So, if you touch the back of the case...

[AmberClad]

...where does the charge go?

I've just been doing some thinking about that tip we've all heard, about grounding yourself by touching the back of the case.

1) Why doesn't touching the case under normal circumstances transfer the charge through the metal IO shield, onto the motherboard? (The type of IO shield that has a tab that touches the board rear IO connectors.)

2) What if the case feet are rubber, and the PSU is mounted using rubber grommets? Is there no grounding in this case?

3) What is the ideal situation involving the touching the case idea? Like, how is it intended to work? How does the charge travel, etc.

 

[JustaGeek]

The charge will flow through the ground wire of the power cord.

 

[Navid]

1) It does. That is why you do it!

You do not touch the case to ground yourself.
You touch the case to bring your body's potential to the same potential as the motherboard.
The case is connected to the ground plane (large plane of metal on the printed circuit board) of the motherboard.



2) Grounding does not matter! Even if you have pulled the plug, you still touch the case.
What matters is to avoid touching a sensitive component if your body is charged with respect to it.
If you are at the same potential, even if at 10,000kV with respect to ground, touching it will not damage it if it also is at 10,000kV since there will be no current (voltage is relative).



3) Let's say you are charged. When you touch the case, some of the charge is transfered to the case. This happens until your body and the case (motherboard) are at the same potential. This could be the ground potential if the case is grounded. Or, it could be a different potential if the case is not grounded.
Charge transfer is current. So, this allows a current to flow.
ESD damage also occurs as a result of current flow! But, only when the current flows through a sensitive component that cannot handle the current flow.

So, if you are charged and you touch a transistor on the motherboard, the current will still flow to the motherboard to bring your body and the motherboard to the same potential. But, the flow of current through the transistor can damage it (ESD damage).

To avoid that, you touch the ground plane directly first (by touching the case or the PSU). A discharge current flowing to the motherboard ground plane cannot damage anything as long as it does not go through any sensitive components.

 

[Zepper]

Yes, but how does the case discharge - thru a true earth ground which is the only place static will go. So, yes having your case grounded thru the power cord is the best. Always check you outlet for good ground. You can get cheap outlet testers for under $5. and I've seen them for under $2. on sale. Check all the outlets around the house - I often find them miswired.

100% agree with justageek.

If you have a static problem in your house, you should get one of those grounding pads to touch - those are grounded thru a resistor so you won't get zapped when you touch it like you will touching a grounded case.

.bh.

 

[Navid]

There is no reason to have the PC plugged in when you work on it.

Do you think the engineers on the space station are grounded when they work on electronic equipment?

 

[Blain]

They have a very long, thin wire going back down to the earth.

 

[Navid]

Originally posted by: Blain
They have a very long, thin wire going back down to the earth.

:laugh:
:thumbsup:

In reality, they probably (I am not an astronaut) connect themselves with a wrist strap to the equipment they are working on. So, they are at the same potential.
But, obviously, they could be at any potential with respect to Earth.
Everything goes back to the fact that voltage is relative.

 

[JustaGeek]

http://209.85.165.104/search?q...=en&ct=clnk&cd=9&gl=us



Touching the Chassis

Computers and peripheral systems may contain a number of static-sensitive devices. Before touching any components inside the system, touch an exposed part of the chassis or the power-supply housing with your finger. Grounding yourself in this manner ensures that any static charge present on your body is removed. You should be aware that this technique will only work safely if the power cord is attached to a grounded power outlet. The ground plug on a standard power cable is the best tool for overcoming ESD problems.

 

[Navid]

I don't consider that link credible. I am sorry.

The only reason for the ground connection in an outlet is to protect humans from being electrocuted if the equipment has a faulty wiring so that the body of the equipment is wired hot.

It is correct to say that if the PC is grounded, you will also be grounded by touching its case.

It is not correct to assume that the only way to avoid ESD damage is to be grounded.

 

[Navid]

Do you know that the outlets in Japan have no ground connection?

Do you think Japan has no understanding of ESD?

 

[JustaGeek]

Originally posted by: Navid
I don't consider that link credible. I am sorry.

The only reason for the ground connection in an outlet is to protect humans from being electrocuted if the equipment has a faulty wiring so that the body of the equipment is wired hot.

It is correct to say that if the PC is grounded, you will also be grounded by touching its case.

It is not correct to assume that the only way to avoid ESD damage is to be grounded.

I don't consider you credible. I am sorry.

Please learn that you don't always have to be right.

No one does...

 

[Zepper]

The only way to actually drain off static energy is thru a true earth ground. I'd have to assume that the neutral wire in Japanese outlets is also a true earth ground then. Neutral in our homes is also supposed to be bonded to ground at the entrance box but it is not assumed to be true earth ground in electronic equipment here. If equipment doesn't have a power switch, I use an extension cord and just hook up the ground pin to make sure the chassis of any equipment I'm working on is true earth.

