Drag chains and grounding mobile equipment, such as carts, can be a challenge in any ESD Control Plan. The required limit for cart work surfaces per ANSI/ESD S20.20 is less than 1 x 109 ohms resistance to ground. In the most recent edition the ESDA November 2013 Threshold E-News Question and Answer section there is a good discussion on what is required to make a drag chain work reliably. Read the details and the rest of the Threshold E-News Here.
Desco’s approach to drag chains is very similar to what the ESDA recommends. If drag chains are used, ideally a 18” long portion should rest on the conductive floor, and frequent compliance verification tests should be performed. To have more confidence in reliably having a path-to-ground, Desco has consistently recommended that drag chains be replaced with a more reliable system if possible:
- Conductive casters – Also recommended by the ESDA in this Q and A
- Using ground wire from the cart attached to a common point ground when stationary
It should be noted that carts are an optional part of any ESD Control Plan. As long as ESD susceptible devices are packaged in ESD shielding containers (shielding bags, boxes, totes, etc) while stored or transported, the cart may not have to be grounded.
For additional information on conductive casters, see ANTI-STATIC CASTERS: The Newest Development in the Ongoing Battle Against ElectroStatic Discharge” by The Darnell Corporation
“The researchers stated: ‘Insulative wheels can accumulate a charge when a cart is moved on an insulative or conductive floor, regardless of the cart being equipped with a grounding chain or not’ if the cart stops at a point where the grounding chain does not make proper contact with the floor.”
“The final conclusion is: For a good ESD protection the carts should be equipped with conductive wheels, preferably all [four] wheels, and the floor must be conductive with short distance between the conductive spots.”