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With or without an ESD Smock, the worker with a properly grounded wrist strap will have generated charges removed from their person via their skin as the electrical conductive path. Even so they can have charges on their clothing that are isolated and do not have a path to ground. If this charged clothing comes in close proximity or into contact with an ESD sensitive device, a charge can be induced to the device, or a discharge can occur.
Wearing the ESD Smock, with cuffs at wrists & buttoned up, a Faraday Cage is created where such charges will be kept to the inside of the Smock fabric & if contact is made with panel to panel conductivity will remove the charge. However, you would have much less tribocharging if your employees were nude and you had them work on dirt floors. Very high charge generation occurs in a low humidity environment, with workers wearing synthetic shoes and clothing walking across carpeted floors.
The former specification EIA-625 in section 11 required a 12” minimum distance from ESD sensitive devices. [Note: Since an electric field has a range of 1/R^2 then the charge at 12” will be 144 times smaller than at 1”]. It reads: “Methods for Minimizing Static Charging, Static charge prevention actions shall be utilized at ESD protected areas and workstations where static voltages greater than 200+/- volts are measured and unprotected ESDS devices are within twelve (12) inches of the charged sources.” You could amend your written ESD control plan deleting the requirement to wear ESD Smocks in your ESD protected areas. You would first want to measure the actual electrostatic charges generated by using a Digital Field Meter. Also, you should observe in your operations, how close does worker clothing come to ESDS devices.
Depending upon the susceptibility of the EDS sensitive devices that you are working on, and the cost to administer ESD control program changes, you can consider the following: 1) Require ESD smocks when humidity is under a certain level 2) Require ESD Smock if worker is wearing synthetic upper body clothing such as polyester, rayon, etc., but not require ESD Smocks if wearing upper body clothing such as cotton, wool, etc. 3) Not require ESD Smocks if worker has no clothing on upper body. 4) Require ESD aprons but with workers wearing sleeveless upper body garments. 5) Not require ESD Smocks, but require that worker clothing be kept a minimum of one foot from ESD sensitive devices.
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