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Question If you have a metro-cart with trays of Printed Circuit Assemblies in a warehouse area where there are no ESD workstations but you do have a wrist strap with an alligator clip attached to the banana plug is it feasible to do either of the following or are both options of no benefit? A. Connect the wrist strap cable to the same metro-cart shelf as the tray of PC assemblies, ensuring that you are at the same potential as the metro-cart shelf before handling the assemblies. B. Connect an alligator clip on the end of the wrist strap cable to a grounded utility box and then with one hand on the shelf that holds the tray of PC assemblies to obtain equalization, proceed to examine the tray of assemblies. 2. Is it harmful to use an alligator clip, designed to be inserted on the end of a banana plug, tested and passed on a wrist strap tester, and finally used to connect the wrist strap to the banana jack on the ESD workstation? and if so, how is that determination of harm made? - Dave, Northford, CT
Answer Electrically connecting your wrist strap to the same shelf as the Metro-Cart may have some benefit before handling your circuit assemblies on the trays. You have to make sure the of the following: that the trays are conductive (<10^11 Ohms), the shelf is conductive and electrically connected to the whole cart, and that the point your wrist strap connects to the cart ties into this circuit. We would not recommend grounding the cart with your hand (as suggested in solution B) only because of contact resistance, grounding integrity, tribocharging, and induction problems. The cart may have fields on non-conductive parts (wheels, frame sleeves, etc.) that could induce charge imbalance back onto the conductive parts of the cart when you let go right before handling the ESD sensitive parts. There is no harm in using an alligator clip on the wrist strap to a common point ground (CPG). You will want to check the integrity of this connection, both mechanical and electrical before use making sure there is an electrical contact between the clip and CPG and the breakaway force (mechanical integrity) is greater than or equal to 1 and less than 5 pounds of pull force.
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