Does a 1MOhm Resistor Provide any ESD Protection?

Thread Starter

intoxicated_pilot

Joined Feb 11, 2021
5
I have very limited knowledge on electricity, so please bear with me on this one....

So I understand that a 1MOhm resistor is included in ESD wrist straps as a standard safety measure.

My question is, does this 1MOhm resistor serve any additional purpose in terms of ESD protection? I've heard an equal amount of people claim that is does, and that it doesn't. The theory I've heard is that the 1MOhm resistor slows the static discharge enough to prevent damage to ICs, and other sensitive components.

For my application, I will be using two ESD mats (two layer), each connected to ground without a 1MOhm resistor. I will also be connected to ground via a wrist strap, with a 1MOhm resistor.

Per this document from Desco;
"ANSI/ESD S6.1 recommends that a non-resistor ground cord be used to ground worksurfaces and floor mats. However, cord may have a 1 megohm resistor for non ESD puropses. Selection of the ground cord is determined by user needs and specifications."

So my understanding is that the 1MOhm resistor is not required for the mats, simply because the top rubber layer provides more than enough resistance to slow the current?

Additionally, I'm wondering if connecting the chassis or the PC I'm building to ground via a 1MOhm resistor would provide additional protection (rather than grounding via PSU connected straight to mains). Say for example, the motherboard I'm installing happens to have a static charge built up, it will likely discharge through the case to ground. Assuming the 1MOhm resistor would slow the discharge, could this provide some additional protection?

Yes, I know this is a bit excessive in terms of ESD protection, but we all know that ESD can cause premature, or intermittent failures that are difficult to troubleshoot to say the least, and if I can do everything I can to prevent this on every system that I build (not cheap $$$), I'd rather take the additional steps. I don't intend to start the decade old debate regarding the actual risks involved with ESD, but rather want to answer the question for my own understanding on the subject.

Thanks!
 

MrChips

Joined Oct 2, 2009
23,520
Providing ESD protection is essential. It is not being excessive.
1MΩ is a typical series resistance and it works.
This resistor is not about the effectiveness of ESD grounding. It is there for protecting personnel from being electrocuted in the event that the grounding strap inadvertently comes into contact with high live voltage.

Grounding of electrical equipment to EARTH is required unless the equipment has a double layer of insulation from AC mains.
If you are building your own equipment a direct connection between the chassis and EARTH connection is required (i.e. no resistors).
In other words, in North America, AC mains connection has three wires. The EARTH connection is the GREEN cable that is connected to the round pin on a standard 3-pin plug. The green wire must be bonded to the chassis of the PSU. Your computer must also have a connection from the computer chassis to the chassis of the PSU. The mother board COMMON GROUND should be connected to this same EARTH ground.

North America
BLACK - LINE VOLTAGE
WHITE - NEUTRAL
GREEN - EARTH

1613097817841.png
 

crutschow

Joined Mar 14, 2008
27,198
Yes, the 1 megohm resistor is just to prevent electrocution of the person wearing the wrist-strap.
It has nothing to do with ESD protection.
 

Thread Starter

intoxicated_pilot

Joined Feb 11, 2021
5
Yes, the 1 megohm resistor is just to prevent electrocution of the person wearing the wrist-strap.
It has nothing to do with ESD protection.
Providing ESD protection is essential. It is not being excessive.
1MΩ is a typical series resistance and it works.
This resistor is not about the effectiveness of ESD grounding. It is there for protecting personnel from being electrocuted in the event that the grounding strap inadvertently comes into contact with high live voltage.

Grounding of electrical equipment to EARTH is required unless the equipment has a double layer of insulation from AC mains.
If you are building your own equipment a direct connection between the chassis and EARTH connection is required (i.e. no resistors).
In other words, in North America, AC mains connection has three wires. The EARTH connection is the GREEN cable that is connected to the round pin on a standard 3-pin plug. The green wire must be bonded to the chassis of the PSU. Your computer must also have a connection from the computer chassis to the chassis of the PSU. The mother board COMMON GROUND should be connected to this same EARTH ground.

North America
BLACK - LINE VOLTAGE
WHITE - NEUTRAL
GREEN - EARTH

View attachment 230175

So I did some more research on the subject, and found a pretty detailed (but rather complicated) answer. I'll copy parts of his answer below.

