Grounding requirement in EPA

#1

Scenario:
I wish to clarify what is a level 2 technical element.
On page 1 of ANSI S6.1,
“Any ESD technical element that is connected in series to common point ground or to a common connection point (equipotential bonding) through another technical element”.

Stainless steel (SS) work table sits on top of ESD floor. SS work table does not have a dedicated connection to any ground and is making use of the contact of the SS frame with the ESD floor as grounding. So the way I see it, the grounding connection is like this:

Grounding point --> ESD floor --> SS work table (direct connection with ESD floor through SS frame).

In this case, is the SS table considered a Level 2 technical element since it is connected in series to a grounding point?

An ESD mat placed on the SS work table. So the connection path becomes:

Grounding point --> ESD floor --> SS work table (direct connection with ESD floor through SS frame)–> ESD Mat

Does that make the ESD Mat Level 3 (is there even such a definition) element? What is the treatment of the ESD mat in terms of grounding? Is such connection allowed/discouraged? I could not find any guidance in the standards.

I’m not a trained ESD guy. I suggested to my supplier to have common grounding point set up like Fig 7 on Page 17 of ANSI S6.1instead of the set up (ESD mat/SS Work table) I described above:

Grounding point --> ESD floor --> SS work table (direct connection with ESD floor through SS frame)–> ESD Mat

Instead my supplier replied that the ESD mat in the above set up is considered Level 2 technical element. I don’t think they are right based on how I understand ANSI S6.1.

I hope someone out there could help. Thank you so much.

Best regards.

#2

Hello CFC and welcome to the forum. The definition form ANSI/ESD S6.1 of a level 2 technical element states, " Any ESD technical element that is connected in series to common point ground or to a common connection point (equipotntial bonding) through another technical element."

By this definition, the mat on the SS table would be a level 2 item since it is connected in series to ground through another technical element. The real question of course is what the requirement is for the mat. For that you need to go to ANSI/ESD S20.20 and you will find that the requirement for a worksurface is a resistance to ground of less than 1x10*9 ohms. So no matter how the worksurface is configured, if it meets this requirement it complies with the requirement in ANSI/ESD S20.20.

I hope this answers your questions.

#3

Hi John,
Thanks for your reply. Please pardon my follow-on questions. Obviously there is some concept that I’m all confused with.

So what you are saying is that regardless of the position in that series connection (to the ground), all the others apart from the first one (ESD Floor) are level 2 elements:
Grounding point --> ESD floor --> SS work table (direct connection with ESD floor through SS frame)[Level 2?]–> ESD Mat (direct contact with SS table) [Level 2?]

So surface resistance to ground in this case is the resistance through the SS table plus the ESD mat; and it should be < 1 x 10^9 ohms?

So in a way, this set up is like the Equipotential bonding in Figure 3 with the SS table as the Common Connection point? But why do this when a common grounding point is available. I would think the Common Point Ground connection should be better since the grounding point is available.

Best regards.

#4

Responding to your first question:

Thank you for your question. The use of a stainless steel (SS) bench is certainly permitted. However, it should have a dedicated grounding wire with no added resistance - see paragraph 5.3.1. It is fine to connect the bench to a common point ground as shown in Figure 7 of S6.1. It would be important to measure the resistance from the top of the SS bench to ground to see the actual resistance. From a safety perspective, it is important for conductive elements to have no (or very little) resistance to ground so that circuit breakers or GFCI devices will open in the event of short circuits from live power sources. At elevated resistance, the SS bench can delay the opening of overcurrent devices and become a serious shock hazard. You will likely see the elevated resistance if you measure from the bench to ground while sitting on the floor. If the resistance is more than 2 ohms, you should use a bonding wire from the bench to the common point ground as discussed above. When the bench is properly grounded you should be able to place a dissipative mat on the surface without additional grounding of the mat (that would be Level 2). Some table mats have an insulative backing so that type of mat will require their own bonding wire going to the common point ground fixture. You need to measure the resistance from the mat to ground to determine whether or not a bonding wire is necessary to the common point ground.

#5

Nick,
Thank you so much for the very clear reply and for highlighting the safety perspective. I really need to keep that in mind more in the future.

In the case I illustrated, I believe the ground is not following the common ground point method. This brings me to question if my supplier has violated ANSI S20.20 even if the surface point to point resistance and resistance to ground point is within specs.

Best Regards.