An access floor contractor was bidding a project calling for “static dissipative” flooring. Like many contractors, the project manager viewed the terminology from a generic perspective. Most laymen equate the term static dissipative (SD) with any flooring type that is marketed for the purposes of mitigating the discharge of static electricity. They do not realize there is a distinction between a conductive floor and a dissipative floor and that there may be a practical reason for choosing one over the other.
Since the architectural specs did not include electrical resistance parameters, cite-specific industry standards, or require that resistive properties be tested before final acceptance, the project manager felt comfortable bidding any type of ESD flooring. In this instance, she proposed a conductive floor for an FAA flight tower, when in fact the FAA requires flooring to measure in the static-dissipative range.
Similar scenarios occur every day. The root causes almost always involve semantics, with specifiers citing incorrect standards for a specific industry, as well as a general lack of understanding about electricity and static-control flooring.
In the construction trade, there is an old saying, “electricity always follows the path of least resistance.” The saying is only partially true. Electricity flows through all paths – intended and unintended. We must keep this in mind when we verify the resistance of installed ESD vinyl or carpet tiles.
If we only follow test method ANSI/ESD STM 7.1, we might overlook an unintended path to ground. STM 7.1 only requires testing the resistance of floor tiles to the ground connection specified by the manufacturer. But what if that ground connection relies on resistors or high resistance adhesive as part of its path to ground, even though the equipment racks on top of certain floor tiles are also grounding the floor?
For this reason, always test the resistance connections between the surface of tiles directly under equipment, and the connection to either the equipment racks or the pedestals of the equipment sitting on the surface. This is a case of prudently exceeding standards and test methods when those standards emphatically warn that they are not intended for evaluating safety.
This creates multiple problems encompassing product liability, economic loss, failure to perform and in compliance with industry standards.
Confusing Conductivity and Specifications
To investigate this dilemma, we need to explore the history of floors used to prevent static-discharge problems.
The roots of the ESD flooring industry hark back to the need for preventing static sparks in medical environments where flammable and explosive gases were administered as anesthesia. Like the static-control wrist straps used in electronics manufacturing today, early versions of static-control products involved some form of single-point grounding and bonding (via tethering) to maintain a single potential between all conductors that came in contact with one another. In general, this was achieved by placing wet towels across the floor to connect the anesthesiologist’s foot with the base of a steel operating table. (Yes, this is real!)
In an article published in 1926, titled “How Can We Eliminate Static from Operating Rooms,” Dr. E. McKesson writes:
“Hence the simplest method of preventing static sparks is to keep the objects concerned in the administration of combustible mixtures in contact—i.e., the patient, the anesthetist and the inhaler. This is usually done and accounts for the relative infrequency of fires from static sparks in the operating room.”1
As throughout the electronics industry today, McKesson recognized that full reliance on a multi-step human process of tethering and un-tethering of personnel and fixtures with cords and wires assumes a perfectly executed process every time. He writes, “But errors of technique are made, and if the conditions are ‘right,’ a fire occurs.”
McKesson recognized the need for a passive grounding system that does not rely solely on a series of connections that may not always occur. McKesson writes:
“An effort has been made at one hospital to make errors impossible by grounding a mosaic floor, consisting of alternate block of tile and bronze in one or two rooms and a solid metal floor in another. That is, when one steps upon this floor the charge on his body flows through a thick wire to the ground. The operating table, apparatus, instruments, anesthetists, surgeons and all are thus grounded or their charges neutralised.”
McKesson wrote this paper for the British Journal of Anaesthesia – advocating for what we now call ESD flooring – all the way back in 1926. And yet, into the 1960s, there continue to be records of hospitals placing wet towels on the floor to provide electrical bonding between the anesthesiologist and the operating table.