I guess I'm just trying to get a better explanation behind why this theory is or isn't true. It's hard to pick a side when an equal number of people online are giving conflicting answers. But if you could provide any further explanation on this, it would be much appreciated. I like to understand the "why" side of things whenever possible.

Thanks!

-------------
https://electronics.stackexchange.com/questions/265920/should-there-really-be-1-mΩ-resistance-between-an-anti-static-wrist-strap-and-a

"The reason for the acceptable range of 1M to 10 M is to current limit static discharge for wrist straps. in addition it reduces current to live voltages. "

"It does this by bleeding body charge slowly to the same potential of the case or gnd reference that the 1M resistor is clamped to, while static charge may be generated by motion or change in body capacitance with a fixed charge, V=C/Q ."

"For Electrostatic Over Stress or EOS prevention all surfaces must be "Static Dissipative" to prevent rapid discharge."
------------------
 

MrChips

Joined Oct 2, 2009
23,520
Sorry, I have not read anywhere where people are in disagreement with the purpose of the 1MΩ resistor.

1) For the purpose of ESD protection it does not matter if the resistor is 0Ω or 10MΩ.
The 0Ω reference is made as being similar to a person touching a piece of metal or metal chassis that is grounded. Yes, you will feel a tingle or even observe an electrical discharge spark but that does not occur here.

2) The purpose of the ESD wrist strap is to bring the body of the person wearing the strap to EARTH potential. If the person is wearing and using the ESD wrist strap correctly the body will be at EARTH potential. Any physical activity such as shuffling feet on carpeted floors, combing hair, or changing clothing, etc. may attempt to cause a build up of static electricity. If the body is properly grounded by continually wearing the ESD wrist strap there will be no build up of electrostatic charge. There is no discharge spark to observe because there is no high voltage.

The ESD wrist strap is to prevent build up of electrostatic charge that may cause harm to sensitive electronic components.

3) Comments made that the 1MΩ is to discharge the high voltage slowly via a tiny bleed current is irrelevant.
The 1MΩ resistor has nothing to do with the discharge rate.

The purpose of the 1MΩ resistor is to protect the person from electrocution in the event that the grounding strap comes into contact with a continuous high voltage source (which would likely be 120-240VAC mains). It limits the current to a safe value from voltage sources up to 5kV.

Summary
The purpose of the ESD wrist strap is to prevent build up of electrostatic charge that may cause damage to sensitive electronic components.
The purpose of the 1MΩ series resistor is to prevent electrocution of the wearer of the wrist strap.
 

Tonyr1084

Joined Sep 24, 2015
5,969
for example, the motherboard I'm installing happens to have a static charge built up, it will likely discharge through the case to ground.
This is highly unlikely. See * below.
This resistor is not about the effectiveness of ESD grounding. It is there for protecting personnel from being electrocuted in the event that the grounding strap inadvertently comes into contact with high live voltage.
100% correct. MrChips has many years experience and he is highly knowledgeable on the subject.
prevent rapid discharge.
Energy transfers from the higher potential to the lower potential. How fast that happens is a function of conductivity or resistance. Having your computer frame grounded affords two opportunities to control ESD and EOS: First, it acts as a Faraday Cage. Your car is a Faraday Cage in a lightning storm. If lightning strikes the car it travels around you and to ground with no harmful affects to occupants and electronics within that cage. Same is true of flying airplanes that get struck by lightning. The second thing having the frame grounded offers is protection from accidental electrical issues that could otherwise electrify the frame.

* Static electricity is the result of two close proximity - non-conductive - objects being stripped away from each other. Scotch Tape is a good example as it is in very close proximity to itself. It's non-conductive; and when you pull a length of tape it develops two charges, one positive and one negative. Which gets which charge is dependent on the material, and sorry, that part I don't remember. But I've been dealing with ESD close to 40 years. The effect that causes this is called "Tribe-Electric Effect". I mentioned lightning. Do you know what causes lightning? Ice crystals bumping into one and other. As they bounce away they develop a small electrical charge. Given the size of the storm cloud this adds up into great static voltages. When it's sufficient to jump the gap between cloud to cloud or cloud to ground you get lightning.

