I have a wooden box and the outside is plated with aluminum. I'm trying to install a computer (the components) inside this box, but I'm worried about the earthing. Usually the screw holes in the motherboard has big dabs of solder around them, as if to earth the case, but this wouldn't be possible in a wooden box, of course. I'm wondering if there is a problem with the aluminum plating, and if it in some way would interfere. Would I have to wire the motherboard earth to the aluminum plates?

Thanks for any help.

  • \$\begingroup\$ It's not essential to earth it if the case is nonconductive on the inside. \$\endgroup\$ – pjc50 Apr 21 '16 at 19:31
  • \$\begingroup\$ @pjc50. Not essential. Does that mean that it would be safer earthed? \$\endgroup\$ – Script_Coded Apr 21 '16 at 19:37
  • \$\begingroup\$ @pjc50 short answers in comments especially for simple questions are likely to prevent normal answers. Please post as answer and maybe extend a bit. \$\endgroup\$ – Grebu Apr 21 '16 at 19:41
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    \$\begingroup\$ @Grebu true, but something like this has a 50% chance of being closed while writing a longer answer. \$\endgroup\$ – pjc50 Apr 21 '16 at 19:45
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    \$\begingroup\$ @pjc50, I consider this question viable and interesting. GND connection via the screws to the casing could improve EMI shielding and (partially) change the path of return currents. \$\endgroup\$ – Grebu Apr 21 '16 at 19:55

Grounding is a really fun topic among EMI/EMC compliance folks. Here is my take on it for computers:

With so much high-frequency content from the various clocks and data interfaces, coupled with the large number of SMPS power supplies on-board, entombing the system inside a metallic case of some type is virtually the only sane way to pass FCC/CISPR radiated emissions testing. The case protects the hardware and nicely attenuates significant amounts of noise, helping you pass testing.

On the motherboards I've designed, mounting holes and "chassis grounds" were single-pointed to digital GND on my PCB itself, through a high-impedance resistor -- all we wanted to do was bleed off charge and not directly conduct ESD events into our circuitry. Some folks may keep them completely separate, and depend on a metal case to link all those paths together. Our metal casing was tied to AC earth ground -- noise got shunted to AC earth, and if we had a short circuit, the case would not remain at a dangerous potential.

The danger with a metal case that is not grounded to AC ground or similar is that if you have a short-circuit inside the unit (AC or DC), the entire case will now float at that potential. You can imagine this being dangerous if you unknowingly had AC live shorted to metal -- anyone who touched that would be in danger.

In your case (hah), I would do this:

  1. Beep out (measure resistance) from the ATX motherboard mounting holes to a known GND point. If it's 0 ohms (short), you're done and fine, since your GND is already connected.
  2. If it's open, check the holes to each other. If they are separate (open) to each other as well, then you also have nothing to worry about -- those were designed to allow screws to mount the motherboard and do nothing else.
  3. I take it it's a wooden box, with aluminium siding on the outside. Does this aluminum make contact with a metallic EMI shield from the motheboard? Or the ATX PSU? My gut says that metal should be earthed to save you in case any kind of high-voltage shorts to the chassis -- better the device goes "pop" than you.

Here is a related SE question: Is it dangerous if I do not connect the ATX shield to ground?

This is a potentially (hah x2) divisive question and I've love to have a discussion on it with folks.

  • \$\begingroup\$ This is an interesting answer. I'm curious about "entombing the system inside a metallic case of some type is virtually the only sane way to pass FCC/CISPR radiated emissions testing." What exactly is that? The words "radiated emissions" caught my attention. So computers give off nasties, and these cases are here to protect us? I would love to hear more information on this. \$\endgroup\$ – XaolingBao Apr 25 '16 at 5:18
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    \$\begingroup\$ Radiated emissions isn't anything dangerous like ionizing radiation -- it simply means electrical noise that leaves the device in the form of electromagnetic waves and propagates as such. Conducted emissions would be noise that leaves via conductors, such as cables and such. \$\endgroup\$ – Krunal Desai Apr 25 '16 at 17:21
  • \$\begingroup\$ Interesting, thanks for the tips. I ask because I am designing a custom case, and I have a similar question to the OP about standoffs. I was curious about the emissions, because I hear that cellphones also emit things that could be dangerous, and figured it could be the same as what's inside the computer case... Except maybe the worry is wireless frequencies from the phones and not electronics * (would routers/wireless devices be similar)? If you're interested, my topic is here electronics.stackexchange.com/posts/comments/… Thanks a lot for your time. \$\endgroup\$ – XaolingBao Apr 25 '16 at 21:04

Earthing and grounding are different things. Grounding references a circuit to the consensus 0V (Earth) whereas Earthing metal is a safety strategy. Earthing requires a high current, low impedance connection which will ensure that in the event of a fault on the mains side, a fuse or circuit breaker will rapidly open. Over the whole installation, there are standards for "earth loop impedance". Each device which is earthed should itself have a sturdy earth connection to ensure that its path to Earth is adequate.

The computer motherboard has grounding connections, not Earth connections. Any significant fault current passing through it will rapidly destroy it, and likely put high voltages on the exposed parts of the circuits such as connectors (e.g. little 5v USB connections, etc).

So grounding is a matter of low voltage circuit function. Earthing is a matter of high[1] voltage supply safety. The computer motherboards and circuit should be isolated from mains except for grounds. The PSU will have a transformer inside which provides isolation. If you look at the PSU PCB you will (or should) see physical gaps in the PCB bridged only by transformers and optoisolators, to ensure that mains finds it hard to jump the gap and become dangerous to the user.

The Earth connection comes into the PSU. It is robustly connected to the PSU case. The PSU case is then earthed by being bolted onto the chassis, providing chassis Earth. This is the only connection you need to be concerned, for safety reasons, being connected to the chassis. Make sure you have a good, low impedance connection from the PSU to the chassis plates. You don't need to "earth" the motherboard at all. It'll run just fine sitting outside a case on an insulating desk or a towel etc. Bear in mind that much electronic equipment is double insulated, meaning there are no connections from low voltage ground (0V) to Earth at all. But ground connection helps reduce stray interference, so it may be beneficial to ground some of those screw mountings anyway. But they aren't there to provide Earthing.

[1] Technically at least here in Britain, "high voltage" is only over 1000V, anything below that is "low voltage". I am using the vernacular usage in which anything dangerous is termed high voltage.


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