A wooden case - Should it be shielded/grounded?

Enclosures and acoustic damping to help quiet them.

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Mr_B
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A wooden case - Should it be shielded/grounded?

Post by Mr_B » Fri Sep 11, 2009 2:02 pm

I'm thinking about making myself a wooden case, because I'm picky and there isn't one available that's exactly what I want. :roll:

Whilst reading up on the subject (of non-metal cases) I came across a few warnings that build-up of static charge could be a problem, as could interference with with other devices around the home.

However, since the wording of these warnings were usually along the lines of: "Dude, you don't shield that thing an' it'll reboot every time your cell phone goes off..." I thought it best to ask here, where there was more chance that somebody would have some actual knowledge and experience.

Is grounding and shielding something I need to take into consideration? If anybody can help me out, I'd greatly appreciate it.

Mr Evil
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Post by Mr Evil » Fri Sep 11, 2009 2:41 pm

The PC must be earthed via the PSU for safety of course.

Shielding is mostly to keep the PC from interfering with other devices, rather than preventing other devices from interfering with the PC. As such, you shouldn't have any problems with the PC itself, but you or those near you may notice interference from the PC.

I doubt that static will be a problem because wood isn't that good an insulator.

audiojar
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Post by audiojar » Fri Sep 11, 2009 3:59 pm

I don't believe normal cases provide much of a shield, as far as electromagnetism goes.

It is nice that you ground yourself whenever you touch the metal frame (PSU chassis ground is earth grounded and thus any metal touching it). So I would make sure to use a wrist band to ground yourself when working in a wood case, because something sensitive could be the first ground path you touch (good practice to always use one anyway though).

Olle P
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Post by Olle P » Sat Sep 12, 2009 1:46 pm

I agree that there shouldn't be any problem. All acrylic cases would be extremely bad if grounding/shielding was always necessary.

That said, if EMI does become a problem you can always cover the inside of the case with a layer of spray-on metal later on.
Mr Evil wrote:The PC must be earthed via the PSU for safety of course.
While having the computer connected to protective earth (PE) is nice, it may or may not be advisable from a safety point of view.

At least here in Sweden PE hasn't been the norm for use in all rooms for more than 20 years or less, so most regular rooms don't have wall sockets with PE. The original idea is that with the room insulated from earth it will still be (somewhat) safe to touch the "line". (The entire idea is flawed since most rooms still have one or more heating radiators that are connected to earth...)
Anyway, by grounding a computer within such a room you create a way for current to move from a faulty device (class 0 or class 1), through your body, and via the computer down to earth.

Cheers
Olle

Mr Evil
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Post by Mr Evil » Sat Sep 12, 2009 2:39 pm

audiojar wrote:I don't believe normal cases provide much of a shield, as far as electromagnetism goes...
Most cases have too many big holes in them to be good shields, but they do still keep a lot of noise in.

Olle P wrote:...While having the computer connected to protective earth (PE) is nice, it may or may not be advisable from a safety point of view...
No, it really must be earthed. If it is not earthed and a fault occurs in the PSU causing the case to become live, then touching it will be dangerous. In this circumstance, there will be a path through you to earth because in most countries, neutral is connected to earth at some point. Even if the mains is floating, leakage capacitance can still be enough to allow a dangerous current to flow. Only double insulated (class II) devices can be left without an earth connection.

audiojar
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Post by audiojar » Sat Sep 12, 2009 5:43 pm

Basically, it has a 3 prong plug for a reason; it was designed to be connected to the safety ground.

I'm not so sure about how much (electromagnetic) noise a standard PC case keeps in though. I seem to remember learning that without a complete shield a large amount "leaks", even from a small opening. The same thing applies to sound waves, just a small crack can leak many dB when sound proofing something.

Either way, it's safe to say that electromagnetic shielding is not a standard part of a PC case. Of course professional audio electronics/cables and such are shielded and balanced for a reason, I'm sure a typical PC does produce some noise.

Zeroignite
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Post by Zeroignite » Sun Sep 13, 2009 10:02 am

Reading over the ATX specifications, it indeed looks that reducing EM Leakage is a major design criterion. That's not to say that every case manufacturer follows these guidlines-- far from it.

http://www.formfactors.org/developer/specs/matxemc.pdf

cmthomson
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Post by cmthomson » Sun Sep 13, 2009 5:45 pm

PC cases are pathetic at containing EMI noise, for the simple reason that removing even one of the PCI slot covers makes a hole big enough for any and all noise to escape.

But that's okay, because all the boards inside a PC are designed to have very low emissions (Class B in the US) on their own. They do this by having all high-frequency traces very close to a nearly-solid ground plane. Now before you think this is because the manufacturers are fastidious about EMI, think again; it's actually to make the board reliable by improving signal integrity of the traces. That said, no PC can ship without meeting Class B standards, typically by a wide (10 dB or so) margin.