Lightning is a good example of a sudden and complete discharge of static electricity. However, if you could build a resistor large enough to conduct that energy to ground it would do so only as long as the pathway to ground is less resistive than the resistance offered by an air gap. Let's bring it down to an electronics laboratory level: When you put your wrist strap on any charge that has built up on your body will be safely conducted to ground because the pathway to ground is lower resistance through the resistor than it is through air. And shuffling around in your work chair, which can generate lots of static is immediately conducted to ground without harmful effects. As has already been mentioned, the resistor is there more so to protect you from accidental electrocution. The items on your static mat don't need that same protection. Even if the mat should become energized it's not likely to be anywhere close to the voltage levels of static electricity. A static discharge of around 3KV is almost completely unnoticeable. You don't notice it unless you're paying extremely close attention to the discharge. Above 3KV is when you start to notice it. 50KV is not uncommon for the human body to store. At the gas station, when you put the nozzle in the tank and start filling it, you go sit in the car to keep warm. Winter time the air is drier and you are more prone to generate high voltages of static energy. When you get out of the car, your butt, in close contact with the seat of the car, you generate a lot of static energy. As you slide off the seat you generate even more. You then reach to touch the nozzle and a spark flies from your hand to the nozzle in a gas vapor rich environment - and voila! You have a gas station fire at your gas tank. Touching the car body before you touch the nozzle discharges that static so that you and the car are at the same potential. Remember, energy transfers from the higher level to the lower level. You can be at 50KV and the car at 20KV. The difference would be a 30KV spark. So your car doesn't have to be grounded, just at a different potential than you.

The subject of ESD goes a long way back. And many people believe they understand it correctly. However, I've encountered people who would shout at me "I'VE BEEN DOING THIS FOR 20 YEARS!" not knowing that I've been at it for 39 years. Part of my employment function was to do ESD audits on wrist straps, people testing their straps on a daily basis, heel straps and testing, conductive flooring as well as conductive work surfaces. Proper grounding and proper cleaning. While just about anybody would say they know the most about ESD, I would have to say the same thing, but with qualification that I not only used ESD preventative equipment I've also audited it, tested it and installed it. Installation of ESD equipment in compliance with IPC 610 from its inception through the latest revision, rev G.

Do you need the resistor for the wrist strap? Yes. The resistor protects YOU. The wrist strap protects the electronics. Is a resistor mandatory on a grounded mat? No. But you can have one if you so choose. Can the mat and operator be grounded to the ground outlet of an electrical plug outlet? Yes, as long as the outlet has an independent pathway to ground. You can not use the neutral line.

Last point on this very long winded answer: You don't have to touch an ESD sensitive component to shock it. Getting close to it can transfer enough energy to either blow the component out completely, or Electrically Over Stress a component. In an EOS event the component doesn't fail. But it's like a rope that has been frayed, or partially cut through. It can't handle the full load it was originally designed to handle. EOS can not be tested for without putting the component to its full functional design specs. Even then it may survive, but live on in a weakened state. That's why ESD and EOS are such big topics in electronics where Life Critical or Mission Critical equipment is involved. Automotive as well as Aviation, Defense and Medical equipment all are held to class 3 electronics. The highest level of manufacturing production.
 

MrChips

Joined Oct 2, 2009
23,520
+1

There is another point that is sometimes totally ignored. Someone says "I've been handling CMOS chips this way for 20 years and have never encountered a failed IC". --- wrong!

Also resistors and capacitors are also vulnerable to ESD.

HP has done a lot of studies by examining the micro structure of semiconductors under electron microscopes.

1613152678523.png

Yes, ESD can cause outright failure of a device and this can be corrected at assembly and testing stages. It is the weakened semiconductor structures that are the more costly in long-term serviceability of the instrument. While a $1 failed component is easy to detect and replace, the same failed component in service in the field might cost $100 to diagnose and repair.

References:
http://www.hp.woodshot.com/hprfhelp/4_downld/lit/other/ana004r.pdf
 

Tonyr1084

Joined Sep 24, 2015
5,969
@MrChips it's been years; 1990 through 1994 when I worked as a micro section technician and did some post failure analysis. (PFA) They have an electron scanning microscope and I've seen many of these images.

