Electrostatic discharge with the case closed?

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the_servicer

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Most discussions of electrostatic discharge are about opening a case and handling internal components. But what about when a computer is just sitting there with the case closed? Are there are any common hazards? I'm wondering about possible precautions during handling, transporting, storing, and normal operation with the case closed.
 
I've heard of people killing usb ports from large static discharges, but generally if they are grounded properly in the case (usually there is a ground wire soldered to the ports on cases with plastic panels, rear i/o shield has tabs which should thouch the outside case of the ports, and the mb and psu should be directly grounded to the case by screws, the psu connected to a properly grounded outlet) there should be no issues.

In a very dry environment, maybe touch something metal on the case (psu, fan guard/grill, etc) to drain whatever charge you have before plugging something in. There may still be a charge on the device you are plugging, but it should be much smaller, and case ground should handle it fine.
 
Each and every time i touch my case i get a electrostatic discharge due to my seat fabric. It's a faraday cage so who cares? I do refrain from french kissing my ports directly though.
 
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When indoor humidity is very low (<25%), it's possible to build up over 50kV! This can arc 1/2" and if not discharged safely most definitely will wreak havoc with sensitive parts. Pulling new chassis' out of boxes packed in foam is one of the worst offenders! The shock can be painful. Fortunately the chassis keeps this out of the circuits.

Peripherals can channel enough energy to cause temporary effects (i.e. system freeze or spontaneously reboot). If there is any questionable ground (i.e. floating above chassis potential) arcs will basically put components at the same potential of breakdown voltage which is usually much higher than needed to do irreversible damage. Essentially any discharge sufficient to be felt or heard (snapping) is going to be destructive if it takes a path through sensitive devices.

The key to reducing ESD is ensuring you're always at the same potential as the part you're working on. NEVER use a wire connected to a wrist strap! Always use an approved lead with resistor. This is because of possible ground faults (hot chassis) and a direct wire will carry potential lethal currents through your body if that were to happen. A 1 megohm resistor will work fine in this case.
 
cpufrost said:
When indoor humidity is very low (<25%), it's possible to build up over 50kV! This can arc 1/2" and if not discharged safely most definitely will wreak havoc with sensitive parts. Pulling new chassis' out of boxes packed in foam is one of the worst offenders! The shock can be painful. Fortunately the chassis keeps this out of the circuits.

Peripherals can channel enough energy to cause temporary effects (i.e. system freeze or spontaneously reboot). If there is any questionable ground (i.e. floating above chassis potential) arcs will basically put components at the same potential of breakdown voltage which is usually much higher than needed to do irreversible damage. Essentially any discharge sufficient to be felt or heard (snapping) is going to be destructive if it takes a path through sensitive devices.

The key to reducing ESD is ensuring you're always at the same potential as the part you're working on. NEVER use a wire connected to a wrist strap! Always use an approved lead with resistor. This is because of possible ground faults (hot chassis) and a direct wire will carry potential lethal currents through your body if that were to happen Check best teams. A 1 megohm resistor will work fine in this case.
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I always thought that it was a good idea to touch the case to discharge any static electricity. However, last night when I was working on my computer when I touched the case, I got a static shock then the computer shut off. And when I turned it back on the power LED no longer worked. So now I'm not so sure that is a good idea. I know you can get those wrist strap things but from the instructions I've read they say to connect them to the computer case, so I don't see how it would help.
 
The real ones have a 1MOhm resistor so they release the charge slowly, at a much lower voltage. The fake/cheap ones might not (among other things, like having cheap wire or no connection at all).

Touching your case is risky, there is no resistor in the path like in such a strap, so depending on the path it takes it could cause damage. There's not much alternative though, unless your desk or desk mat is grounded -- you could touch that instead.
 
ESD or Lightning, both act the same way and are therefore somewhat predictable as in: both choose the path of least resistance.

It is safe to use a case if the closet ground/ least resisting path is the psu case-screw and not a button, port or the board itself.

If you live in a cold area a house radiator is obviously even better.
Those are grounded a.f..
 
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jameswilliam said:
I always thought that it was a good idea to touch the case to discharge any static electricity. However, last night when I was working on my computer when I touched the case, I got a static shock then the computer shut off. And when I turned it back on the power LED no longer worked. So now I'm not so sure that is a good idea. I know you can get those wrist strap things but from the instructions I've read they say to connect them to the computer case, so I don't see how it would help.
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Sounds like a floating ground scenario where your case was not earthed. If it's plugged in (obviously it was as you said it was running), this should not happen as the PSU screws will ensure that part is indeed earthed as long as your power supply is connected to a properly grounded outlet. If the front cover isn't bonded to the chassis, this could cause issues as well. A static shock strong enough to feel and make a snapping noise is thousands of volts and will jump over small gaps to reach a conductor that's earthed or capacitively coupled. In the latter, if that means taking a path where residual currents travel through sensitive semiconductors there will be incidental damages. CMOS devices are quite ESD vulnerable!

