ESD Systems - Questions And Answers

I am having a difference of opinion with our Manuf. supervisors. They claim that PCB's inside of a card cage are protected from ESD and therefore they do not need to wear their wrist straps when working on them. I say that the card cage is not a Faraday cage and they should be wearing the straps whenever they are working on any assembly with PCB's in them. Can you give me your opinion on this ??...Thanks - Anonymous, Westwood, MA

For PCB's inside of a card cage to be protected from external ESD, the card cage would have to be fully enclosed and conductive. For protection on the inside, the cage should be electrically dissipative and antistatic. You mentioned that the card cage is not a Faraday cage and they should be wearing the straps whenever they are working on any assembly with PCB's in them. You are absolutely correct! Even if the card cage acted as a Faraday cage, you would still have to wear a wrist strap, when handling the PCBs inside the grounded cage. A Faraday cage works, because the contents are protected from external static electrical forces using the skin effect in conjunction with a sufficiently enclosed conductor (charge is forced to the outside surface). Once a foreign conductor (your hand) is introduced into the cage, you run the risk of damaging any sensitive ESD components within because the integrity of the Faraday Cage effect has been compromised. The hand (fingers, wrist, arm) if not grounded can hold a charge that can be easily be discharged inside the cage into the conductive traces of a Printed Circuit Board. Whenever handling sensitive components "out-of-the-package" [whether the item comes out or your hand goes in] all items should be at the same potential (ground) or an ESD event can occur, i.e., wear a wrist strap.

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