One day I was looking at a cross-section of gold plated edge contacts when I noticed they looked thinner than normal. Spec was for a minimum of 50 microns gold plating. So I zoomed in (optically) and measured approximately 25 microns. Told my supervisor what I found and he said "YOU CAN'T MEASURE THAT OPTICALLY!" So he came and snatched my sample and sent it to the scanning microscope. Three days later he came back and said "I KNEW it wasn't 25 mic's." "OK," I said, "What was it?" Somewhat sheepishly he said "it was 24.6 microns." "OK, for 4 ten millionths of an inch I was wrong ? ? ?" And yes, I DID do that measurement optically.

In most of my PFA's I usually didn't find the mode. But in a few notable instances I was able to salvage a scrapped project and force a chip manufacturer to "Re-Manufacture" ten hybrid chips, costing a grand each.
 

Thread Starter

intoxicated_pilot

Joined Feb 11, 2021
5
3) Comments made that the 1MΩ is to discharge the high voltage slowly via a tiny bleed current is irrelevant.
The 1MΩ resistor has nothing to do with the discharge rate.
Energy transfers from the higher potential to the lower potential. How fast that happens is a function of conductivity or resistance. Having your computer frame grounded affords two opportunities to control ESD and EOS: First, it acts as a Faraday Cage. Your car is a Faraday Cage in a lightning storm. If lightning strikes the car it travels around you and to ground with no harmful affects to occupants and electronics within that cage. Same is true of flying airplanes that get struck by lightning. The second thing having the frame grounded offers is protection from accidental electrical issues that could otherwise electrify the frame.
So I did some more research on basic terms (voltage, current, resistance, electrons, etc...) , and my understanding is that current, put simply = the flow rate of electrons, or the number of electrons that flow past the point of measurement every second. Particularly one amp = 6.242(10^18) electrons per second. If the entire purpose of a resistor is to limit this flow rate, or current, then the number of electrons that flow through the resistor (and consequently, through the entire path) will decrease. I understand that this ability to limit current is what saves us when exposed to dangerous levels of electricity, the 1MΩ resistor of course limits current flowing through our bodies, preventing us from potentially burning to death (electrocution). Similar to how we as humans can only withstand some level of current, electrical components such as ICs, capacitors, etc... I can only assume also have their own current tolerances which they are designed for. Because of this, the 1MΩ in theory, could limit the current that flows through the electrical component, keeping it within its current/or heat tolerance, ultimately avoiding EOS, or complete failure.

I think the problem lies within how I'm asking the question. Ultimately, 1MΩ resistor is primarily a function for safety, and assuming that all surfaces, people, and electronics are kept within the same potential, none of this really matters, and EOS should not occur. However, I will give two scenarios (which are not limited) where I think the 1MΩ COULD make a difference regarding ESD protection;

A) The person is attached to a wrist strap with 0Ω of resistance, and the component happens to (somehow) have a positive charge which was previously built up. The person grabs the component, which is then discharged to ground immediately. In this situation, the 1MΩ resistor would have limited the current, and the component would experience less stress as a result (lower current, less heat, etc..).

B) Similarly, the person is installing a motherboard into a chassis which is grounded via the PSU, and connected directly to mains (0Ω). Despite wearing a wrist strap with 1MΩ of resistance, a specific component on the motherboard comes into close proximity, or contact with the chassis, which discharges through the chassis, instead of the 1MΩ wrist strap. Had the chassis been connected to ground via 1MΩ of resistance, the component would have experienced a lesser current, and potentially stayed within it's designed tolerance.

Obviously these examples only apply assuming the component somehow already has a charge built up. Say for example I grab a component without first grounding myself, and place it on a shelf, only to later pick it up with a wrist strap. Of course, the charge would have to remain within the component on it's own without somehow dispersing, if this likely, or even possible? Assuming someone is careful about keeping components in their ESD safe bags, placing them on ESD safe surfaces, etc... this should never occur, but still, just some hypothetical examples to help illustrate the idea.