Late in 1950, a Wisconsin company called Natural Products began work on plastic conductive flooring. The following year they would introduce Statmate and rename the company Vinyl Plastics Inc (VPI). VPI’s non-metallic conductive floors gained immediate and widespread acceptance as a highly effective grounded flooring solution in hospitals. Unlike metal, these early conductive plastic floors could be made with inherent and controlled electrical resistive properties. This was and is critical to electrical safety.
Circa 1950, the NFPA had determined that floors in hospitals should not measure below 25,000 (2.5 x 104) ohms or in excess of 1,000,000 ohms (1.0 x 106). Vinyl floors could be manufactured to meet this requirement. This ohms range of 2.5 x 104 to < 1.0 x 106 marks the launching point at which today’s confusion about conductivity, resistance ranges, and the suitability of conductive floors begins.
Resistance Tests Per NFPA Guidelines Are Not Equivalent to ESD/STM 7.1 Tests
Although metal floors were durable and provided effective conductivity, they offered absolutely no safety in the presence of alternating current (A/C). To ensure safety along with a reliable level of conductivity, NFPA bulletin 56 (issued in the 1940s) required a specific electrical resistance range for conductive floors. Electrical resistance was to be tested using an ohmmeter, with 500 volts of applied current. This was because, in 1950, meters – 500 volts was chosen to test for resistance with an emphasis on electrical safety. Wall-mounted meters, such as the Conductometer were installed in ORs and tested both flooring and footwear at 500 volts. Today we test with 10 volts of applied current.
Why does this matter? Ohm’s Law: the higher the applied voltage, the lower the resistance. Likewise, the lower the applied voltage, the higher the resistance.
Since ANSI STM 7.1 requires 10-volt electrification, resistance tests of the same material will measure much higher than an NFPA test using 500 V of applied current. Likewise, the results of an NFPA test using 500 V of applied current will be much lower than the results of a test following guidelines of 7.1 applying 10 V. The point is that the test methods are not equivalent; therefore, measurements are not equivalent.
The Electrostatic Discharge Association (ESDA) and the electronics community have chosen an upper limit of less than 1,000,000 ohms for defining a conductive floor.2 This conductive range is quite different from the range set by the NFPA. Yet many floorings suppliers state that their floors measure above 25k ohms per NFPA – but also market their floors as measuring between 25k and one million ohms per the current ANSI/ESD STM 7.1 10-volt test method.
This is not possible. A floor measuring 25,000 ohms at 500 volts will present as a much less conductive surface with 10-volt electrification. The chart in Table 1 shows measurements taken by an independent lab. As indicated in the chart, gray ESD carpet measuring 75,000 ohms with 10 volts of applied current measured only 16,000 ohms at 500 volts. While the floor tested per S7.1 measured slightly above the stated 25,000 ohms, when tested at 500 volts, it failed to meet the NFPA’s requirement for resistance.
Table 1 shows examples of the discrepancy between resistance test results performed per NFPA and ANSI/ESD test methods.
| Carpet Tile Test Results for product marketed as measuring 2.5 x 104 – 1.0 x 108: | ||
| Color | ANSI/ESD STM 7.1 @10 volts | NFPA @500 volts |
| Grey | 7.5 x 104 | 1.6 x 104 |
| 7.2 X 104 | 1.4 X 104 | |
| Silver | 7.5 x 104 | 1.4 x 104 |
| 6.9 X 104 | 1.3 X 104 | |
| Dark grey pattern | 5.0 x 104 | 1.4 x 104 |
| 6.0 X 104 | 1.0 X 104 | |
| Carpet Tile Test Results for product marketed as measuring 1.0 X106 – 1.0 X 109: | ||
| Color | 10 volts | 500 volts |
| Patterned carpet | 1.8 x 106 | 1.1 x 106 |
| Blue Carpet | 1.5 x 106 | 8.0 x 105 |
Table 1: Carpet tile resistance test results showing the discrepancy between NFPA and ANSI/ESD test methods
What Is a Static-Dissipative or Conductive Floor?
This history of conductive flooring and evolving resistance test methods brings us to the concerns we face today. What is a static-dissipative floor, what is a conductive floor, and which version should be referenced in a specification?
