Choosing the Right ESD Floor
NOTE BEFORE READING:
THE FLOORING SECTION OF ANSI/ESD S20.20-2007 IS UNDER REVIEW FOR REVISION.
- THIS 2005 ARTICLE IS OUT OF DATE DUE TO CHANGES IN INDUSTRY STANDARDS, TEST METHODS, GROUNDING & SAFETY RECOMMENDATIONS.
- THE UPDATED VERSION OF Choosing the Right ESD Flooring WILL BE PUBLISHED IN SUMMER of 2012.
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The original article Choosing the Right ESD Flooring for Laboratory and Technical Spaces was published in the May 2005 issue of Conformity Magazine. In its published form, the article targeted specifiers interested in establishing a comprehensive ESD prevention program for electronics manufacturing in the factory and laboratory environment. Factory ESD programs require controlled footwear, special static free packaging, employee training about electricity, frequent scheduled testing of the electrical properties of work surfaces, carts, floors and packaging as well as a comprehensive grounding strategy for all conductive and dissipative materials. These areas are called ESD protected areas or EPAs for short.
The original article was never intended as an advisory for specifiers of grounded flooring destined for installation in end user environments. End user environments present significantly different challenges than those encountered in a factory situation. The static control strategies are completely different between these two types of environments.
At the time of publication, the article favored conductive flooring materials over static dissipative alternatives. I have recently been asked to explain why we changed our stance on conductivity towards more slightly more resistive materials. The change in our technical stance was driven by three main factors:
1. In 2005 most grounding standards approved the use of conductive flooring materials measuring below 1.0 X 106. This is no longer the case. In late 2005 several major standards organizations like the FAA revised ESD grounding documents. The revisions included a requirement for an increase in the minimum resistance limits of the static control flooring from the conductive range to the static dissipative range. As of 2005 the minimum resistance is 1.0 X 106. The FAA STD 019e states, "Conductive ESD control materials shall not be used for ESD control work surfaces, tabletop mats, floor mats, flooring, or carpeting where the threat of personnel contact with energized electrical or electronic equipment exists."
2. In 2005 static dissipative carpet tiles were manufactured with too much electrical resistance to meet some of the requirements of ANSI/ESD S20.20 for electronics manufacturing factory environments. Since that time, the resistive properties of some static dissipative carpet have been altered to effectively address the same performance criteria previously addressed by highly conductive flooring. These resistive changes were mandatory in order for carpet to be considered “fit for use” in operational equipment environments and also meet the 2007 revision to ANSI/ESD S20.20.
3. The previous bias towards installing conductive materials for factory environments gave many architects and designers the false impression that a highly conductive floor might be the correct choice for non-factory applications including call centers, schools and computer rooms. New revised post 2005 telecommunications and FAA standards have eliminated this confusion. As has been well documented in national grounding standards, conductive flooring measuring between 2.5 X 104 and 1.0 X 106 is not recommended for end user environments like call centers, control rooms and flight towers. This stance is strongly reinforced by data published in a 2012 white paper about ESD control materials and electrical safety hazards. Ronald Gibson, the author, provides empirical and real life evidence showing that more resistive ESD control materials – e.g. static dissipative flooring - have the ability to resist potentially dangerous electrical currents significantly better than less resistive materials like conductive flooring.
Go here to read the White Paper about conductive versus static dissipative carpet
A follow-up article incorporating the new versions of the standards referenced in the original article had been intended for some time. For example, ANSI/ESD S20.20 was revised shortly after the original publication of the article, as were many other grounding documents. Unfortunately, Conformity Magazine dissolved before a revision could be submitted.
The original research and testing of all ESD flooring was performed on a limited number of samples in a warehouse in Wilmington, Massachusetts under uncontrolled humidity conditions. The effects of relative humidity on the performance of static control flooring are well documented in the public domain.
A new updated version of Choosing the Right ESD Flooring for Laboratory and Technical Spaces will be published in late summer 2012. The updated article will include several revisions and additional conclusions based on independent lab testing at low humidity in a controlled environment. For example, conductive carpet generates over 1kV at low humidity. In the original study this was not obvious because high humidity conditions masked this limitation.
