How do you ground ESD mat?

[tkamiya]

I bought an ESD mat.  It has a grommet to attach a ground wire.

How do you do this?  Is there an industry standard?

I am an electrician, so I should know and I do.  But what is electrically approved per code doesn't make sense for type of equipment involved.  The code says ground should enter a dwelling from one and only one place.  That means service entrance.  So I can wire ground to the prong of American standard outlet.  BUT, ground wire goes all the way back to breaker box and then back to the main feed to a service entrance, then FINALLY to the ground rods.  That's pretty high impedance.  It's also possible to pickup hum this way. 

I can actually ground fright outside the window.  It will be a short path, and that ground is BONDED to the main ground outside.  But, if I do, if there are any current on ground (which it shouldn't but....) it will create a voltage differential and then ground loop.  Also, this is against the code.

There must be an industrial standard in electronics industry.  Does anyone know?

[DaJMasta]

ESD mats don't mind a high impedance path, you're not looking for a very conductive mat or everything you put on it gets shorted.  Grounding directly to the outlet ground is acceptable.

http://documents.desco.com/pdf/tb-2000.pdf

[tkamiya]

Thank you.  That's what I'll do.  Just connect to the center screw of an outlet.  You are right - it's not very conductive.

[Synthtech]

Aren’t they supposed to ground via a high value resistor for electrical safety? Both commercial ground mats and all of the wrist band grounding station kits that I purchased came with integrated series resistors.

[DaJMasta]

I've seen 1Meg as the standard series resistor, and I assume it's for reducing the risk of electrocution in the event of a very high voltage by limiting the current, but I'm unsure if that would be useful on a mat.  In theory, I think the resistance of the mat (which is much higher than the resistance of the strap itself) would be sufficient, but maybe if it was on the low side you'd run into potential shorting issues near the snap without the resistor?


I don't know if it's needed, but it seems like it would function fine with a series resistor.

[tkamiya]

I never, ever, used those wrist straps.  I'm so conditioned to touch a chassis before touching anything else.  I just envision my wrist falling off (burning off?) in case of a severe shock. 

[richnormand]

The wrist straps I have all have a 1Meg before the clip button and then to the electrical ground plug.

Same with the mats on the bench. They all have a 1Meg resistor before the exposed contact button on the mat.
Just think of an old style live chassis contacting a straight-to-ground button ....

Just make sure the limiting resistor is present at all stages of your esd protection system.
The only things I have straight to ground are the test instruments and soldering irons. That is when an isolation transformer comes in when needed.. Dave has a good video on that aspect.

[Mr. Scram]

I never, ever, used those wrist straps.  I'm so conditioned to touch a chassis before touching anything else.  I just envision my wrist falling off (burning off?) in case of a severe shock.

Turning around after touching a chassis could charge you with kilovolts.

[tkamiya]

Kilovolt hand falling off my hand....

[DaJMasta]

But that's why the resistor is there.  A kilovolt through your 200-400k ohm body resistance is starting to get to the place where it could stop your heart if it's passing through it (about 5mA on the low body resistance end), but put 1M ohm in series with it and all of the sudden you're good until 5kV or more (well, not good, but unlikely to be dead).

Also eliminates any chance of accidentally touching a chassis that is not grounded, and is at potential.  Unlikely, but it's a fault that can arise.

[oventech]

All the mats at work are tied to ground thru the ground prong. The wrist straps and mats have 1 meg resistance to the ground connection.

[tpowell1830]

But that's why the resistor is there.  A kilovolt through your 200-400k ohm body resistance is starting to get to the place where it could stop your heart if it's passing through it (about 5mA on the low body resistance end), but put 1M ohm in series with it and all of the sudden you're good until 5kV or more (well, not good, but unlikely to be dead).

Also eliminates any chance of accidentally touching a chassis that is not grounded, and is at potential.  Unlikely, but it's a fault that can arise.

Let's not confuse the purpose of an ESD mat: It is to dissipate any static charge that may have developed on your body in order to protect electronic devices (mostly devices like mosfets that have sensitive inputs) so that the static charge does not destroy them.

The mats are not there for safety purposes. You are more likely to get a shock because you are grounded through your shoes or hands, if near a high voltage/current source. The ESD mat will not save you. Electrical safety should be a practice that you use your head when near a potential, which is covered in a totally different standard. Do not get the idea that ESD mats are there to protect you.

[BravoV]

+1 , ESD mat purpose is to protect stuffs, not human.

[JugglingElectrons]

I've seen 1Meg as the standard series resistor, and I assume it's for reducing the risk of electrocution in the event of a very high voltage by limiting the current, but I'm unsure if that would be useful on a mat.  In theory, I think the resistance of the mat (which is much higher than the resistance of the strap itself) would be sufficient, but maybe if it was on the low side you'd run into potential shorting issues near the snap without the resistor?


I don't know if it's needed, but it seems like it would function fine with a series resistor.

The 1M series resistor is to create a higher impedance for dissipating any charge buildup to protect ESD sensitive circuits. If you were to directly connect yourself to earth ground, any static buildup on your body discharges too quickly and can create arcs. This is just as bad as discharging yourself onto a metal surface. Using a 1M resistor allows the charge to bleed off from your body to ground in a more controlled manner. The value of the resistor isn't essential as long as it's not too small to be conductive or too large to be insulative. Too much resistance ends up being like an open circuit and the charge builds up faster than it can dissipate.

The point to point resistance of the mat is also quite high in order to serve ESD protection and to provide a nice isolated surface for the board to lay on. If the resistance of the mat was too low, any exposed pins touching the mat would have a shorted path to another pin and potentially get damaged which is what DaJMasta is referring to (imagine powering up a board on top of sheet metal). Likewise if the mat point to point resistance was too high, it would build charge faster than it could dissipate it and there would be an ESD concern. You'll still want to give the ESD mat a path to ground so that it can also dissipate any built up charge on its surface. The button snap on to male banana cable that connects my ESD mat to earth ground measures 970k for the series resistor inside.

Like BravoV and tpowell1830 said, the ESD mat and ESD straps are meant to protect ESD sensitive circuitry, not protect people.