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I have a 12V DC-powered SBC to mount inside a metal case. How important is it that I connect its ground(?) to the metal case on the interior? How is this going to protect against ESD events?

From my understanding, whenever I touch the metal case, will I not be "exchanging" (thus equalizing) charges? Wouldn't connecting the case to the ground of the PCB inside allow the flow of this charge within the PCB? Also, would that not expose me to any kind of danger from the current flowing within the board, as the typical 12V AC/DC adapter doesn't have a connection to the mains ground, from what I see.

I am a bit confused because I do not know how important it is to actually do the grounding, plus I don't really know how exactly to do it. Do I just find the ground pins and connect it through some wiring to some random point on the case?

(This looks like the case, with the 4 screws at the corners, which is where I will mount the board that looks like this)

ForeverNoob
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Since your power adapter has no ground/earth connection, there really is no ground connection at all.

Any "ground" of your circuit is just a local ground, it has no relation to mains earth/ground as there is no connection.

You do not have to connect the metal case to your circuit. Note however that if you do connect it to your circuit's (local) ground you will be relying on the mains isolation of the adapter to prevent your metal box from becoming mains live. If for some reason that mains isolation is compromised (for example a fault in the adapter) then your metal box becomes dangerous to touch.

Often it is preferred to use a mains adapter with a ground connection (3 pins at the mains side) and a grounded output meaning the - pole of the DC output is connected to mains earth. Then as soon as you connect the power supply, your metal box is grounded. This is what is used in nearly all professional measurement equipment: metal box which is grounded directly to mains earth.

However, looking at the metal box, it appears that it has a black coating which could provide some protection against direct human contact with the metal.

I think in your case (this mini-PC you're building) you do not need to worry about the grounding, it is quite common to connect the case to the PCB's ground and power that from an adapter without a ground.

Regarding ESD: ESD has little relation to grounding and really you should not worry about ESD. The input and outputs of that (single board computer) PCB will have sufficient ESD protection. As long as you're not deliberately ESD-charging yourself and then discharging yourself through for example USB socket data pin using a metal pin or wire, then you do not have to worry about ESD.

So: just build your project as everyone else would: mount PCB in metal case, power with the adapter you have, stop worrying about ESD.

Bimpelrekkie
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  • Thank you very much for the information. I am still wondering, what are the drawbacks of not connecting the PCB's ground to the case (because this is the part that I don't know how exactly to do)? My understanding is that, then, the PCB might accumulate static charge and if I want to remove it from the case in the future, I will not be able to bring myself to its charge level safely (is there any way to do such a thing, perhaps touch its ground for example?). – ForeverNoob Apr 05 '18 at 22:07
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    It will be difficult to properly isolate the PCB ground from the case as there are also some connectors touching or nearly touching the case. You should **stop worrying** about static charge as all the sensitive parts are on the PCB and they have sufficient protection at the exposed connectors for them not to be damaged by ESD. If it were possible to isolate the case from the PCB ground that would also not pose any problems as again all possible discharges will happen at the PCB's ports which are designed to handle ESD discharges. – Bimpelrekkie Apr 06 '18 at 06:52
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    About the only time when you should worry about ESD is when you place the CPU and RAM modules onto the PCB. In that case: just touch any grounded point on the PCB, all the ports have grounded shields so touch one of those: **Done**. Now immediately place CPU / RAM etc. That's it. – Bimpelrekkie Apr 06 '18 at 06:57
  • I will be installing an m.2 ssd disk and a fan, but, actually, I will be wearing a mains-grounded anti-static wrist strap and have everything (screws, metal case, PCB, SSD and screwdriver) on a mains-grounded anti-static surface mat. I think this would be enough protection for that. One last question would be, don't mounting holes usually provide such connection to the board ground on single-board computers? – ForeverNoob Apr 06 '18 at 10:04
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    Your ESD precautions are about as ESD safe as you can get. Yes, the mounting holes indeed provide the ground connections. It is the same in standard desktop PCs. – Bimpelrekkie Apr 06 '18 at 15:24
  • Thank you very much! I have been overly worried because of, among other things, the relatively high cost of this SBC, otherwise, I would simply go on and mount it. Besides, there also an [acrylic enclosure](https://ksr-ugc.imgix.net/assets/005/706/979/93fe891b857c37dceb39848e47fef4e4_original.png?w=680&fit=max&v=1460541262&auto=format&lossless=true&s=59b4c6c8f2e5278e9d429ef07a63b1f5) and I was ready to use that, but I don't really know if it's safe for frequent transportation of that thing. – ForeverNoob Apr 06 '18 at 19:04
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    I would not choose that acrylic housing as it is an "open" case and doesn't provide full protection. Even if you left it in one place (no moving around) it will collect dust etc. Especially since you want to move this thing around, I'd get the metal case for sure. – Bimpelrekkie Apr 07 '18 at 11:50
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You can reach in and touch the GND pins/fill on the PCB with a finger, at any time, safely.

IMHO

Once you have a sustained contact with GND, then you may perform whatever operations you need to do, safely.

