Conformity Magazine - March 2009 - (Page 19) ESD Open Forum The following test procedure is designed to measure the resistance of a “system” comprised of the following parts: Conductive portion of the hand tool | Hand tool handle | Hand | Wrist Strap | Wrist strap resistor The above test can be used to determine which tools to use in an ESD Protected Area. The above steps are not complex and require only basic ESD control and monitoring equipment. They may be used to identify and separate existing inventories of hand tools for proper use according to their inherent properties. Again, these tests are not intended for the initial qualification and selection of hand tools. The ESD Association is in early stages of drafting a Standard Test Method for the qualification of non-electric hand tools. In the April ESD Open Forum column, we’ll provide an easy to repeat method for demonstrating the Charged Device Model (CDM) charging process using basic ESD monitoring equipment. About the Author Tim Prass is ESD Subject Matter Expert for Raytheon Technical Services Company LLC, and Working Group Chair of the ANSI/ESD STM 13.1 Standard Test Method for Hand Tools. He can be reached at (317) 306-4279. About the ESD Association Founded in 1982, the ESD Association is a not for profit, professional organization directed by volunteers dedicated to furthering the technology and understanding of electrostatic discharge. Additional information may be obtained by contacting the ESD Association at (315) 339-6937, or by e-mail at info@esda.org. Test Equipment 1. 2. 3. Megohm meter with test leads and alligator clips Wrist strap (test first to confirm proper function) Ground source (static-safe work station, or other) Test Procedure 1. 2. 3. 4. Hands should be clean, dry, and free from lotions and oils unless ESD approved. Attach a test lead from the megohm meter to the conductive tip of the hand tool using an alligator clip. Plug the wrist strap banana plug into the megohm meter as the second test lead Press and hold the test button of the megohm meter for the prescribed test period (10 to 15 seconds). (See Figures 3 and 4) If the tool handle yields a resistance that can be categorized as conductive (less than 1x106 ohms) or dissipative (between 1x106 ohms to less than 1x1010 ohms), and if the tool handle does not generate excessive static tribo-charging, the tool is safe for general use in an ESD protected area. 5. A fair amount of variability exists in this test, introduced by moisture content of the skin, relative humidity of the test area, strength of the grip, and skin contact with wrist strap. In both of the above examples, the effect of the wrist strap’s built in 1 megohm resistance has little bearing on the outcome of the test. After segregating the tools here is a guide to what actions can be taken to remediate the tools. Resistance Test Results Table 1 marCh 2009 Conformity 19 Figure 3: Screwdriver with a system resistance Figure 4: Needle nose pliers with a system of 6.21x1012 ohms (insulative – charge will not resistance of 2.29x106 ohms (borderline flow to ground) conductive and OK) Tribo-Charging Test Results Low Static Generating High Static Generating Treat with topical anti-stat and recheck resistance measurement Implement both actions, replace the tool, or remove it from the ESD protected area Dissipative/ Conductive No action required 1x1010 ohms Wrap handle with conductive tape connected to the metal/conductive portion of the tool
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