11
\$\begingroup\$

Should an anti-static mat be connected to earth directly or with 1E6 ohm resistance in between?

2 layer mat. Upper side: dissipative (10E7 ~ 10E10 ohm/m²). Bottom: conductive.

To be a little more precise: connection to earth will be thru a CGP (common ground point) which is at earth potential. So, 1E6 ohm resistance or not, between mat and CGP?


In response to Lorenzo Donati: so, the workstation environment explained in Op Amp Applications Handbook, chapter 7, page 95, looks like this:

enter image description here

Why not the following setup? Note the wrist strap grounding, which i edited from the original image.

enter image description here

Now, instead of 2E6 ohm resistance to earth, there is 1E6 ohm between the wrist strap and earth. Is this enough?

Page 96: "Again, a 1E6 ohm, from the wrist strap to ground, is required for safety".

\$\endgroup\$
  • 5
    \$\begingroup\$ All the similar questions you asked have answers and comments that point out to the resistor as being a device that serves to protect the user and/or devices from larger currents. What is it exactly that you not understand? \$\endgroup\$ – Wesley Lee Dec 14 '16 at 11:12
  • 1
    \$\begingroup\$ Still you're completely focused on the value of the resistor(s) while almost everyone here says it's irrelevant. I can guarantee you that you can make all resistors and sheet resistances 10x smaller or 10x larger and the ESD protection and user safety will be practically the same. \$\endgroup\$ – Bimpelrekkie Dec 14 '16 at 14:06
  • \$\begingroup\$ Allright, so none of the 2 setups above is better than the other? \$\endgroup\$ – Marty Dec 14 '16 at 15:09
  • \$\begingroup\$ @FakeMoustache, 10x smaller would not be sufficient to protect yourself from ground fault shocks, i think. But i get your point :-) Thanks for all your answers so far. \$\endgroup\$ – Marty Dec 14 '16 at 15:46
  • 1
    \$\begingroup\$ 10x smaller would not be sufficient to protect yourself from ground fault shocks Why ? 100 kohm at 240 V is only 2.4 mA. That's a nice "tingle" but dangerous, no, I don't think so. \$\endgroup\$ – Bimpelrekkie Dec 14 '16 at 15:48
26
\$\begingroup\$

You (and the some of the other answers) focus too much on the actual value of the resistance to ground; the fact is that the actual value is irrelevant regarding ESD. The ESD charge just needs a path. If that path is high-ohmic (few mega-ohms) it will only take slightly longer for the charge to find its way to ground. But it will still be a fraction of a second which is still fast enough.

What is important is your safety. You do not want a large enough current to flow through your body when you inadvertently touch a mains live voltage! That is why the 1 Mohm resistor is important in the wrist-strap, but also in the ESD mat as your hand might be resting on it. What if your hand was close to the earthing connection of the mat and you touched a live voltage? Then that 1 Mohm resistor will limit the current, and you will only feel a slight "tingle".

\$\endgroup\$
  • 8
    \$\begingroup\$ +1 for the when part! \$\endgroup\$ – AndrejaKo Dec 14 '16 at 9:37
  • 6
    \$\begingroup\$ If you prefer to rely on that and not use the 1 Mohm resistor to ground feel free to do so despite all the good reasons to use that 1 Mohm resistor. There are only advantages in using that 1 Mohm resistor and no disadvantages. If you think that ESD protection will be worse because of that resistor then you have not understood ESD very well. \$\endgroup\$ – Bimpelrekkie Dec 14 '16 at 10:56
  • 1
    \$\begingroup\$ @FakeMoustache, ESD protection is about high voltage, low current. So, resistance is needed. Not in the last place for the operator's safety. I was just wondering whether the resistance of the dissipative layer would be enough. Especially after watching this operator connecting the mat directly to earth. \$\endgroup\$ – Marty Dec 14 '16 at 12:44
  • 1
    \$\begingroup\$ @Marty -- "no current" at equilibrium with no fault condition. In case of a fault, the resistor is there as extremely cheap insurance. That's it. \$\endgroup\$ – Wesley Lee Dec 14 '16 at 13:59
  • 1
    \$\begingroup\$ ESD protection is about high voltage, low current. So, resistance is needed. For the ESD protection itself, no, resistance is not per se needed. Only if you do your utmost to make the ESD discharge current as large as possible (metal grounded table, you on isolating shoes all charged up firmly holding a metal object, then discharge yourself through a component on that metal table) only then will that resistor save your component. In practice there will always be some series resistance. The 1 M ground resistor is mainly for safety reasons. \$\endgroup\$ – Bimpelrekkie Dec 14 '16 at 13:59
10
\$\begingroup\$

Here is an excerpt from chapter 7 of

enter image description here

enter image description here

Note: copying images because PDF files are protected and text cannot be copied.

BTW: here you will find the entire book freely: OP AMP APPLICATIONS.

