ESD Compliance in a Server Room

Static-control flooring provides protection against electrostatic discharge (ESD) in multiple industries servicing disparate applications that range from eliminating annoying shocks to protecting aircraft flight-tower operations from equipment malfunctions. Often referred to by the term ESD flooring, this category of flooring can protect static-sensitive electronic devices and equipment from harmful (but, due to its invisibility, seemingly inconsequential) levels of static discharge, far below the threshold of human sensitivity. In other instances, ESD flooring is installed to prevent static sparks from causing ignition of flammable chemicals, munitions, explosives, and energetic materials.

In their article "Are Data Centers Drying Up,"1 authors Beaty and Quirk discuss alternatives to humidification, like ESD flooring, for preventing real-life ESD problems in data centers, such as:

Specified and used properly, ESD flooring prevents the generation of static electricity and provides a path to ground for charged objects, including people, materials, machines, and transport equipment. ESD flooring also grounds any object with intrinsic conductivity that makes contact with the floor. For data centers, multiple ASHRAE-funded studies strongly suggest the use of at least moderately conductive flooring systems in controlled areas to reduce the overall level of electrostatic charge accumulation, regardless of environmental moisture or the type of footwear used in the space.

Depending on the industry and application, different static-control requirements and test methods take precedent. For example, static-control requirements for handling explosives usually fall under the jurisdiction of either the Department of Defense (DoD) contractor manual, DOD 4145.26, or Department of Energy (DoE) Standard, DOE 1212-2019. In contrast, organizations handling static-sensitive electronic devices follow the guidelines of ESD Association standard ANSI/ESD S20.20.

It's critical to match the right standard and static mitigation strategy to your specific application. When comparing the value, jurisdiction, and viability of any organization and standards, it's worth noting the possibility of legal complications should the wrong floor be installed. In a January 2012 article published by In Compliance Magazine, nationally recognized liability attorney Kenneth Ross says that in a lawsuit:

"…Industry standards and even certifications like UL are considered minimum…the standard establishes a reasonable alternative design, and the manufacturer has to justify why it didn't comply." 2

Although this advice applies specifically to safety risks, it presents a second problem on a much wider scale. What about product performance? Static discharge is a very real problem, but it is mostly an invisible problem. How does the end user know they actually installed a compliant solution? Does the end-user organization rely on supplier literature and specifications, or does the organization do its own testing? What are the metrics for establishing product compliance, and does their space resemble the conditions under which the product was designed to operate? ESD footwear, for example, greatly enhances the performance of ESD flooring but may be impractical for spaces such as call centers and server rooms.

Given that standards vary, how do you determine which standards and test methods should be referenced for which environment? To understand why this is important, consider the different requirements for resistance testing between UL 779, DoE/DoD, and the ANSI/ESD test requirements. DoE and DoD resistance testing of conductive flooring is usually performed with an ohm meter set at 500 volts. The ANSI/ESD and ASTM requirement for the same resistance test specifies applying either 10 volts or 100 volts, depending on the resistive properties of the material under test.

Flooring manufacturers do not typically provide product specifications based on 500-volt resistance testing, and most flooring specifiers don't ask for results obtained at different voltages. Why would using different voltages in a resistance test present a problem? In the case of the DoD, the government set a minimum flooring resistance of 40,000 ohms tested at 500 volts to assess "safety" from electrocution. According to Ohm's Law, increasing applied voltage lowers resistance. A floor that measures 40,000 ohms using test method ANSI/ESD STM 7.1 at 10 volts will measure well below the 40,000-ohm requirement when subject to 500 V applied voltage.

ASTM and ANSI both evaluate the resistance of conductive floors at 10 and 100 volts. If a specifier chooses a conductive floor based on test results obtained using ASTM F150 or ANSI/ESD STM 7.1 test methods, the floor may not meet DoD (500 volts) requirements.

What happens if resistance testing isn't performed until after the floor has been installed? This occurred at a U.S. Air Force base earlier this year. The facility handles explosives, and the floor, tested post-installation, was not in compliance with government requirements. The supplier has spent over $100,000 in labor and materials to remove their ESD floor and install a new floor that complies with the government standard. Either floor would have eliminated static satisfactorily, but the Department of Defense doesn't provide waivers for non-compliant materials used in explosives applications.

A large cable television provider enlisted a local flooring contractor to provide costs for a complex project involving the removal of old flooring in a large operational data center/server room and replacement with a static-control solution—the project presented many challenges.

The bond between the old floor tiles and the concrete (see Figure 1) had deteriorated due to age and adhesive breakdown. Flooring directly under racks could not be removed because the facility operates 24/7. Removing the floor surrounding the racks was risky due to potential problems with dust containment. These obstacles and preexisting conditions steered the cable company towards solutions that could be installed directly over the existing floor.

