Grounding Methods for Mobile Worksurfaces?

Technical Questions
#1

Other than drag chains and ESD wheels, what other methods are there to properly ground mobile worksurfaces?

My current employer is going through all the carts on the production floor and wants to standardize them, with ESD safety being part of the end goal.

#2

If you have an conductive or static disspitive floor, drag chains or conductive wheels on the carts can be the only way to ground them without a lot of process control.
The carts could be connected to ground while loading and unloading via a wire but that would require some process steps to ensure that would happen.
If you package the ESD senstivie items during transport, then grounding may not be required. Packaging in an EPA could just be trays or totes. The tray or tote would have to be grounded while loading or unloading. This could be accomplished if the packaging is on a worksuface or grounded in some way during the load and unloading process.

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#3

Zachariah,

Drag chains and ESD caster wheels are the two most common ways to ground mobile equipment in an EPA. Generally, the ESD caster is the better option as it makes better contact with the floor. Drag chains often do not make adequate contact with the floor or they do for a while and then as the chain and floors get dirty, the conductive path is interrupted. As for other options, I have two suggestions

  1. Grounding cord: In this method, you would have a grounding cord that you would attach to the mobile equipment when at rest and before you would handle any ESDS items on the cart. This option does require some discipline and training to ensure operators always remember to connect the ground cord before handling the ESDS items

  2. Strip of ESD Mat: This idea came from a person at my facility, and it seemed to work pretty well compared to drag chains. In this method you cut a thin strip (maybe an inch or so wide) of ESD mat material and tie it to the bottom of the mobile equipment diagonally so that the material drags along the ground but will not interfere with your step. This gives a much greater surface contact area than a drag chain. You would need to verify this works by measuring the resistance to ground. You would also want to check this more frequently at first as dust may collect on the strip. We found it was good for well over a year although we had a pretty clean EPA

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#4

Andy

Curious as to what the RTG measurement range was with the “strip of mat” vs. the drag chain. Also, is there concerns for fod generation from the mat as it drags along the floor? Finally, was this a two layer or single layer mat?

Thanks

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#5

Chris,

I don’t remember the RTG measurements as this was over 5 years ago but I believe they were around 10^7 to 10^8 ohms.
There could be a FOD concern, but we didn’t have any. It seemed the strip of mat didn’t pick up a lot of dust.
We used a 2-layer mat

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#6

Thanks Andy. Was just curious.