metal case, or anything connected to the PC, such as mouse, keyboard, display, etc, the electricity can be felt, but leakage current will be limited to below safe values.

It is a problem is multiple devices such as PC and a display are connected together, as if the display also has a similar filter, it may double the leakage current, and all other devices connected to PC with similar mains filter add up too, so the shock can be quite strong, but not lethal.

It is also a problem if you connect devices together, and their voltages are at different potentials. If you have one grounded device, connecting it will discharge the filter caps and equalize the potential, so it is best that the device has a connector which connects grounds first, so that the potential does not equalize via any data pin.

So yes, you will get shocks from any device that requires to be connected to a grounded outlet but is not connected to a grounded outlet. Even if the device is working properly. It is still connected incorrectly to ungrounded socket.

Some old installations don't have grounded outlets and that is a problem when using new devices that require grounded outlets.

Many devices read in their manuals that they must be connected to an earthed outlet, so basically, if you get shocks, you are using the device in a way that is forbidden by the manual, so that is something you have to deal with if you don't have grounded outlets.

Some laptop power supplies come with ungrounded plug, so they do not need ground. But as these types of power supplies have to work without ground, they may have two issues. Their output is not grounded but floating, so it may float at the power supply switching frequency, and the floating voltage may be quite large, so it might also cause feeling of electricity when touching and emit electromagnetic interference. This is usually prevented by putting a filter cap between mains input and low voltage output, but then as it removes high frequency floating voltage, it causes the output to have capacitive coupling to mains voltage floating, so there may be mains frequency AC voltage present on the output, just like when ATX supply is left ungrounded.

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  • 1
    Afaik metal casings are almost always connected to ground (and devices with metal casings require a three-prong plug and, obviously, outlet that provides ground). Necessary to prevent the casing from accidentally being on high voltage just because the insulation somewhere rubbed through. Devices with plastic casings may choose to not have a ground connection. May 16 at 12:01
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    @Peter-ReinstateMonica Plenty of equipment exist with metal cases and two prong ungrounded plugs, so simply just the metal case does not require grounded plug. Blu-ray players, AV receivers, set-top boxes are like that, for example.
    – Justme
    May 16 at 12:07
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    @Justme, I think the rules depend highly on where you're situated. Here in the UK for instance, two-wire mains leads are permitted only for double-insulated devices, meaning that pretty much anything with exposed metalwork must be earthed. May 16 at 12:35
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    Yes, they must be double-insulated. In my house, all the double-insulated devices have plastic casing, but I believe metal housings can be made conformant (probably not as cheaply). May 16 at 13:16
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    To recap, yes, connecting any device that has mains filter caps that requires earthed socket to an unearthed socket will give shocks. No it is not same as touching live wire directly, because current flows through filter caps, they should limit current to safe non-lethal value. Also any device connected to ungrounded computer can give you shocks too from their metal case (keyboards, etc)
    – Justme
    May 17 at 4:17
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This would be a good cross-post to Electrical Engineering!

There is more than meets the eye.

Answer First: It is a reasonable assumption in most regions of the world and most applications But it is not a strict requirement of ATX or PC Systems. It is theoretically possible to make an isolated Motherboard and ATX PSU system. But I suspect that it would be difficult to make it compliant with national governing bodies for Electromagnetic Interference (e.g. FCC, EN, CE) and tested by Nationally Recognized Testing Laboratory for Safety (e.g UL, CE) while being competitive and universally compatible. . Isolated systems, which are common in industrial sectors, are not marketed as generic PC parts, but as custom systems or marketed for specialized applications. .

Claim #1

Contentious Claim #1 There is no ATX et. al or System level specification that dictates that Chassis/Earth and Common must be bonded

Intel ATX 12V Supply Design Guide

"ATX Specification" as archived by instructables

In the latter document, the word "Ground" is mentioned very sparsely, without clear guidelines. "Earth" is not mentioned in the document.

For example, in Section 3.3.5, grounding is mentioned in passing in discussion about back panel I/O clearance and paint masking on cases for EMI Compliance. There are useful diagrams of the back panel.

enter image description here

The former document, an Intel design guideline for ATX Supplies.

In section 8.4 mentions "Earth Fault" as a "Recommended" Catastrophic Failure Protection", something that shouldn't happen if the Power Supply fails.

Intel also describes the conditions for Safety Compliance which mentions "Earth"

enter image description here

These are not the authoritative documents, but the lack of any specific reference to earthing or grounding leads me to claim #2

Claim #2

Contentious Claim #2: Bonding Chassis to Earth and shielding everything is the most expeditious way to comply with electromagnetic interference requirements (EMI)

What may not be transparent to you, but the PC world , as an industry, is specified such that if you build a system following the MFG instructions and you use certified parts your final system will be compliant with the regulations in your region.

This is why every ATX Motherboard must come with a back I/O face plate.

This specification is easy to describe but difficult to define.

One aspect of EMI testing, involves radiated emissions, you point an antenna at the system in a shielded room.

enter image description here

source: science direct

Another key requirement is Electrostatic Discharge, where a high voltage spike (1-4KV) is applied to various pins and metal parts of your device/system and it is expected to not respond or recover on its own.

enter image description here

Source: https://www.nutwooduk.co.uk/archive/keitharmstrong/emc_testing3.html

As a whole, EMI testing is similar across the world. It is not surprising that a global product like PC and PC components has settled on a common denominator that keeps everything compatible and keeps the regulators happy. PC's are very (radiofrequency-wise) noisy constructions, and following this pattern keeps everything quiet in the neighborhood.

The last aspect is how to reconcile this with safety

Claim #3

Contentious Claim #3 : If you must have chassis bonding to facilitate emissions, and you also must provide a path to electrostatic discharge to ground, then a non-isolated supply Class I is the most obvious standard.

Class I supplies require a PE connection, which was already (per claim #2) required to be electrostatic compliant and must be connected to ground for shielding compliance, and thus you see this little symbol on power supplies and systems.

enter image description here

source: https://www.idealpower.co.uk/what-are-the-differences-between-iec-power-supply-protection-classes/

The basic difference is that Class I supplies only have a single layer of isolation between the Power circuitry and the chassis, and thus a protective earth path is required for safety, and all metal sufaces must be bonded to protective earth. Subsequent, because it must also be bonded to EMI shields, and the shields are bonded to ground, this is where the connection between Protective earth and Ground is again reinforced, This is then reflected in the test by the Nationally Recognized Testing Laboratory which will specify (on a per-region/country basis) the requirements for the relevant label (e.g. UL in NA, CE in Europe, CCC in China)

enter image description here enter image description here

Comparison of Class I and Class II from XPPower https://www.xppower.com/resources/blog/iec-protection-classes-for-power-supplies

Interestingly, the Intel design guideline does mention a power supply feature in section 3.3.2 about the Power On Pin, but does not mention isolation as a requirement for any other pins.

enter image description here

enter image description here

A search of "Class II ATX supplies" (with roman numerals, not class 2) shows much fewer options in the standard locations.

This leads to the conclusion

Conclusion

Contentious Conclusion Universal Compliance with EMI requires using the metal chassis for shielding and safety regulations reinforce the requirement that the chassis must be bonded to earth. The combination leads to the general assumption that the Protective Earth, Chassis/Metal Case, and system"Ground" or "Common" must be isopotential, in most compliant situations.

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  • But your claim #1 is not true. The first PDF you linked in chapter 3.5.6 does dictate that chassis and common must be bonded. Normal ATX power supply specs too.
    – Justme
    May 16 at 17:51

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