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Question Wanted to find out the difference in ESD effectiveness of using a cloth ESD wrist strap versus a metal, expansion board. It seems to me that the metal, expansion band would be better at eliminating static build-up than the cloth strap, due to a larger conductive surface area. - Anonymous, Fresenius Medical Care-North America, Walnut Creek, CA
Answer The key to personal grounding is to have an adequate path to ground so that there is never a potential difference with respect to ground on the human body at times longer than 150 ms body movement time. This is accomplished well with either the cloth or metal expansion wrist strap. The resistance of the cloth strap is typically under 1 kilohm (I measured 400 ohms on one in the lab) which is more than adequate to derive a discharge time well under 150 ms, in fact is well under 2 microseconds, which is more than adequate. In both cases (metal expansion and cloth wrist straps) the total discharge time is mainly influenced by the skin's resistance, the contact resistance to the band/buckle, and the body/band capacitance to ground. Using either band, the human body is more than adequately protected from it's ground connection via the wrist strap. The concern is if the band's resistance or equivalent resistance of the body to band starts to exceed 1x108 ohms. Most wrist strap testers fail high over 1x107 ohms.
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