Bottom line: don't worry about EMI in an unshielded PC (either inbound or outbound).

As for safety (earth) ground: the only place this matters is in the PSU, because that's the only place where there are high voltages. All commercial PSU's have built-in grounding, so you as a consumer don't need to do anything about it.

Olle P
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Post by Olle P » Mon Sep 14, 2009 6:53 am

Mr Evil wrote:If it is not earthed and a fault occurs in the PSU causing the case to become live, then touching it will be dangerous. In this circumstance, there will be a path through you to earth because in most countries, neutral is connected to earth at some point.
Yes, neutral is, and should be, connected to earth... but I fail to see how that makes me connected to earth if I'm not connected to the neutral line.
It's me (and just about everything else in the room) that's "floating", not the power supply. Touching a live wire will then be painful, but not inherently dangerous.

The PSU is designed to be a class 1 product, and just like all other class 1 products it will turn into a class 0 product when connected to a socket without PE. It will thus have the same (low) level of safety as all other class 0 products that are still perfectly legal to use in such environments.
(At least it's legal in Sweden.)

Cheers
Olle

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Post by Mr Evil » Mon Sep 14, 2009 7:10 am

cmthomson wrote:...But that's okay, because all the boards inside a PC are designed to have very low emissions (Class B in the US) on their own...
PCs definitely generate enough to cause measurable interference. There are worse offenders of course, but the general rise in use of noisy electronics has been very noticeable over the years, to the point where if I want to perform sensitive measurements on electronic circuits, I have to avoid doing them at certain times of day or they are swamped in noise.

It's not the end of the world if a PC goes unshielded, but it's better if it is. If you're making your own case, then you can make sure there aren't any holes to compromise the shield.



Olle P wrote:Yes, neutral is, and should be, connected to earth... but I fail to see how that makes me connected to earth if I'm not connected to the neutral line.
It's me (and just about everything else in the room) that's "floating", not the power supply. Touching a live wire will then be painful, but not inherently dangerous.

The PSU is designed to be a class 1 product, and just like all other class 1 products it will turn into a class 0 product when connected to a socket without PE. It will thus have the same (low) level of safety as all other class 0 products that are still perfectly legal to use in such environments.
(At least it's legal in Sweden.)

Cheers
Olle
If neutral is bonded to earth and a fault causes the case to become live and you touch the case and you are earthed in any way, then you form a complete circuit, with the current limited only by your body's impedance.

If neutral is floating, then touching the case may not be dangerous because you don't form a complete circuit. However, there is always some leakage capacitance to earth, which can be enough to allow dangerous currents to flow.

It may well be legal to leave a PC without an earth connection in some countries, but it's still a bad idea from a safety point of view (as are all class 0 devices) and I would never advise anyone to do it.

Mr_B
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Post by Mr_B » Mon Sep 14, 2009 9:00 am

Thanks for all your input guys, I think I will be having a bash at shielding now.

How much will escape through any ventilation holes I'll probably never know, but I'll start off using the equations in that link that Zeroignite posted (thanks for that) to work out the aperture sizes. If I find that too restrictive to airflow I can always re-shape things later.

walkingjohn
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Post by walkingjohn » Mon Sep 14, 2009 11:13 am

You can learn more by looking here and elsewhere for "faraday cage". Basically, in my limited understanding, radiation of long wavelength relative to any openings is blocked by a conductive enclosure.

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Post by cmthomson » Mon Sep 14, 2009 12:49 pm

Mr Evil wrote:
cmthomson wrote:...But that's okay, because all the boards inside a PC are designed to have very low emissions (Class B in the US) on their own...
PCs definitely generate enough to cause measurable interference.
Well of course. Class B is definitely measurable (how else would you know you meet or don't meet it?), many tens of dB above the noise floor.

If you've never been to an EMI testing facility, you might be surprised by how hard it is to create a quiet environment. These facilities are usually miles away from the nearest house or non-deserted road, sit inside Faraday cages, and still have to do multiple scans because a car comes nearby, or a plane flies within a few miles, or ...

All that said, if you want to build a containment for a PC, make sure that the largest non-conductive hole is smaller than about 4mm in diameter. A good guide is the hex grills used on most cases and power supplies. Any slot-shaped hole not only lets EMI out, it can often focus and amplify it.

