ANSI/ESD S20.20 Has Three Categories of ESD Garments – Why?
ANSI/ESD S20.20-2007 has three Garment categories with the following names and required limits:
- Static Control Garment < 1 x 10^11 ohms
- Groundable Static Control Garment < 1 x 10^9 ohms
- Groundable Static Control Garment System < 3.5 x 10^7 ohms
Per section 8.2 Personnel Grounding “All personnel shall be bonded or electrically connected to the grounding / equipotential bonding system when handling ESDS items. The personnel grounding method(s) shall be selected from Table 2. NOTE: Use of a garment to achieve personnel grounding shall be documented in the ESD Control Program Plan. The garment shall be electrically conductive from one sleeve to the next and must also meet the system resistance requirements defined in Note 2 in Table 2.”
Note 2 reads “For situations where an ESD garment is used as part of the wrist strap grounding path, the total system resistance including the person, garment and grounding cord shall be less than 3.5 x 10^7 ohms.”
So when using a garment as part of the path-to-ground for grounding personnel, we recommend Desco Statshield ESD Smocks which reliably meets the Groundable Static Control Garment System required limit of < 3.5 x 10E7 ohms. Count on Statshield ESD Smocks to provide long and reliable life protecting your ESD sensitive products improving your operation’s quality, productivity, and reliability. Statshield ESD Smocks have the best warranty in the industry (see 2 Year Limited Warranty) covering a minimum of 2 years or 100 launderings whichever comes first. Review the independent test lab report where the highest reading was 2.49 x 10^6 ohms after 100 launderings.
What about the other two higher resistance categories? Basically, they are offered for sale in the marketplace, so the additional two categories are included. They might be acceptable for guests and those not handling ESD sensitive items. However, it is best practice for all ESD garments to be grounded.
All three categories may suppress or otherwise affect an electric field from clothing worn underneath the garment without being attached to ground. However, without grounding, a charge may accumulate on conductive or dissipative elements of a garment, if present, resulting in a charged source.
Most clothing is insulative, and being a non conductor, electrostatic charges cannot be removed to ground. The ESD garment has a “Faraday Cage” shielding effect suppressing the charges so as not to damage ESD sensitive items being handled. The ESD garment fabric has resistance so charges on the ESD garment fabric can be removed to ground.
Per ESD Handbook ESD TR20.20-2008 section 188.8.131.52.6 Proper Use “After verifying that the garment has electrical conductivity through all panels, the garment should be electrically bonded to the grounding system of the wearer so as not to act as a floating conductor.”
So we recommend Desco Statshield ESD Smocks a Groundable Static Control Garment System, and that all ESD garments be grounded. Groundable static control garment systems may also be used in conjunction with a continuous or constant monitoring system in a manner similar to those used in continuous monitoring of wrist straps in an ESD protected area (EPA).
The Desco Statshield® premium smock incorporates our “hip-to-cuff” grounding feature which allows for hands-free grounding with no tugging at the operator’s wrist. This feature allows connection of a ground cord to a 4mm snap stud on the hip. A seam of carbon-suffused threads provides a secure and direct electrical connection from the snap stud on the hip to conductive elastic cuffs, providing a highly reliable connection via the user’s skin. The smock will quickly and effectively ground the person when used in this manner.