Specifying Static-free Floors
Understanding personal safety risks when grounding conductive flooring to protect ESD sensitive electronic equipment
by Dave Long
Originally published in Construction Specifier Magazine
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Other considerations for writing a conductive tile, ESD carpet and static dissipative flooring specification
Other factors affecting flooring applications
Before product selection and installation, it is essential to assess two different environments that require static-control flooring: the real world and electronic manufacturing plants.
The real world includes mission-critical environments like computer rooms, data centers, 911 dispatch centers, and magnetic resonance imaging (MRI) suites. All these operations rely on the uninterrupted use of sophisticated electronic equipment, and are free of static-control protocols like wearing wrist straps and specially grounded footwear.
Since the floor is the only line of defense against the generation of static on employees, it must inhibit charges and be fault-tolerant—that is, possess the ability to continue operation even when equipment fails. In these environments, there is little clarity on performance criteria, and this can get specifiers in trouble when they receive requests for ‘anti-static flooring.' Most of the specification sheets from flooring manufacturers do not reveal what will happen under conditions involving people wearing ordinary footwear.
A typical manufacturer's spec sheet will publish static generation test measurements obtained in a walking test in controlled laboratory conditions using static-control shoes in tandem with a grounded floor (ANSI/ESD S97.2). The other common test references how much charge is generated on a subject wearing test-specific shoe sole covers composed of neolite and leather (i.e. American Association of Textile Chemists and Colorists [AATCC] 134, Electrostatic Propensity of Carpets). However, neither test method sheds light on the implications for real-world environments.
Independent research consultants like Dangelmayer Associates and The ESD Journal have determined EC rubber will likely inhibit static charges on people wearing most types of shoes. At the same time, they have proven static-control options like epoxies, high-pressure laminates, and vinyl (conductive or dissipative) generate significant charges with people wearing most standard footwear. In other words, these materials, regardless of conductivity and grounding, are static-generating antagonists under these conditions.
In mission-critical areas, static-control carpeting offers excellent static mitigation properties. Research has shown carpet tiles manufactured with conductive yarns, conductive pre-coats, and special backing will inhibit static generation on people wearing standard or static-control footwear.
However, there is a caveat: Some carpet yarns are too conductive and could put people in danger if they were to come in contact with AC-line voltages from servers and data storage equipment. In fact, the manufacturing process of ESD carpet produces significant variations in conductivity, with resistive properties varying from less than 10,000 to more than 100,000,000 ohms within the same batch of carpet tile. The first number is below the lowest resistance limits allowed by NFPA 99 for safety; the second is indicative of a material that will not drain static as fast as it is generated.
As far as specifiers are concerned, it is imperative to identify material with consistent electrical properties for both safety and static mitigation performance—and ultimately to preclude liability.
Any time static-control carpet is considered, it would be prudent to specify a safe resistance range that offers some margin for error at both ends of the spectrum. The material cannot be too conductive, but it should still be effective in mitigating ESD. This author suggests quantitatively specifying a material resistance range above 100,000 and less than 100,000,000 ohms—the ‘sweet spot' illustrated here.
Before specifying any carpet, the material should be thoroughly tested for resistance to ground, as well as resistance between two surface points on the same tile. The logic behind requiring two tests is to weed out overly conductive, unsafe materials. Regardless of lab tests or information on a manufacturer's specification sheet, the supplier should certify the resistive properties of the floor after installation and before it is turned over to the owner. This is particularly important because an acceptable resistance range is different in an electronics manufacturing facility than in a 911 call center.
When dealing with the world of electronic manufacturing, international standards require use of specialized footwear, packaging, and wrist straps. However, these standards are not easily enforced, and workers are often lax. The upshot is in the static-free manufacturing world, as well as end-user environments, it is best to strive for maximum static protection.
Standards and certification
Many methods and industry standards have led to the creation of standardized technical specifications that match flooring application requirements. As noted, electronics manufacturers require anyone handling static sensitive parts to follow rigorous grounding protocols.
The electronics industry designates spaces where grounding is necessary as electrostatic protected areas (EPAs). Grounding protocols used in EPAs are outlined in ANSI/ESD S20.20-2007, but this standard offers little instructional value for controlling static in real-world environments like data centers, call centers, and healthcare facilities.
A close reading of ANSI/ESD 20.20-2007 should eliminate the argument over whether ‘static-dissipative' or ‘conductive' flooring is better suited for static-control flooring. The requirements summary section, in Table 2, Section 8.2, sensibly encourages material selection based on multiple variables and several test standards, and not based on choosing a broad range of resistance designated as ‘conductive' or ‘static-dissipative.' Instead, the standard calls for specific numerical values. This is also the way an architectural specification should characterize materials with electrical properties.
ANSI/ESD S20.20-2007 requires that no person have the ability to develop a body voltage in excess of 100 V. Body voltage is measured using test method ANSI/ESD S97.2. In theory, it should not be possible for persons to generate more than 100 V if they are part of a system resistance measuring below 35 megohms. However, there are several circumstances where this assumption has proven invalid.
Circuit-board manufaturing areas often combine specialized carpeting and vinyl tile products.
It is important to ensure the floor specified meets or exceeds the standards mentioned throughout this article. However, post-installation certification is also important. Most facility managers do not know whether they comply with anti-static flooring standards—and they may be at risk if they are non-compliant.
Ideally, facilities should ensure they are protecting worksites on three levels: personal safety, static-control performance, and environmental health. To ensure flooring meets electrical safety specifications after installation, some manufacturers offer audits.
Meanwhile, specifiers should recognize the ESD flooring industry will continue to grow, presenting significant challenges and opportunities at the same time. In this arena, education will remain the most powerful tool. In the final analysis, all industry professionals should be working toward the same goal—providing the best flooring options for clients. To realize this outcome, the entire project team needs to appreciate what is at risk and begin to speak the same language.
- This is known as "Moore's Law." Intel Corp.'s co-founder Gordon Moore predicted the number of transistors on a chip will double about every two years.
- See David E. Swenson et al's "Resistance to Ground and Tribocharging of Personnel, as Influenced by Relative Humidity" from the 1995 EOS/ESD Symposium Proceedings.
- This can be downloaded at no cost at www.esda.org.
Dave Long is president and CEO of Staticworx, a manufacturer of electrostatic discharge (ESD) flooring products that protect worksites with customized solutions. Based out of Watertown, Massachusetts, with an office on the West Coast, he works directly with contractors, design professionals, and end-users in selecting flooring such as rubber, carpet, vinyl tile, epoxy, and adhesives. Long is a member of the ESD Association, the National Emergency Number Association (NENA), and the U.S. Green Building Council (USGBC). He blogs regularly at www.staticworx.com and esdtile.com. Long can be contacted via e-mail at firstname.lastname@example.org.
This article provides a comprehensive overview of static-control flooring applications, grounding considerations, key standards, and test methods referenced in a specification. It focuses on how to compare performance characteristics and properly match a floor's electrical properties with the space's static mitigation requirements. This article emphasizes the important balance between effectively eliminating static charges through grounded flooring with the need to guarantee electrical safety for space occupants.