Figure 1: Deteriorating floor in cable company server room

Several different ESD flooring materials were evaluated. The primary objective was to find a material that could be installed without adhesives. This limited the options to interlocking tiles or a floating solution such as rubber, vinyl, or ESD carpet tile. The carpet option was dismissed due to the need to move heavy equipment without adding rolling resistance. This led directly to the decision to install a hard-surface interlocking floor.

The next question: did they want dissipative or conductive flooring? To ESD program managers in electronics manufacturing facilities, this may seem like a simple choice, but this application required grounding people who were handling and changing circuit boards in an operational environment. The client wanted to know how high the resistance could be before it was too high to effectively decay charges and what resistance might be considered too low or unnecessarily conductive, thus posing a potential safety risk. The floor also needed to inhibit charge generation on a person wearing regular footwear in an environment with varying humidity.

Per ANSI/ESD STM 7.1, conductive flooring is defined as any flooring with a resistance to a groundable point of less than one million (< 1.0 x 106) ohms. A dissipative floor measures from one million ohms to less than one billion ohms (< 1.0 x 109). This test's ANSI/ESD S20.20 qualification phase is typically performed in a lab at low relative humidity (RH). An ohm meter is used to measure the aggregate resistance of all the components required to install the floor. With glue-down floor tiles, this entails installing tiles to a test substrate with the proper adhesive and then measuring the resistance from the tile's surface to a ground connection buried into or under the adhesive. The measurements obtained from this simple lab test determine whether a floor is categorized as conductive or dissipative.

To attain a compliant resistance, floors with conductive surfaces are sometimes installed with dissipative adhesive. As long as the adhesive assures a path to ground above 1.0 x 106 and less than 1.0 x 109, this type of flooring system would be characterized as a static-dissipative flooring system. Lab testing cannot predict whether or not this may be problematic in the field because labs don't present variables found in the intended installation environment. For example, a dissipative flooring system that relies on dissipative adhesive to control its resistance to ground could be rendered conductive if installation conditions introduce concrete moisture vapor transmission or if grounded equipment placed on the flooring surface creates an unintended ground path.

Depending on the construction of the flooring system, certain types of floors could also measure differently in the field than in the qualification test. A composite floor such as carpet tile or a floating vinyl floor, for example, might be manufactured with a more conductive surface layer than the layers below the surface. Performing tests on a mock-up installation can catch such possible pitfalls ahead of time, preventing surprises after the floor has been installed.

In the case of data centers and server rooms, there are no official standards for choosing the right electrical resistance for ESD flooring. But we can look for static-control guidance from manufacturers who build this equipment. Most use some type of ESD flooring. Since their ESD-prevention programs are designed to meet ANSI/ESD S20.20, they install flooring with a resistance measurement below 1.0 x 109 ohms to ground and charge generation (per test methodANSI/ESD STM 97.2 lower than 100 volts on personnel wearing ESD footwear.

Given that S20.20, IEC 61340-5-1 (the international equivalent of S20.20), and FAA standards all set an upper limit of < 1.0 x 109, the point at which the performance of static-control flooring is significantly diminished, it's logical that this would be a universally accepted upper threshold.

For decades, NFPA publications set a minimum electrical resistance of 25,000 ohms for floors installed in operating rooms. This resistance value was determined using an ohm meter set at 500 volts. UL 779 requires an average minimum resistance of 25,000 ohms. DoD 4145.26 sets 40,000 ohms as the minimum in areas with 110-volt service and 75,000 ohms near 220-volt service. (For DoD, a ground fault interrupter meets the same requirement.) A post on an IBM data center website, updated in May 2022, says:

"For safety, the floor covering and flooring system should provide a resistance of no less than 150 kilohms when measured between any two points on the floor space 1 m (3 ft) apart." 3

FAA 019f, Motorola R56, and ATIS 0600321 all require ESD flooring to measure above 1.0 x 106. Like the company in the case study that needed to protect grounded personnel, people employed by facilities covered by these standards work near electrified equipment. These industries created their standards with the intention of protecting workers from the risk of electrocution. While we don't know of a case where someone was electrocuted by an ESD floor, it's a theoretical possibility that has been upheld in laboratory testing.

