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As I understand it, the case of a PC is metal so that it can form an EMI enclosure around the electronic parts. My view is reinforced by there being conductive fingerstrips around the mating edges.

This is a home built PC without any form of case:-

wall pc

I suspect that the black backboard with the four arms is scratch built too. It's probably plywood. This is 100s of Watts (motherboard + 2 high end GPUs) of totally exposed electronics running at ~4GHz spread out over at least a square foot. That's got to be an ariel somehow. There are many other examples if you Google "wall mounted PC". And there are of course all of the fashionable single board computers like Raspberry Pis running inside shoe boxes at 1.2GHz. Or glass PC cases.

Since EMI regulations get stricter all of the time, I thought that EMI is a problem. I read that it can be a major headache for equipment designers. Is it not really the problem the authorities make out? Or will this PC's owner never receive a mobile signal, be unable to watch clear TV or be sterilised within hours?

I've read EMI/RFI emissions and computer cases, Non-metal cases and emi standards/best practices for PC cases. The general tone of answers to these questions speaks to enclosing the PC in a shield of some sort, be it metal or conductive paint/ tape. Why bother?

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    \$\begingroup\$ Not produced in a large enough volume to merit FCC action. \$\endgroup\$ – Passerby Jan 28 '18 at 1:57
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    \$\begingroup\$ Does it ship with an FCC label or not? If not, it was probably never tested. If so, it may be worth studying how they were able to pass without a case. I am sure it is possible, but it would require lots of extra effort and attention to detail. \$\endgroup\$ – mkeith Jan 28 '18 at 2:21
  • \$\begingroup\$ Your metal enclosure may only stop radiated EMI at fairly high frequencies, and all slits in the casing for any opening will leak like a sieve. Your problem is conducted EMI via all cables and your case won’t do diddley-squat to prevent it from escaping. \$\endgroup\$ – winny Jan 28 '18 at 8:57
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I'm pretty sure, that this device will cause a considerable amount of EM emissions, but it is rather unlikely that this will lead to a problem for the owner.

Here's why:

  • This digital circuitry contains a bunch of highly integrated circuits providing many functions on its interfaces. It will generate a wideband noise rather than emissions of single frequencies. Wideband noise is much less likely to produce problems on other appliances/electronics
  • A tight enclosure of a PC does not emit nothing but a reduced wattage. Hence an open case will only make more noise of the same kind, and most electronics are already designed to cope with that type of emissions.
  • Even if there's an emission capable of let's say interfering in the ISM band, it will probably only reduce the usable bandwidth on a certain transmission by lowering the SNR.
  • Even if there are serious interferences suppressing a specific application e.g. in your neighbourhood it is still unlikely that they will find out, that you are the culprit.

My assumption is, that the designs of these "custom made PCs" did not undergo proper EMI-testing, because it is so unlikely that they will be ever made liable for violating CISPR regulations or similar code.

Nevertheless, there's a question of morals. Is it ok to break rules, just because you'll never be hold liable for it? Probably not. This is a typical problem of collective behaviour. To keep things working, the majority of people (and their computers) have to adhere to regulatory code. Otherwise the emissions of too many bad designs would sum up in a way preventing the use of correctly designed hardware.

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That looks like a computer module, not a consumer product, in which case the rules are different.

There is a big difference from a consumer or commercial product and an OEM module designed to be integrated into something else. If it is a complete product intended for wide distribution it must, or is supposed to be, tested to meet the requirements of regulatory body of the region where it is to be sold.

Products designed to be integrated into other products do not need to meet those requirements. The company integrating the module is instead responsible for ensuring the final product complies with the requirements. That does not mean all such products are without certification, most at least pass safety rules.

That may sound like a loop-hole but it really is not. The fact is even if the thing was tested and complied, there is no guarantee that after you stick it in a box with connectors and other parts that it will still pass testing. As such you need to certify the whole assembly anyway.

Further, everything is made of parts, made by other companies, and some of those parts are in turn made by someone else. Each in their own right is a potential source of EMI when you connect them together no matter how good the individual certification is.

If you are building your own stuff, and decide to pack someone else's module in a plastic box, you do not need to get it tested, however you do run the risk of having the local radio emissions authority come knock on your door because your neighbor complains when he cant watch his favorite TV show.

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  • \$\begingroup\$ Ah, it's not a computer module. My fault. I should have given a sense of scale. It's a full motherboard based PC. This thing is probably a full metre across. Those six blue round discs will be ~120mm fans and the two silver rectangles at 8 o'clock are hard disks. I suspect that the black mount (with the four arms) is a 60" flat screen telly bracket. \$\endgroup\$ – Paul Uszak Jan 28 '18 at 12:48
  • \$\begingroup\$ @PaulUszak " It's a full motherboard based PC. " that does not mean it is not a module designed to be integrated into something else.. Just the same as you buy a motherboard for building a PC, it isn't EMC qualified either, the box you put it in is. \$\endgroup\$ – Trevor_G Jan 28 '18 at 12:56
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There's electronics in that PC, but there's shielding, too. The hard drives, for instance, are in metal cases, and the wiring going TO the hard drives is hidden (perhaps in a metal channel?).

The power supply (upper left) is in a steel case. The RAM modules appear to have their own metal shields (a pair of plates affixed at the factory). The (big, fancy) dual video cards have some kind of housing as well, which might be both shielding and a ductwork for cooling air.

With short wires, kept close to the wiring ground plane, electromagnetic emissions are minimal and (if the circuit board designer cares to do so) can be made zero by printing shielding layers into a wiring board. Really, an overall grounded case is only one last layer of a defense-in-depth strategy of keeping all signals from interfering with each other.

The remedies for electromagnetic interference are well-understood and don't always require an overall metal box.

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