As a result of recent changes in grounding and safety standards, the edited version will include NFPA 99 test measurements of flooring. We have added NFPA 99 testing in order to evaluate the safety and liability exposure of grounded carpet used in the vicinity of school computer labs, electrical appliances in switch rooms, call centers and dispatcher areas, networked offices and other uncontrolled public access spaces. When using the NFPA 99 test method we discovered that carpet measuring in the conductive range (as defined by the ESD Association) would likely not meet NFPA 99. The updated article will also include commentary by consultants and engineering experts in the filed. There will be a collection of pdfs of relevant language from grounding standards, a commentary on the upcoming revision to ANSI/ESD S20.20 and a reference chart designed to help specifiers match suitable ESD flooring options with the safety standards and requirements of specific types of work spaces. The chart will include a checklist on whether or not a particular type of floor meets the following safety and grounding standards:
This short video highlights some of this information.
· ANSI/ESD S20.20-2007 and pending changes in 2012
Each of the above standards were revised after the original publication of Choosing the Right ESD Flooring for Laboratory and Technical Environments
· IBM Data Center Recommendations
· NFPA 99
· DoD Manual 4145.26-M for explosives
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Most ESD Flooring Performs Differently In the Installed Environment
Some processes used during tile and carpet manufacturing operations alter finished flooring surface in one way or another. Some processes generate residues. The processing of high vinyl content PVC tiles may create temporary glazes for shine and appearance. These inadvertent surface alterations may temporarily impact the static control performances of new flooring. For example, PVC tiles have a shiny wear layer that disappears after scuffing and heavy use. Eventually the PVC surface must be revitalized through high speed buffing or often, waxing with ESD finishes. The new walking surface may exhibit entirely different tribocharging characteristics from the original factory wear layer. Rubber flooring may be contaminated with silicon mold release that will hinder the product's dissipative performance until it has been cleaned several times. Data collected in the present study showed a significant improvement in tribocharging characteristic between cleaned rubber flooring and freshly installed flooring that was still contaminated with factory applied mold releases. In the case of carpet, antistatic coatings are often incorporated into fibers that reduce static during manufacturing. Although antistatic coatings are not permanent, simple washing will not remove them. However, the coatings disappear over time from traffic and humidity changes. Coatings may remain on the carpet just long enough to prevent walking body voltages over 100 volts during lab testing and initial floor certification. A few months after installation, that same carpet may generate thousands of volts on mobil personnel.
Accelerated Life Testing Can Reveal Product Weaknesses
Conductive carpets should always be tested before and after the "chair caster test." The chair caster test simulates approximately one to two years of task chairs rolling over a carpet's surface in a normal lab or assembly environment.
The test had been previously used by carpet mills to evaluate surface wear and appearance of yarn systems. The test revealed significant weaknesses in carpet tile designs with PVC backing. Rolling chair casters destroy or "mash" the carbon fibers in conductive carpeting and render a once conductive product into an insulator. Only one conductive carpet design met the recommended requirements of ANSI/ESD S20.20 after the chair caster test. A Properly conducted test should expose floors with conductive yarn systems to at least 100,000 cycles. Our testing demonstrated that a lifetime-time electrical warranty can not be interpreted as immunity to caster rolling damage.
Many conductive PVC floors are purchased by buyers with no intention of ever applying waxes of any kind. This belief may be short lived since it costs more to mechanically burnish floor tiles than it does to wax them. Despite the beautiful look of a factory shine -it would be wise to evaluate ESD floor finishes and glazes at the same time as the overall ESD floor tile evaluation. One embarrassing experience from a plant tour by unimpressed visitors could initiate an overnight philosophy change in maintenance practices. Before buying any resilient floor it is usually a good idea to install a small patch in a high traffic area and monitor performance and appearance. This inexpensive test drive could prevent a costly miscalculation in total cost of ownership and worse - compromised ESD performance due to unanticipated performance problems caused by maintenance and normal abuse.
Test Flooring Based On Real World Conditions
Will the operation require the use of heel straps or other types of conductive footwear? Many labs and technical environments can not mandate or monitor compliance with the use of heel straps. Unfortunately, these environments still require some form of ESD mitigation due to the use and handling of extremely sensitive electronic equipment. It may be prudent to purchase flooring based upon meeting not three but four electrical testing parameters:
Resistance to ground, total system resistance, voltage measurement in combination with a person with conductive footwear and voltage measurement in combination with person wearing uncontrolled footwear. Our testing demonstrated that certain materials can meet the first three criteria and perform reasonably well in the fourth test. The decision to address or ignore the fourth criteria is strictly a function of confidence in procedures. A periodic analysis of heel strap compliance in similar operations usually predicts what can be expected in similar operations.
