Protektive Pak - Questions And Answers

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

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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