.bh.

 

[mindless1]

Touching the back of the case DOES discharge onto the motherboard, but the key thing is difference in impedance. There is a primary ground path through the case and several PSU ground wires instead of the trivial amount flowing through secondary paths which are at a safe level.

Touching the case will certainly prevent some ESD damage scenarios but it is not enough to prevent all. The purpose of the case being grounded is that you are not only at the same potential as the case, but also at a common, reference potential so anything else you touch is again referenced/differenced against that ground -0- volt value. This too is useful in some scenarios but not all, or rather, merely being "referenced" against ground or case through a part is ALSO what causes that damaging ESD event too, because you are not the only object that being referenced against the case ground and/or earth ground.

Suppose you have two cases side by side both unplugged, and a part lying out. If you are at same potential as one case by touching it, does it necessarily mean you are at the same potential as the part or the 2nd case? No. Can you hold one case frame and pick up the part on the table safely to install in that system? No (but "maybe", if it were in a properly designed ESD safe bag still and you handled the bag first while touching the case frame to provide this drain path). You don't know that they are at the same potential, or rather should not assume they are unless you're a gambler or in an environment known to have very low potential for discharges (it never happens and you accept risk vs value of parts).

You want a ground strap with a high resistance, so that whatever you touch, the difference in potential is drained down to earth or chassis potential at a low current so all are equalized. You could clip the ground lead from the ESD strap to the case then let go of the case with your hand and pick up the part so it slowly drained to case potential, or you could leave the case grounded and have your ESD strap grounded also whether it be to case or directly to earth ground. Provide that slow discharge path when possible, that's why a strap isn't just a piece of wire.

 

[Navid]

Originally posted by: JustaGeek

Originally posted by: Navid
I don't consider that link credible. I am sorry.

The only reason for the ground connection in an outlet is to protect humans from being electrocuted if the equipment has a faulty wiring so that the body of the equipment is wired hot.

It is correct to say that if the PC is grounded, you will also be grounded by touching its case.

It is not correct to assume that the only way to avoid ESD damage is to be grounded.

I don't consider you credible. I am sorry.

Please learn that you don't always have to be right.

No one does...

I never said I am always right.
We are just having a conversation. We state our opinions. That is all.
That link is also someone's opinion. It could be right. It could be wrong.

 

[Navid]

Originally posted by: mindless1
Touching the back of the case DOES discharge onto the motherboard, but the key thing is difference in impedance. There is a primary ground path through the case and several PSU ground wires instead of the trivial amount flowing through secondary paths which are at a safe level.

Touching the case will certainly prevent some ESD damage scenarios but it is not enough to prevent all. The purpose of the case being grounded is that you are not only at the same potential as the case, but also at a common, reference potential so anything else you touch is again referenced/differenced against that ground -0- volt value. This too is useful in some scenarios but not all, or rather, merely being "referenced" against ground or case through a part is ALSO what causes that damaging ESD event too, because you are not the only object that being referenced against the case ground and/or earth ground.

Suppose you have two cases side by side both unplugged, and a part lying out. If you are at same potential as one case by touching it, does it necessarily mean you are at the same potential as the part or the 2nd case? No. Can you hold one case frame and pick up the part on the table safely to install in that system? No (but "maybe", if it were in a properly designed ESD safe bag still and you handled the bag first while touching the case frame to provide this drain path). You don't know that they are at the same potential, or rather should not assume they are unless you're a gambler or in an environment known to have very low potential for discharges (it never happens and you accept risk vs value of parts).

You want a ground strap with a high resistance, so that whatever you touch, the difference in potential is drained down to earth or chassis potential at a low current so all are equalized. You could clip the ground lead from the ESD strap to the case then let go of the case with your hand and pick up the part so it slowly drained to case potential, or you could leave the case grounded and have your ESD strap grounded also whether it be to case or directly to earth ground. Provide that slow discharge path when possible, that's why a strap isn't just a piece of wire.

Everything you have said here makes absolute sense.

I touch the case to share any possible charge on my body with the case.
If I want to pick up a graphics card now from the table, which as you said could be at a different potential, I pick it up by only touching its ground plane (its metallic frame if it has one, or its bracket). This way, the card also moves to the same potential safely.
I never touch a card by holding the components on the board.

I always pick up a motherboard holding on to the edges only (ground plane) for the same reason.


Question:
If the only acceptable way to work on ESD-sensitive devices is to be grounded, how does an astronaut do it while away from ground?

 

[Modelworks]

You can discharge static electricty by touching any metal that carries an opposing charge.

Drag your feet on carpet then touch a metal door knob, get shocked ?
Yep, that door knob has NO path to ground, when mounted in a wooden door, yet the static still discharges.

