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Question We have a large manufacturing floor that has ESD flooring. Every year we do a 100% ESD audit on all our tables, equipment, carts, etc. And we do other ESD audits quarterly. What we have found is that many (the majority) of our carts and racks are failing for ESD. They pass from shelf to shelf but are not making that connection from shelf to floor. A year ago everything passed okay. A few months ago when we discovered this issue, we replaced many of the whips and chains that looked old/dirty. We even stripped some of the floor of it's ESD wax, thinking this may have been the issue. However, the racks/carts that we replaced chains on are still not passing whether they are on waxed or unwaxed floors. Have you heard of this happening before? Any idea of what we can do?
Answer

Many users report that grounding carts to an ESD Floor via drag chains does not work reliably. Best practice is using conductive casters, and to use a ground cord to a common point ground after the cart is moved into position where it will become stationary.

The minimum recommended technical requirement range for Mobile Equipment is less than 1 X 10^9 ohms per ANSI/ESD S20.20 Table 1. The Floor should also be less than 1 x 10^9 ohms, and the measurement from cart shelf to ESD floor RTT (resistance top-to-top) cannot be met if the Floor is high.

Per ESD-ADV53.1 ESD Protective Workstations Electrical Requirements "Workstation elements shall be connected to, and maintain electrical continuity to, the common point ground as follows:

Worksurfaces - Resistance: Between 1 x 10^6 ohms and 1 x 10^9 ohms, Surfaces of shelves and drawers intended to be used for unprotected ESD sensitive devices - Resistance: Between 1 x 10^6 ohms and 1 x 10^9 ohms"

However, we have also heard at a recent ESD Association seminar that a significant solution is to state in your test instructions is that the RTT measurement is to be taken after the cart has been moved a bit. This will jiggle the chain and allow it to make better contact. To take this a bit further is to take the measurement when the cart is being held by a grounded worker if that is how it is moved.

The rationale is that the major concern is tribocharging when the cart is being moved. That then the RTT to the ESD Floor via the drag chain will be more reliable, and/or that the path-to-ground via a worker wearing ESD Footwear on the ESD Floor will be reliable.

 
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