ESD Systems - Questions And Answers

1. How do ESD smocks / garments work? 2. Do they have to be in contact with the skin to be effective? 3. Do they have to be grounded to be effective? 4. What are other methods for measuring their effectiveness (i.e. overall charge generation on a person while wearing a smock, I am familiar with ESD STM 2.1-1997)? Thank you very much. - Anonymous, Morgan Hill, CA

ESD Smocks, some believe, represent the single most important step to demonstrate committment in an ESD control program. Per ANSI/ESD S20.20, all process essential insulators that have electrostatic fields that exceed 2,000 volts should be kept at a minimum distance of 12 inches from ESD susceptible items if considered a threat. Clothing, particularly when made from synthetic fibers, are significant charge generators. Worse for ESD control, the fabric is an insulator, so the result can be very threatening: an isolated charged insulator which cannot be grounded. Although ANSI/ESD S20.20 does not require ESD smocks, they are a very practical means to fulfil the above noted requirement. Our Statshield® ESD Smocks with dedicated hip-to-cuff grounding remove charges from the Smock with hands free grounding eliminating the need for a wristband. Statshield® ESD Smocks have conductive knitted cuffs providing worker comfort and a reliable path-to-ground from the person’s wrist. Fabric is 88% polyester, 12% carbon conductive nylon fibers woven in chain-link design.

ESD Smocks are an ESD protective product that should possesses the following ESD control characteristics:
- Antistatic, low charging so minimizes the generation of electrostatic charges
- Dissipative, so when grounded will remove charges to ground
- Shielding creating a “Faraday Cage” effect, so will restrict charges generated on user’s clothing to the inside of the ESD Smock.
- Groundable, so user can easily and reliably connect to ground

The most critical test per ESD STM2.1 is the panel-to-panel conductivity test [see Figure 1C in specification]. A Resistance Test Kit can quickly measure resistance of the fabric and ensure panel-to-panel conductivity by placing five pound electrodes on different fabric panels. To ensure that the fabric is low tribocharging, a Static Field Meter can be used to measure charges generated by causing contact and separation with other materials. In addition, the Static Field Meter can demonstrate shielding by measuring a charged object, and then covering the charged item with the ESD Smock; being shielded the measured charge should be greatly reduced.

Note: Some other ESD smocks have as little as 1% suffused carbon and lose their ESD protective qualities after a few washings. Since ANSI/ESD S20.20 requires that “All conductors in the environment, including personnel, must be …connected and attached to a known ground”, there is no occasion “when grounding the garment is not required” in an ESD protected area. If not grounded via a wrist strap coil cord, the Smock should be grounded via the person’s wrist removing charges via ESD footwear to ESD protected flooring. The conductive fabric in an ESD Smock is a conductor. If not grounded, the ESD smock can be a potentially threatening isolated charged conductor.

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