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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 type. Would the metal expansion band be better at eliminating static build-up than the cloth strap, due to its larger conductive surface area? -Anonymous
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 longer than 150 milliseconds (ms) body movement time. Such rapid grounding is accomplished well by elastic band, Speidel® Twist-O-Flex®, or metal expansion wrist straps. The resistance of the silver impregnated nylon elastic band is typically under 1 kilohm which is more than adequate to provide a discharge time well under 150 ms.In a recent test, the elastic band resistance measured only 400 ohms and removed the charge in under 2 microseconds (0.002 milliseconds). In all cases (elastic band, Speidel® Twist-O-Flex®, or metal expansion) 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. The concern is if the wrist band’s resistance or the equivalent resistance of the body to band starts to exceed 1x108 ohms. This should be detected in testing as most wrist strap testers fail high over 1x107 ohms.
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