Concerning the foam, it is intended to be non static electric charge generating, though it isn't necessarily extremely conductive / dissipative, it is usually only very slightly conductive / charge dissipative. It is intended to provide a useful cushion to prevent damage to the motherboard while preventing accumulation / generation of harmful charges. It is quite OK to use as a cushioning / rest material for the motherboard and other parts as long as one has kept the overall environment reasonably free of static charge accumulations.
Gently rubbing your grounded (wrist strap) hand over the surface of the foam / box / bags / packaging will ensure that it is all OK to come in contact with the parts.
If you want more conductive / dissipative foam, the pink stuff is usually better than the grey stuff, and the dense spongy black stuff that is actually used for holding naked chips is the best. Purpose designed anti static cushioned work-mats are of course ideal also.
The motherboard bags are often somewhat or substantially conductive at least on their interior layers, though the outside and possibly the inside of the bags may not be particularly conductive. They're basically to provide a shield so that charges won't accumulate in one spot differently than another spot and risk zapping the product. Usually the heavily metallized "metal foil appearance" bags are the most conductive and the ones that are basically slightly tinted colored transparent plastic are the least conductive. They're all substantially better than nothing or other alternatives when it comes to packaging the products, though the use of them (or the flat foam ESD motherboard pad) as a work surface is not ideal (compared to a grounded mat / grounded metal table) except from the perspective of keeping dust/dirt off and providing some slight ESD safe cushioning.
I have to disagree with the below comment.
He's leaving the power supply plugged into the wall because of it being connected through a 3 prong power cord to a 3 prong properly grounded AC outlet, and hence the case of the PSU would be connected to earth ground through the house's AC wiring.
Almost all large appliances are, in fact, similarly connected to earth ground through their power cords and their metal chassis are explicitly grounded so as to prevent the possibility of a "hot" wire coming into contact with the chassis and permitting the whole chassis to become a "hot" shock risk source. If you're suggesting that a plugged in appliance with its chassis earthed through the AC outlet is dangerous to be around, I suggest you check out your refrigerator, washing machine, et. al. The national electrical code specifically encourages or requires such practices, in fact, for appliances one may come into contact with.
Furthermore, there is not a significant risk of electrocution from a computer's power supply even if it were turned on because the output voltages are transformer isolated from the AC mains voltages, so the most voltage present on any of those wires is approximately +12/-12V DC, less dangerous by far than handling a car battery.
Also nobody was suggesting to leave the PSU's hard power switch in the ON position during such service / construction; in the other thread it was said more than once that the PSU's "hard power" switch should be left in the OFF position, and noted that it would even be a reasonable idea to plug it into the wall through a modified cord which connects ONLY the ground lead and not the AC HOT/NEUTRAL leads if the PSU did not have a "hard power off" switch.
The only significant dangers in leaving a PSU connected to the wall during service are
a) accidental energizing of the PSU outputs via turning on the hard power switch (if present) in a circumstance that would short out or fry the electronics when they're partially assembled. PSUs put out "standby power" of +5V even when their "soft power" switch is OFF or disconnected, and that is dangerous for the electronics (when being assembled or only partly connected) as well as the PSU (possible short circuiting). This is the worst and most likely danger, and it is entirely eliminated by just turning off the "hard power switch" on the PSU's rear (assuming it has one).
b) accidental contact with energized circuits inside the PSU -- basically one would have to be wearing a metal necklace / chain / dog-tag or something small enough to penetrate the PSU case and come in contact with an internal high voltage circuit to present any danger even if the PSU's "hard power switch" was ON (which it should not be); this is a rather unlikely risk, and wouldn't be much worse than leaning over your stove top, toaster, TV set, etc. with the same kinds of jewelry / whatever. When the PSU's "hard power switch" is OFF (as it must be) only a tiny place within the PSU has any high voltage on it, and that'd be ALMOST impossible to accidentally contact even if one was trying to poke a wire/chain through the PSU case.
Originally posted by: VirtualLarry
Do NOT leave the PSU plugged into the wall, it does nothing to help with grounding, but it puts you at a serious hazard for electrocution.
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