\$\endgroup\$
  • 1
    \$\begingroup\$ +1 exactly the point I want to make in my answer. Thanks for backing that up with with info from a book. \$\endgroup\$ – Bimpelrekkie Dec 13 '16 at 22:00
  • \$\begingroup\$ Then why would a company sell grounding cords without resistor and why would they sell common ground point with no internal resistor between the mat and the ground point? \$\endgroup\$ – Marty Dec 14 '16 at 10:58
  • 1
    \$\begingroup\$ @Marty: please, read the answer from FakeMoustache. Not everything companies sell is, by itself, something that provides adequate safety margins. The resistance of the mat might be a sufficient mean of limiting ground faults currents, but you cannot predict exactly what resistance will be on the current path during a fault event. The difference between 10mA and 2mA through your heart muscle could mean death vs life! \$\endgroup\$ – Lorenzo Donati Dec 14 '16 at 11:17
  • \$\begingroup\$ @Marty grounding cords are also used when there's no mains present. An easy example is adding RAM to a PC, where the PC chassis acts as the antistatic table mat, and is disconnected. A field technician can connect directly to the case. Another example -- testing battery-powered devices, or those fed off an isolating power supply \$\endgroup\$ – Chris H Dec 14 '16 at 13:25
  • \$\begingroup\$ Well this picture isn't all that pedagogical, as there is a resistor between the wrist strap and the ESD table, which in turn has another resistor to ground. The resistor to the wrist strap seems superfluous (but harmless) - I'm guessing it is only there to protect some nutto from connecting the wrist strap directly to the ground. \$\endgroup\$ – Lundin Dec 14 '16 at 13:37
2
\$\begingroup\$

Through the resistor! This resistor allows charge to leak to earth to ensure you don't destroy components with ESD.

Low-impedance paths to earth are for safety. They ensure that fault current has a reliable, robust path to earth.

In theory, if your ESD mat were connected directly to earth, it could pass large fault current if any mains-referenced voltages touched it. Boom.

Addendum: The top of your mat is static dissipative. If you connect earth directly to that surface, it will dissipate charge. However, you can't and shouldn't rely on that as a safe, known, or reliable resistance to earth. Electrical safety often seems redundant or unnecessary, but it's all about knowing your setup is safe, rather than assuming that it should be safe.

\$\endgroup\$
  • \$\begingroup\$ The upper side is dissipative already. 10E7 ~ 10E10 ohm/m². That impedance is not low enough? \$\endgroup\$ – Marty Dec 13 '16 at 19:55
  • 5
    \$\begingroup\$ High* enough? Maybe. Maybe not. That's not a figure to rely on for your safety. It is a measure of sheet resistance, not a measure of series resistance to earth. What if it's humid? What if you spill something on the mat? For safety, you must have a known resistance to earth. A 1Meg resistor costs nothing and is a known series resistance to earth. \$\endgroup\$ – vofa Dec 13 '16 at 20:01
  • \$\begingroup\$ Yes, i meant: high enough :-) \$\endgroup\$ – Marty Dec 13 '16 at 20:03
  • 3
    \$\begingroup\$ This MAT GROUND INSTALLATION is done directly to ground It is unclear if there is a 1 M ohm resistor somewhere in the grounding cable. You won't know until you measure that wire. \$\endgroup\$ – Bimpelrekkie Dec 13 '16 at 20:14
  • 1
    \$\begingroup\$ Are you certain there isn't a resistor in series with the wire? Imagine that a non-static voltage is present on your mat. Like, if a mains wire touches the dissipative surface. The sheet resistance measurement is meaningless. The thickness of the mat at that location determines the series resistance to earth via the conductive layer, and that's variable. A 1Meg resistor costs nothing and guarantees your safety. \$\endgroup\$ – vofa Dec 13 '16 at 20:14
2
\$\begingroup\$

According to the standard ESD test arrangement (as in description of IEC 61000-4-2), the arrangement must have a solid aluminum ground plate. The plate must be grounded via TWO 500kOhm resistors (in series) to ground. This limits ESD discharge currents while providing reasonable discharge (RC) time.

The "anti-static" mats are safety devices, and whoever manufacturers them are responsible for their protective function. Therefore they can ground their products in whatever form or shape, including solid ground wire (and relying on limited sheet resistivity of their product), or else, provided that the product performs its function. To avoid liabilities, safety devices must be installed in full accord with manufacturing specifications.

However, if you are crafting protective measures on your own, it would be your responsibility to do things right, including either a controlled sheet resistivity, or mega-Ohms resistor inside grounding wire, or some combination.

\$\endgroup\$
  • \$\begingroup\$ 61000-4-2 describes how to build an ESD test bench where you can carry out tests with an ESD gun, including air discharges. This isn't necessarily the same thing as building an anti-stat work bench. There are no metal plates on a work benches. \$\endgroup\$ – Lundin Dec 14 '16 at 13:45
  • \$\begingroup\$ @Lundin, That was my point exactly. I believe that's where the confusion about necessity of 1M resistor comes from. What is your objection? \$\endgroup\$ – Ale..chenski Dec 14 '16 at 19:00
  • \$\begingroup\$ In the case of 61000-4-2, the resistors are explicitly there for the ESD discharge, while on regular ESD material I believe they are there to protect against ground failure on the power net. So that's two different purposes. \$\endgroup\$ – Lundin Dec 15 '16 at 7:18
  • \$\begingroup\$ @Lundin, in both cases the conductive path is to prevent the top surface form charge build-up. In 61000-4-2 case the surface is a solid conductor (actually covered on the top by a sheet of insulator), so the resistors are needed to limit current from a possible discharge. ESD discharge never goes into this reference plane directly. In second case the current is limited by relatively high impedance of the surface itself, but the purpose is the same - prevent charge build-up due to movement of equipment and parts. \$\endgroup\$ – Ale..chenski Dec 15 '16 at 7:45
0
\$\begingroup\$

With the 1 MOhm in between. That limits the current of the discharge to ground.

\$\endgroup\$
  • 1
    \$\begingroup\$ but the upper side is dissipative already. 10E7 ~ 10E10 ohm/m². \$\endgroup\$ – Marty Dec 13 '16 at 19:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.