ESD Systems - Questions And Answers

I work in the Quality department of an electronics manufacturer. One of our technicians has complained that wearing multiple ESD grounding devices (2 foot straps and a wrist strap) diminishes their protective ability because the 1 Meg. ohm resistors are in parallel. That the operators are at risk of "fatal" electric shock! Can someone in your organization review the validity of his claims and explain why his theory is accurate or not? If there is truth in this, then perhaps you can make some recommendations on how we can correct this situation. - Anonymous, Irvine, CA

His hypothesis is theoretically correct, where if all points (both feet and wrist) are grounded to the same ground point (at 0 ohms), then the equivalent resistance will be 333 kilohms, but this is practically impossible. An ESD Floor averages 10 megohms to ground, but can be as low as 10 kilohms using conductive floor mats and typical contact resistance to the floor through foot straps can be as high as 10 megohms (not including the resistance of the floor to ground). My suggestions would be to employ foot straps with 2 megohm resistors, so when wearing a pair of foot straps and standing on both feet on a metal plate to ground the equivalent resistance to ground will be no less than 1 megohm. Attaching the wrist strap in this same circuit will yield an equivalent resistance of 0.5 megohms, which would yield a maximum current at 125 AC Volts of 0.25 mA (the UL safe limit). People can be put at risk under this condition if exposed to greater than 125 VAC. What voltages are used at your ESD Safe workstations? According to the DOD-HDBK-263 Section 7.3.1.3, Table VIII, (Ref.: MIL-STD-454) the table list reflex action as the response to an AC current between 4 and 21 mA. Underwriters Laboratory states that our wrist straps and foot straps are “UL Safe” if the AC Voltage does not exceed 250 Volts when employing a 1 megohm resistor in series with the grounding device. This implies that an AC current is UL safe if it is less than 0.25 mA.

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