Olle P
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Post by Olle P » Tue Sep 15, 2009 5:18 am

Mr Evil wrote:If neutral is bonded to earth ... and you are earthed in any way, then you form a complete circuit, ...
Which is why no earth connections are allowed within that room in the first place!
Mr Evil wrote:It may well be legal to leave a PC without an earth connection in some countries, but it's still a bad idea from a safety point of view (as are all class 0 devices) and I would never advise anyone to do it.
It's actually illegal to connect it to earth in a room without PE sockets, since that would make the computer the earth connection required to close the loop if some other (class 0) product is broken nearby.

The only legal alternatives are to either have an electrician replace all wall sockets in the room to PE sockets, which can be a bit costly, or to move the computer into a room that already has PE sockets, typically the kitchen, bath room or other "wet" area unsuitable for computers.

(And I do agree that class 0 products are crap from a safety point of view, but the real problem here is that the wall sockets typically don't carry PE.)

Cheers
Olle

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Post by Mr Evil » Tue Sep 15, 2009 10:47 am

Olle P wrote:Which is why no earth connections are allowed within that room in the first place!..
In which case the second part of what I said applies: Even if neutral is not connected to earth, there is going to be leakage capacitance to earth. This could well be enough to be dangerous, and even if it's not, you are quite likely to get the occasional tingle or spark if you touch it, simply because the voltage relative to earth is uncontrolled.

Olle P wrote:...It's actually illegal to connect it to earth in a room without PE sockets, since that would make the computer the earth connection required to close the loop if some other (class 0) product is broken nearby...
Ok, I don't know the law in your country as you do. Of course you shouldn't try earthing such a system (nor should you make any additional earth connections to a system that is already earthed) - all equipment should be earthed only through the wall socket. I wouldn't be happy using anything less than a class II device there though.
Last edited by Mr Evil on Tue Sep 15, 2009 10:49 am, edited 1 time in total.

Mr Evil
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Post by Mr Evil » Tue Sep 15, 2009 10:48 am

EDIT: oops.

Olle P
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Post by Olle P » Wed Sep 16, 2009 8:54 am

Mr Evil wrote:In which case the second part of what I said applies: Even if neutral is not connected to earth, there is going to be leakage capacitance to earth.
But neutral is connected to earth and insulated from the case, so under normal conditions there's very little leakage current.

With a single fault neutral might become connected to the casing, effectively grounding the case, which shouldn't be dangerous since it's still insulated from the line.
Another possible single fault is to have the line connected to the case, which is a bit more troublesome but still shouldn't be really dangerous until a second fault is applied.

Cheers
Olle

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Post by Mr Evil » Thu Sep 17, 2009 11:20 am

Olle P wrote:But neutral is connected to earth and insulated from the case, so under normal conditions there's very little leakage current...
When neutral is connected to earth, then like I said before, a fault that causes the case to become live is potentially fatal.

Olle P wrote:...With a single fault neutral might become connected to the casing, effectively grounding the case, which shouldn't be dangerous since it's still insulated from the line...
Neutral being connected to the case shouldn't be dangerous, but you should always allow for the possibility of neutral and live being reversed (a common wiring error).

Olle P wrote:...Another possible single fault is to have the line connected to the case, which is a bit more troublesome but still shouldn't be really dangerous until a second fault is applied...
No, that's the bad one. It's ok if this happens when the case is properly earthed because it just blows a fuse, but if the case is not earthed then touching the case would be dangerous because it's now earthed through you.

Das_Saunamies
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Post by Das_Saunamies » Sun Sep 20, 2009 3:24 am

Friend built his computer in a wooden box and made a footrest out of it. Seems to work fine without any sort of shielding where he uses it and at our LAN parties - as long as no one steps on it or pours coke in, the latter of which has happened. :D

Another funny story: tried to buy a side panel with a window for my ancient, then-huge Compucase 6919 tower case, but the importer informed me that it did not pass the EMI shielding requirements and thus could not be sold in this country. Guess it matters and is tested!

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Post by cmthomson » Sun Sep 20, 2009 4:20 pm

Das_Saunamies wrote:Another funny story: tried to buy a side panel with a window for my ancient, then-huge Compucase 6919 tower case, but the importer informed me that it did not pass the EMI shielding requirements and thus could not be sold in this country. Guess it matters and is tested!
Well...

It might matter and it might be tested, but then again, it might just be legislated...

Das_Saunamies
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Post by Das_Saunamies » Sun Sep 20, 2009 10:58 pm

Of course it was "just" legislation that kept it from being sold in the country, but I like to think they don't have all those rules for nothing. :wink:

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Post by EsaT » Fri Sep 25, 2009 12:32 am

Das_Saunamies wrote:Another funny story: tried to buy a side panel with a window for my ancient, then-huge Compucase 6919 tower case, but the importer informed me that it did not pass the EMI shielding requirements and thus could not be sold in this country. Guess it matters and is tested!
Akasa Eclipse got sales ban from Safety Technology Authority for failing EMC testing.

