Desco Europe ESD - Questions And Answers

Wrist Strap Testing: Most wrist strap testers are calibrated to provide a "pass" if the resistance value measured is between 750 kohms and 10 Megohms. We buy wrist straps with 1 megohm resistor. I am told that the tester will measure the entire circuit, including the user. My question is: Could somebody "pass" the wrist strap tester with a wrist strap that doesn't include the 1 megohm resistor? It seems that if a large resistance can be provided by the user's body, etc, that you could measure at least 1 megohm even without the wrist strap resistance. Thanks for your help? - Brian G. Severtson, Naval Warfare Assessment Station, Corona, CA

You are right, it is conceivable for a person to have a skin resistance approaching 1 megohm. If the coil cord's resistor was shorted, then a Wrist Strap Tester would not detect this failure. However, we have never experienced this actually happening. Ensuring that this event not occur might fall into the costly program overkill mode. To ensure that the wrist strap's coil cord is not shorted (bypassed the 1 megohm resistor) then simple test only the coil cord first. Plug the banana plug into the wrist strap tester and connect the snap end to the return circuit (metal touch plate) to test only the coil cord. Since the coil cord should have a 1 megohm resistor in it (in the molded snap end) then the coil cord should pass on its own (> 750 kilohms). Only after passing that test, to then test the worker with the Wrist Strap in the normal manner. It is recommended that workers test themselves at least once daily. If the concern about detecting coil cords with defective resistor could be evaluated statistically, then an alternative approach could be utilized. Randomly selected coil cords could be tested to establish if the resister is "blown" in a shorted position. This would be accomplished as above noted with testing only the coil cord above. The sample selected to be tested would be dependent upon the population of coil cords and the level of confidence that you require. If a failure is detected or if there is a substantial period where no failures are detected, the sample size or frequency of testing could be adjusted accordingly.

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