IMHO

analogsystemsrf
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    I agree until some kids forget. In 1975 before EOS/ESD training , we would just touch fingers , zap then transfer a card to someone while holding the ground pads But then I even had mysterious TTL failures. Then when I implemented plant wide training and ESD protection in the factory, I showed how to generate 200V 100pF just by raising one leg on a carpet. after zeroing to gnd. Then in 1980 with our Home network of the future (Project Ida), touching the TV caused ESD to go into basement computer card and hot CMOS latchup. so that had to be fixed. – Tony Stewart EE75 Apr 06 '18 at 20:07
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Having done about a dozen EMC compliance tests on various products and designed many systems myself, my confidence that this sytem would pass all the system tests with your choice of unearth bonded 12V supply is a very high risk of product failure to conform to all the EMC tests.

This is not JUST for ESD diversion, but also for unintentional radiation and susceptibility, ie. for the sake of good EMC performance

If you have plans to ship a lot of these, you better have a plan B when they fail.

schematic

simulate this circuit – Schematic created using CircuitLab

One must consider all interfaces, both human and cables to determine how to prevent unwanted ingress (EMI going in) and egress (EMI going out. Susceptibility tests include 5k~15kV 100pF ESD, radiated ESD, pulsed RF on plane radiator or inside Faraday room, conducted CM noise from power lines, radiated CM noise ingress to high Z with interface cables seeing strong interference.

Both radiated and conducted {ingress,egress} tests are often Corp Stds and IEC, FCC and other international standards.

There is no universal solution but if you follow the best practice used by PC's it will be bonded close to chassis and incming earth bond.

It can still be affected by ESD if IO cables do not have CM chokes and feedthru shunt caps. Exposed surfaces can also still radiate RF.

There is no general solution for EMC design, just many principles to choose to shunting disturbances and unwanted intrusive microwave ESD transients and local radiated E fields or high current H field transients.

Get the Henry Ott EMC Book from your internet library or buy it.

NOTE: The power supply you choose does not have a large ferrite CM choke, often used but may be SMT on PCB inside, or not. ( just like every VGA cable has)

Tony Stewart EE75
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  • I did not "choose" the unearthed 12V supply. I am not even sure if it is not grounded. I just saw that the plugs of all these 12V AC/DC adapters don't have a proper contact for the ground hole in the mains socket, so I assume they are not grounded to mains earth. Is this how it is supposed to be, or did I miss something? Are 12V AC/DC adaptors earthed? – ForeverNoob Apr 06 '18 at 10:09
  • I am 100% sure. Portable electronics are never earth bonded for ground fault safety risks, but may have earth bonded Y filters in the brick which is isolated from the DC even if the AC has 3 prongs. This unit should be designed like a PC for EMI, and not just because it is one. If you don't use a earth bonded DC , some will fail. period. This isn't my 1st rodeo and there are so many laptops with nasty 100k~1MHz leakage inspite of factory test ok., yet may be ok for ESD but radiate on the interface cables and some have noisy headset mics. take heed. – Tony Stewart EE75 Apr 06 '18 at 12:03
  • So, all I can do from that point on (having the board) is try to ascertain whether the AC/DC adaptor I use is grounded, right? How can I know if an AC/DC adaptor for power supply is grounded? Do I have to tear it open? – ForeverNoob Apr 06 '18 at 18:19
  • No but you have to buy a DMM and measure earth ground pin on the 3 pin AC plug to the DC jack. None of these types of Wall-warts will have that. Now go shopping ..Bangood, Amazon... – Tony Stewart EE75 Apr 06 '18 at 18:57
  • This is my problem. The plug for these AC/DC 12V adapters always has [two pins](http://static.webshopapp.com/shops/081702/files/043111340/power-supply-12v-25a-converter-ac-dc-adapter-for-a.jpg). I don't know if this thing can still provide connection to ground. – ForeverNoob Apr 06 '18 at 19:09
  • Well we're told not to help with shopping questions. Do you have all the power specs? – Tony Stewart EE75 Apr 06 '18 at 19:20
  • The specific SBC uses 12V DC at 3A. I am not asking about which adaptor to buy, I am only asking if such adaptors with two pins can also ground to mains earth or are unable to do so by design. – ForeverNoob Apr 06 '18 at 19:40
  • THey can but are not in general as I said due to risk of open ground faults and mobile users getting a shock.. But like PC's they are earth grounded and not mobile. – Tony Stewart EE75 Apr 06 '18 at 19:53
  • How does this look, it has earth gnd avail, but you can use a 1nF cap for some RF suppression between 0V and Earth gnd and still be ground fault at 60Hz..err 50Hz protected with 100K bleeder R. https://www.amazon.ca/Switching-Monitoring-Industrial-Transformer-220VAC-DC12V/dp/B0781VTH33/ref=sr_1_26?s=electronics&ie=UTF8&qid=1523044469&sr=1-26&keywords=12V+4A (see answer again) THis one needs a cordset with pigtail – Tony Stewart EE75 Apr 06 '18 at 19:56