Ever get shocked on a car door when you exit the car ?
That car is insulated from ground as well by the rubber tires and the sole of your shoes unless you are standing in water.
Yet the static still discharges.

Unless the metal your touching is acting as a capacitor, its almost always going to have a negative charge.

 

[Navid]

Originally posted by: Zepper
The only way to actually drain off static energy is thru a true earth ground.

If your body is charged to 10,000kV, and you ground yourself, you will drain off the energy with respect to Ground.

But, you will actually be charging yourself, with negative energy, with respect to a device that is at 10,000kV itself.

In that case, if you touch that device, you could damage it even though you are perfectly grounded. You would not have damaged it if you did not ground yourself.



I am not saying that being grounded is bad!
All I am saying is not to let "being grounded" give you a false sense of immunity against ESD.

 

[seemingly random]

Originally posted by: AmberClad
...where does the charge go?

I think you're just a troublemaker - always asking provocative questions...

But really, did you get a definitive answer? I didn't. I'll probably just continue on ignoring all the ESD warnings - haven't had anything go bad in 10 years. I usually try to not touch any parts right after I've pet the cat though. Now 20 years ago was a different story.

 

[Zepper]

Well you want a path for the static energy to prefer over other things - better a true earth ground over reverse charging your capacitors, etc. Almost all electronic components within the PC (capacitors, semiconductors, etc.) are connected on their negative poles to ground. I'd much rather any static charge I place on my case proceed out the ground wire rather than doing something else.

.bh.

 

[seemingly random]

How about the situation where a pc is being built outside of a case? I take the mb out of the bag without trying to touch any contacts and place it on a piece of cardboard. The same for the cpu, ram, etc. What do you ground yourself with/to?

 

[AmberClad]

Originally posted by: seemingly random
I think you're just a troublemaker - always asking provocative questions...

Are you referring to the NewEgg thread ? That thread, just like this one, started innocently enough, and I'd like to point out that I'm not really responsible for any flaming and arguing that ensues as a result :laugh:.

*hides the box of popcorn*

 

[seemingly random]

Originally posted by: AmberClad

Originally posted by: seemingly random
I think you're just a troublemaker - always asking provocative questions...

Are you referring to the NewEgg thread ? That thread, just like this one, started innocently enough, and I'd like to point out that I'm not really responsible for any flaming and arguing that ensues as a result :laugh:.

*hides the box of popcorn*

Sure...

I bet you've got several more up your sleeve.

"the PSU is mounted using rubber grommets"? Never heard of this one.

 

[AmberClad]

Originally posted by: seemingly random
"the PSU is mounted using rubber grommets"? Never heard of this one.

Something like this.

 

[Navid]

Originally posted by: seemingly random
How about the situation where a pc is being built outside of a case? I take the mb out of the bag without trying to touch any contacts and place it on a piece of cardboard. The same for the cpu, ram, etc. What do you ground yourself with/to?

Think of it as an imaginary stadium field that can move vertically. I know this sounds crazy. But, it helps to explain.

The height of the stadium, with respect to ground, is the voltage of the motherboard ground plane.
The height of each player is the voltage of the RAM or the graphics card or the CPU ...... with respect to ground.
The relative height of each player with respect to the height of the field is the voltage of each component with respect to the motherboard ground plane.

When you install the motherboard and attach the PSU cable and then plug in the PSU, it is like you bring the stadium down to ground. All the players are standing on the stadium so they move with it safely.

When you touch your RAM on the motherboard without first touching the case, and if you are charged, is like picking up one of the players and lifting him up in the air without lifting the stadium and letting him go! That is fatal!

Now, let's say you are working on a PC that is not plugged in. That is like the stadium is in the air with all the players playing on it.
If you touch the case before working on the PC is like you move up to the same height as the stadium before working with the players. That is safe.

But, if you ground yourself and work on this (ungrounded) PC and touch a component on it, is like you push one of those players from the stadium (lifted up in the air) and let him fall to the ground (fatal).

Now, let's say you ground yourself and the PC but you pick up a RAM module from its ESD bag and the RAM happens to be charged, and you install this RAM into the grounded PC.
That is like the stadium and all the players are on ground. But, all of a sudden a player arrives with a chopper and jumps out of the chopper (still in the air) and falls to the stadium level! Fatal again!


Always touch the ground plane of the motherboard first (this is accomplished by touching the case). Analogy: Always move to the same level as the stadium first.
Always touch the ground plane of any component first before installing it. Always hold components by holding their frame. Do not touch the leads or tracks. Analogy: Always make sure that the copper lands on the stadium ground before the player gets out.

 

[seemingly random]

Navid: I don't get "ground plane". Is this a physical thing - one of the layers of the mb or component on the mb - or a concept? Sorry, not an EE here.

Take my example and simplify it even more - no pc case or psu - just the taking the mb out of the bag and placing it on a piece of cardboard. How does one get grounded?