Choosing the Right ESD Flooring
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- THIS 2005 ARTICLE IS OUT OF DATE DUE TO CHANGES IN INDUSTRY STANDARDS, TEST METHODS, GROUNDING & SAFETY RECOMMENDATIONS.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
(continued from prior page)
Conclusions
1.) The data suggest that a dissipative PVC floor without conductive footwear offers marginal static control advantages and in fact could represent an unnecessary liability.
2.) Based on the cross section of shoe soles and conditions, it is unlikely that moving people will generate over 400 volts as a result of interactions with a conductive rubber floor or over 600 volts on a conductive carpet tile with conductive thermo-plastic backing. It is more likely that the walking body voltages will be around the respective mean values of 250 volts and 450 volts.
3.) The normal distribution of both rubber (RUB_KV) and carpet (CAR_KV) flooring graphs suggests that triboelectric performance may be more predictable for certain flooring materials than other materials. In the case of rubber this may be the result of triboelectric tendencies influenced by material work function (see ESD Association ADV11.2-1995 for further explanation.) Conductive carpet's antistatic tendency may be explained by the principle of "charge backflow." (Also see ADV11.2)
4.) The bi-modal nature of graphs of the PVC (PVC_KV) testing illustrates tremendous voltage variation depending on shoe soles.
5.) Any performance evaluation of ESD flooring materials should consider the amount of procedural control and the level of compliance an organization can reasonably expect in their environment. Unless access to and procedures within technical spaces are carefully controlled, conductive rubber flooring or conductive carpet may offer a significantly lower threshold of risk from static charge generation than dissipative PVC. In uncontrolled environments the data suggest viewing flooring materials as preventive/environmental controls that should attenuate static on moving bodies independent of heel strap compliance.
6.) A fault tolerant floor grounding system can be achieved by installing ESD flooring materials with a combination of antistatic and conductive properties.
These findings present significant implications on the selection criteria of flooring for uncontrolled environments where sophisticated electronics are used for health, safety, security and other mission critical activities.
All tests were performed between 62 and 68 degrees F at RH between 19 and 24% Please note: Testing was based on an evaluation of materials for specific client applications at the time the testing was performed. The selection of materials should in no way be considered exhaustive. Further testing is underway using a larger sample of shoes and additional flooring materials. More clothing variable will be introduced.
Excessive tribocharging induced by a particular flooring material should not be interpreted as reason to dismiss all materials in that material composition category. When new, all materials measured less than 35 megohms in combination with a person wearing conductive heel straps per ESD-STM-97.1 Carpet products were exposed to accelerated life tests (100,000 caster cycles) because most carpet warranties exclude chair caster damage from warranty claim coverage. Presently there are no manufacturers promoting performance after chair cast exposure. Failure Total electrical failures of two out of three carpet designs reinforces the need for this testing.
A. New ESD carpet tile with PVC backing 50cm X 50cm
B. ESD carpet tile (A) after 100,000 cycle chair caster durability test
C. New ESD carpet tile with conductive thermoplastic backing 24" x 24"
D. ESD carpet tile (B) after 100,000 cycles chair caster test
E. ESD vinyl laminated to access floor panel with conductive adhesive
F. Conductive two layer rubber with black reverse side 24" X 24"
Shoes used for tests:
- Dress leather loafers with synthetic soles
- Dress laced shoes with thick rubber soles
- Hiking boots with rubber soles
- Dress shoes with leather soles
- Running shoes with plastic soles
© 2005 Conformity
About the Author:
David H. Long is the principle of Staticworx® , based in Newton, MA. He can be reached by phone at (617) 923-2000 or by e-mail at
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