The purpose of the resistor in a grounding strap is for (human) safety rather than preventing ESD. Imagine working on something where you're bonded directly to a good earth ground (ground wire or water pipe) and you touch something in a case that has full 120V potential! The risk of lethal shock is very high. With a million Ω resistor in series the current is reduced to very harmless levels. At the high potential ESD static can reach in low humidities, the resistor will still keep you well drained at low potential. Even though a computer chassis internals is much different than a radio or other appliance, a wiring issue (eg. ground fault) can result in something carrying line voltage you'd never know was there. That is where the resistor would potentially save your life.
 
cpufrost said:
Sounds like a floating ground scenario where your case was not earthed. If it's plugged in (obviously it was as you said it was running), this should not happen as the PSU screws will ensure that part is indeed earthed as long as your power supply is connected to a properly grounded outlet. If the front cover isn't bonded to the chassis, this could cause issues as well. A static shock strong enough to feel and make a snapping noise is thousands of volts and will jump over small gaps to reach a conductor that's earthed or capacitively coupled. In the latter, if that means taking a path where residual currents travel through sensitive semiconductors there will be incidental damages. CMOS devices are quite ESD vulnerable!

The purpose of the resistor in a grounding strap is for (human) safety rather than preventing ESD. Imagine working on something where you're bonded directly to a good earth ground (ground wire or water pipe) and you touch something in a case that has full 120V potential! The risk of lethal shock is very high. With a million Ω resistor in series the current is reduced to very harmless levels. At the high potential ESD static can reach in low humidities, the resistor will still keep you well drained at low potential. Even though a computer chassis internals is much different than a radio or other appliance, a wiring issue (eg. ground fault) can result in something carrying line voltage you'd never know was there. That is where the resistor would potentially save your life.
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That resistor wont do much for 120V AC. The kind that would fit in a wrist strap might be able to handle 1 watt of power, but an AC line (with no loads in series) is capable of multiple amps, over a kW of power. All the strap does is provide an easier path THROUGH YOU to ground. DO NOT wear a strap when working on powered electronics.

A faraday glove/suit is different, it provides a path AROUND you. You still do not want to touch powered electronics while wearing one, however. It is meant to be an additional layer of protection in case you accidentally touch a powered circuit.
 
Nobu said:
That resistor wont do much for 120V AC. The kind that would fit in a wrist strap might be able to handle 1 watt of power, but an AC line (with no loads in series) is capable of multiple amps, over a kW of power. All the strap does is provide an easier path THROUGH YOU to ground. DO NOT wear a strap when working on powered electronics.

A faraday glove/suit is different, it provides a path AROUND you. You still do not want to touch powered electronics while wearing one, however. It is meant to be an additional layer of protection in case you accidentally touch a powered circuit.
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The resistor is around a million ohms (1 meg OHM) limiting the power to around 15 milliwatts! (120V example). That's just over a tenth of a milliamp which is less than most people's threshold to feel it. To get 1W, the resistor would be around 15,000 ohms and the current would be approaching 10mA which is quite noticeable, even painful to some. The resistor is there for safety and will absolutely keep static potentials well below damaging thresholds. Yes one should not work on something "hot" but it's common to leave the power cord plugged in to maintain proper earth ground of the chassis while the power switch remains in the off position. A wiring fault or floating/lifted ground, for example, can cause a hazardous condition where the strap would cause a current flow back through your body into what you're touching. Without the resistor, this is dangerous.

It's always a good idea to use a multimeter to verify grounds are indeed at zero potential and never have both hands inside a chassis touching two areas that have potential between them.

PPE based on Faraday principle are typically used by HV lineman working on energized transmission lines to bring equipment up to working potential and prevent burns from arcing from initial contact.

This can be seen in this old video here:


View: https://www.youtube.com/watch?v=Z3q9WdjD5wc&t=69s
 
True, but AC is a funny beast, and it doesn't always go the way you expect it to. You might think it'll travel through the wire, when instead it decides ground is a better path. The wire provides a connection, but the inductance and capacitance and your mutual connection to ground becomes the hazard instead. I'm referring to ground in general here, not just chassis ground where the strap is connected.

Sorry, I always had a poor grasp of ohm's law. But my training told me explicitly not to use a ground when working on powered equipment, and I know it was for good reason.

Edit: Rather, I understand the law well, but I often forget it when it's important.
 
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