Again, please excuse my ignorance on the matter, everything I have explained above could be completely false, but I figured that explaining my current understanding may help find what I'm missing here. I appreciate your detailed responses so far. As I mentioned earlier, this isn't just for my own application, but also because I'm genuinely curious to learn more about the subject, and better my understanding.

Thanks!
 

nsaspook

Joined Aug 27, 2009
8,382
...
Again, please excuse my ignorance on the matter, everything I have explained above could be completely false, but I figured that explaining my current understanding may help find what I'm missing here. I appreciate your detailed responses so far. As I mentioned earlier, this isn't just for my own application, but also because I'm genuinely curious to learn more about the subject, and better my understanding.

Thanks!
Experience and wisdom in ESD (and in life in general) only means you can still destroy some valuable thing in some new and exciting ways. However 99.99% of most destruction will be prevented.
 

Thread Starter

intoxicated_pilot

Joined Feb 11, 2021
5
The items on your static mat don't need that same protection. Even if the mat should become energized it's not likely to be anywhere close to the voltage levels of static electricity.
This is one thing I forgot in my last response. What if I happen to come into contact with dangerous levels of current while standing on an ESD mat, or resting my hand on one? I know that standard 2-layer ESD dissipative mats have a top rubber layer with very high resistance, and a bottom conductive layer which connects to ground. I would assume a 1MΩ resistor would be necessary for safety if it wasn't for the top layer of the mat?
 

MrChips

Joined Oct 2, 2009
23,520
When it comes to the function of the 1MΩ series resistor we are no longer concerned about ESD or any damage to sensitive electronic equipment. The only sensitive equipment of our concern is the human wearing the anti-static strap.

In many cases when a human limb comes into contact with live wires, involuntary muscle contraction causes the limb to pull away from the source of high voltage. A person tethered to a grounding cable no longer has this option. Of primary concern is the voltage potential difference placed across the human heart. This can lead to ventricular fibrillation and cardiac arrest. Currents greater than 75mA can cause ventricular fibrillation resulting in death in minutes. Currents as low as 5mA can trigger conditions leading to serious injury. 12mA is the suggested "safe" limit.

The most common source of HV for someone wearing an ESD strap is 240VAC line voltage. With a peak voltage of about 350V, a 1MΩ series resistor would limit the current to 350μA, which is just lower than 0.4mA.

Summary
The purpose of the 1MΩ series resistor in an ESD grounding strap is to limit the current to lower than 1mA in the event that the strap comes in contact with live 240VAC mains.
 
You do want to discharge yourself to the equipment reference which should be earth before picking up stuff.

To give you an idea of the "failures" that I have repaired just by waiting or discharging the battery is pretty amazing.
The failures were not permanent.

#1. A bicycle computer dragged across a car rug, Short battery terminals about 5 minutes with battery out. Function restored.

#2. An HP calculator used a "," for a decimal point. Same as #1.

#3. A factory digital clock in a car had no display. problem occurred when car was jumped. Same as #1, but difficult to do.

#4. Backlight on a Carrier Infinity thermostat was always on. Changing the backlight ON/OFF fuction would not work. It used a supercap. I turned off the furnace for 3 weeks and added a bidirectional Transorb on the 24VAC secondary. Fixed with no recurring issues.

ESD can come back and haunt you.
 

Tonyr1084

Joined Sep 24, 2015
5,969
Not to be a pedantic [censored], but that's 'triboelectric effect'. :)

And something in a related tangent: Cat_demonstrating_static_cling_with_styrofoam_peanuts.jpg
Looks like auto-correct got me on that one.
What if I happen to come into contact with dangerous levels of current while standing on an ESD mat, or resting my hand on one? I know that standard 2-layer ESD dissipative mats have a top rubber layer with very high resistance, and a bottom conductive layer which connects to ground. I would assume a 1MΩ resistor would be necessary for safety if it wasn't for the top layer of the mat?
Heel straps or conductive footwear has the built in resistance. As for resting your hand on a mat - I think you got it - there's a considerable amount of resistance per square centimeter. However, I don't think you will experience a shock. But to be honest, I don't recall looking into that scenario.
 

metermannd

Joined Oct 25, 2020
189
Tony: It's all good. I've been bitten or nearly so by auto-correct in the past (I think there's even a site full of submitted auto-correct induced typos). :)
 
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