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How do you measure ESD level on carts that use drag chains to ESD protective flooring? What is acceptable industry resistance measurement in Ohms scale?
How can products on carts be ESD protected?
Per ANSI/ESD S20.20-2007 Table 3, the required limits of Mobile Equipment (Working Surfaces) to be < 1 x 10E9 ohms resistance to ground.
To measure RTG [resistance to ground], use a Surface Resistance Test Kit placing a 5 pound electrode on each cart’s shelf worksurface and connect the sensing [black] lead to equipment ground.
To verify conductive casters or drag chains, many companies measure Resistance Top to Top (RTT ) measurement. Place a five pound electrode on the ESD Protective Floor and one on shelf worksurface of cart. Shelves should have dissipative mating which is electrically connected to the drag chain.
The ESD Association Handbook ESD-TR20.20-2000. Under paragraph 5.3.8 Mobile Equipment, it states:
“When mobile equipment is used to transport unpackaged ESD sensitive items it must be capable of being grounded as products are loaded and unloaded from it. When the mobile equipment is in an ungrounded state (i.e. between ESD protected areas), care must be taken to ensure that personnel do not handle the products unless they first bring all items to an equipotential balance by bonding together the product, mobile equipment and the personnel that handle the ESD sensitive products.
Mobile equipment can be grounded directly to an ESD protective floor materials via drag chains, conductive wheel(s) and cable or ball assemblies. When mobile equipment is grounded through floor materials it is necessary to ensure that there is a path to ground from the surface (where the unprotected parts are stored) to ground no matter which connection method is chosen. One of the benefits of this type of system is that the connection to ground is constant and normally does not require operator intervention. One of the drawbacks is that the connection to ground can be lost if a solid contact with the ESD floor material is not achieved. Dust and dirt buildup (i.e. on the surface that contains ESDS [ESD sensitive] parts, the floor and/or the grounding mechanism), can result in a loss of the electrical connection between the ESDS item and ground.
For ESD programs that do not utilize ESD floor materials, mobile equipment can still be used effectively to transport ESD sensitive products. In this situation, the mobile equipment will need to be connected to ground via a wire connected to the ESD grounding system prior to product loading and unloading. The mobile equipment should be connected to AC ground per the recommendations found in the grounding section of this handbook.
In both of the above cases it will be necessary to include all pieces of mobile equipment in periodic process audits.”