My 2 Cents ->
1. If a populated PCB is put on a ESD safe Rack, is it able to be placed on a Plastic Shelf? = I would say no, here is why: if there is an esd event on the rack/tray where will it go with no path to ground? being the rack is a generator you are going to have esd events for sure!
2. If a populated PCB is put in a covered ESD box, is it able to be placed on a Plastic Shelf? = Short answer is yes, but do your homework on the box/tote to know its a true faraday cage.
3. Could we put ESD vinyl mats on the plastic shelves and ground those? = do the math on this! figure out what the cost of the matting/ ground snaps/ ground cable connections and plastic racking is along with the time spent messing around with all of this and compare that to the cost of the cheapest ESD rack you can find and i bet you will find its cheaper to go with a ESD rack. Check Craigslist and FB marketplace and i bet you can find some metro (or knock off) racks that will work for you, we find most of ours this way and usually only have to replace the plastic sleeves or run a screw through each rack to the post to make it conductive. if you are a CM and have the chance of getting audited by a customer this will remove doubt and eliminate a point of argument.
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