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Question We installed our first ESD Conductive tiles in one part of our Cleanroom. Standard tests (RTG and tile to tile) are good but we noticed that when we rub the tiles with any insulator or the conductive shoes, it generates a negative charge as high as 400 V (using a field meter) and the reading stays for a couple of minutes. Is this a normal behavior of ESD conductive tiles? Shouldn't it be always 0 volts? What could be the problem on our ESD conductive tiles?
Answer I am less concerned that it can be charged to –400 volts, and more concern that it “stays for a couple of minutes”. Is the Conductive Tile properly grounded? Was conductive adhesive used to adhere the tiles to the existing floor? What does RTG (resistance to ground) to equipment ground measure? What does RTT (resistance top to top) measure? Uninstalled Conductive Tile product could be both conductive and high charging meeting its specifications. See the Tech Brief. The most important property of ESD Conductive Tile is that it measures in the conductive resistance range so when properly grounded electrostatic charges will be removed to ground. The following citation is from a Packaging standard, but the principle is applicable: Per ANSI/ESD S541 section 7.2 Note: “There is no correlation between resistance measurements and the ability of a material to be low charging.”ESD protective products posses one or more of the following ESD control properties:
  • Low charge generation (antistatic)
  • Dissipative or conductive resistance (so when grounded, charges are removed to ground)
  • Shielding able to create a Faraday Cage
So the product you have could be designed to just have the conductive property.
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