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Post by Das_Saunamies » Fri Sep 25, 2009 12:35 pm

Good to know there's more stories like the side panel out there, thanks Esa.

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Post by Gorsnak » Mon Sep 28, 2009 4:11 pm

Haven't had a lick of trouble wrt RFI in my case, and I put in no shielding whatsoever. I can say quite authoritatively that using 2.4GHz cordless phones, cell phones, etc next to the case results in no problems at all.

It is grounded via the three-pin power cord, and the psu chassis is connected by an extra ground wire to the mb tray and drive cage. I personally wouldn't skip that step.

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Post by Linqon » Tue Sep 29, 2009 12:32 am

Intresting topic!

I built a wooden HTPC just recently (gallery, buildlog, text in swedish only, sorry) and was noticing something strange.

For controlling the computer I use an old Logitech MX700 keyboard+mouse. When I switched to my wodden case from a Lian li aluminum case the range of the cordless controllers changed for the worse.

Hardware in the computer is the same. Placement of the transmitter is even slightly better now, still the performance of the cordless mouse is alot worse now. I'd say the working range is 50-75% shorter now, about 1.5 - 2 m, and it needs a direct path between the transmitter and mouse or it wont work at all.

It seems the mouse is using radiofrequency at 27MHz. Could my problems be due to lack of shielding in my case?

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Post by Das_Saunamies » Wed Sep 30, 2009 8:19 am

Well it is RF technology, so interference can't be ruled out. I don't know enough about the frequencies emitted by a PC to say for sure, but if the only thing that has changed to cause this is the enclosure, then I'd say that's a suspect right there.

I'm sure you're familiar enough with the gear now to know if there were options like channel and signal strength that you could tweak to try and correct this.

With wireless it's damn hard to know what really constitutes "a good position" for receivers and transmitters, at least if you haven't studied the field. I know that using my bluetooth cellphone gear and 3G cellphones sometimes feels more like voodoo than digital technology. :lol: All I can suggest is to try and keep the transreceiver as far away from the case as possible.

Oh, and a funny story... my D-Link DI-524 wireless G router shoved in the bedroom closet has more range than same unit on living room shelf. Did not reset device, transmitting power was set to maximum in both cases to test range... go figure! I'm guessing the signal bounces better now, as in the living room it was in a corner made of two reinforced concrete walls.

Linqon
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Post by Linqon » Wed Sep 30, 2009 1:43 pm

Das_Saunamies wrote: With wireless it's damn hard to know what really constitutes "a good position" for receivers and transmitters, at least if you haven't studied the field.
Yea, turned out that was the thing. I had moved my receiver up on a table to give it a line of sight to my kb/mouse. Thought that must be the optimal position, right? Wrong. That was apparently not an improvement. When I moved it back to its original position, which means there is a wooden table between receiver and kb/mouse, it works just as good as before.

Weird. Anyway, that rules out any shielding issues on my part.

Das_Saunamies
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Post by Das_Saunamies » Wed Sep 30, 2009 4:31 pm

Cool, lucky it was that easy to solve. :)

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Post by Olle P » Tue Oct 06, 2009 1:19 am

Mr Evil wrote:... you should always allow for the possibility of neutral and live being reversed (a common wiring error).
Since the standard European connector is symmetric there's no "reverse". It's perfectly fine to have either pin connected to neutral/live.
Mr Evil wrote:
Olle P wrote:...Another possible single fault is to have the line connected to the case, ...
No, that's the bad one. ... if the case is not earthed then touching the case would be dangerous because it's now earthed through you.
How could it be earthed through me when I'm isolated from earth?
If I'm not isolated from earth then that's by definition a second fault, since the entire room (with the exception of the power outlets' neutral points) is supposed to be electrically isolated from earth.

Cheers
Olle

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Post by lm » Tue Oct 06, 2009 5:09 am

Olle P wrote:Yes, neutral is, and should be, connected to earth... but I fail to see how that makes me connected to earth if I'm not connected to the neutral line.
It's me (and just about everything else in the room) that's "floating", not the power supply. Touching a live wire will then be painful, but not inherently dangerous.
Simple answer: by your feet.

You know these voltage-testing pens, that have a small lamp inside them, and then you push one end to a hole in the wall socket and push the other end of the pen with your thumb. If the lamp lights up, then this hole of the wall socket has the live wire.

So why this works? Well, a small current flows through the lamp, through you, through your feet to the floor, and from there to ground by some route.

If you just push the pen to the hole, but don't push the metal with your finger, the lamp will not light up.

If you jump, the light will also turn off.

Even this should make it very clear: How could you get a painful zap from touching just ONE wire, if there were NO path to ground at all?

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