It's paramount to keep in mind that resistance measurements made with an ohm meter should never be relied upon to determine how much current will pass through a static-control floor. One study in particular, by Fowler Associates in Simpsonville, SC, demonstrated a significant variance in the actual measured electrical current on ESD flooring materials versus the predicted electrical current based on resistance measurements obtained using an ohm meter.4 The only flooring products marketed to protect workers from electrical current are highly insulative and serve no static-control purpose. ESD flooring is not designed to prevent the flow of electricity. It is exactly the opposite. ESD flooring facilitates the flow of charges to ground.

This leaves us with requisite policies such as following national and local electrical codes, limiting electrical work to only qualified personnel and organizations along with developing, implementing, and enforcing an electrical safety program. This isn't to say that we shouldn't consider a minimum resistance. It just means that we shouldn't rely on electrical resistance as a safety measure. But whether resistance is a reliable predictor of leakage current or not, flooring manufacturers should take Ken Ross's advice into consideration, i.e., a standard (UL, NFPA, DOD, FAA) establishes a reasonable alternative design, and in the case of an accident, the "manufacturer would have to justify why it didn't comply."

Server rooms differ from electronics manufacturing spaces, and the criteria for selection differ as well. One significant question when selecting an ESD floor for a server room as opposed to a manufacturing environment is whether or not the floor can mitigate static charges without ESD footwear. In electronics facilities, all personnel on the floor are required to wear some type of ESD footwear. The use of ESD footwear would be impractical in a server room. This limitation creates a strong need for installing a floor that generates minimal charges regardless of footwear or low relative humidity.

According to a major ASHRAE-funded study:

"While it may prove impossible to control with certainty the footwear worn by personnel who enter or work in data centers, facility owners and managers should be aware that footwear can lead to issues in the daily operation of the data center. Just about any conventional polymer-based sole material may lead to high charge levels, some more than others – regardless of humidity. A conductive floor will help mitigate electrostatic charging even on shoes with the highest potential for generating static." 5

The type of static-control flooring material also plays a part in charge generation. Among the most compelling documented statistics is the probability of generating a charge over 500 volts while walking on a static-control floor wearing ordinary shoes (see Table 1). The probability of 500 volts occurring on a static dissipative vinyl floor was calculated at 35%; for a conductive vinyl floor, the probability dropped to 8%. The probability of a conductive rubber floor allowing a charge over 500 volts was only .1%.

* Using ESD-mitigating flooring and footwear, the risk of ESD upset and damage can be reduced to an insignificant level, even if the humidity is allowed to drop to low values, such as 8%. Unfortunately, controlling the footwear in most data centers is very impractical.

Historically data centers have relied upon humidification to control static. The ESD Association removed humidification as a requirement in the 2007 version of ANSI/ESD 2020. We can and should draw from other standards to address the specific needs of these spaces.

ASHRAE research project RP-1499 shows that the installation of static control flooring in data centers and server rooms can control, reduce and prevent problematic levels of static generation and, as a result, enable a significant reduction of long-standing humidification and energy requirements in these spaces.

Combatting these problems with a one-and-done infrastructure solution like ESD flooring makes sense, particularly compared with wasting energy to cool a highly humidified space. In "The Effect of Humidity on Static Electricity Induced Reliability Issues of ICT Equipment in Data Centers" (Endnote #5), authors Wan, Swenson, Hillstrom, Pomerenke, and Stayer strongly suggest the use of:

"at least moderately conductive flooring systems in controlled areas to reduce the overall level of electrostatic charge accumulation, regardless of footwear or environmental moisture. Flooring has to be installed anyway, and the cost associated with a conductive rather than insulative floor is minor compared to continuing operational costs to sustain proper moisture levels (low humidity)."

When evaluating an ESD floor, multiple performance factors should be investigated, including maximum and minimum electrical resistance, electrical codes and industry standards, charge generation at the lowest operational relative humidity, and performance with and without ESD footwear. Whether the data center is under construction or already in operation will impact and possibly limit ESD flooring options.

Some organizations prefer that conductive flooring not make electrical contact with racks (see Figure 2) due to the potential impact on system analysis due to a ground path from the rack to the floor. Another consideration is whether contact with grounded racks might alter a floor's surface-to-ground resistance properties. Experimental installations can expose these possibilities prior to specifiers making a final selection.

Figure 2: Server room covered with interlocking conductive floor. Note the floor edges are trimmed to avoid contact with the racks

Combined with static-control chairs and grounding straps, static-control flooring can provide a highly effective, single-expense solution for all types of ICT spaces.

data centerDavid Longesdflooringserver roomstandardsstatic control

Dave Long is the CEO and founder of Staticworx, Inc., a leading provider of flooring solutions for static-free environments. He has 30-plus years of industry experience and combines his comprehensive technical knowledge of electrostatics and concrete substrate testing with a practical understanding of how materials perform in real-world environments.

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