The first answer is actually a question. What are the test methods you’re using to measure resistance and what standards do you need to meet for compliance in your industry? One example is NFPA 99. Almost every flooring manufacturer mentions NFPA 99 compliance; NFPA 99 deleted any mention of floor testing years ago due to the elimination of flammable anesthesia. Unless the manufacturer specifications account for and incorporate test data obtained at 500 volts, they are misapplying a defunct test method.
The perhaps bigger problem is that different industries have different resistance standards. We often see ANSI/ESD S20.20 cited in specifications for ESD floors for 9-1-1 dispatch centers. ANSI/ESD 20.20 relates specifically to electronics manufacturing and handling environments and requires the use of ESD footwear in the qualification of ESD flooring. ESD footwear is never used in call centers and dispatch areas. In these applications, the mention of 20.20 is irrelevant and potentially misleading. Floors in these environments should reference either Motorola R56 or ATIS 0600321, both of which require floors to measure between 1.0 x 106 and 1.0 X 10 10. Many airport flight towers are also equipped with static-control floors. Like Motorola R56 and ATIS 0600321, FAA-STD-019f, Lightning and Surge Protection, Grounding, Bonding, and Shielding Requirements for Facilities and Electronic Equipment, prohibits the use of flooring measuring below 1.0 X 106 due to concerns for the safety of people working near energized equipment.3
Unlike end-user spaces, there is no lower resistance limit for flooring used in an ANSI/ESD S20.20 ESD program. Conductive floors are an important element in an ANSI/ESD 20.20 program due to the need for worker mobility, rapid charge decay, prevention of tribocharging, effective grounding of mobile workstations, and the ability of personnel to handle highly sensitive products without the use of wrist straps. ANSI/ESD S20.20 states that the resistance measurements obtained through the use of ANSI test methods are not to be used to determine the relative safety of personnel exposed to high AC or DC voltages. Although most flooring manufacturers do not produce flooring measuring below 25,000 ohms it is imperative that the end-user understands that the burden of liability involving both safety compliance and product suitability of electrically grounded flooring rests on both the manufacturer’s and specifier’s shoulders.
It should not be implied that conductive flooring is unsafe when appropriately utilized in an ANSI/ESD S20.20 certified program. These programs require regular testing of both floor conductivity and footwear conductivity, these spaces are accessed only by trained personnel and conductive flooring should never be installed in areas where high potential testing or equipment is in operation. However, before any conductive floor is installed, buyers should understand that a conductive or static dissipative floor is a system that requires multiple installation materials, special footwear and specific steps during the qualification and verification processes. As further confirmation that flooring should not be viewed as a discreet component, we need to look no further than the newly proposed tile in the 2020 draft of test method ANSI/ESD STM 7.1., Flooring Systems – Resistive Characterization.
Test Methods Versus Performance Standards
Most ESD flooring specifications reference some type of resistance testing procedure, such as those found in ANSI/ESD STM7.1, ASTM F150, DOD 4145.26 or NFPA 99 (formerly NFPA pamphlet 56). Many buyers mistake these test methods as representing performance standards. Performance standards guide the specifier in determining what test results are acceptable. Test methods tell us how to determine if we have compliant products.
For example, FAA-STD-019f states that a floor must measure between 106 and 109 ohms. Motorola R56 states that the floor should measure between 106 and 1010 ohms when tested per ANSI/ESD S7.1. ATIS 0600321 cites the same resistance requirements as Motorola R56. Although not an actual standard, IBM’s Physical Site Planning document states:
“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. They require a test instrument similar to an AEMC-1000 megohmmeter for measuring floor conductivity.”4
Like the hand crank meggers and other instruments used to test insulation resistance, the AEMC-1000 does not offer a 10-volt output but it does allow testing up to 500 volts. Since IBM’s upper recommended resistance is 1010 and no test voltage is mentioned, one might believe that this test was intended to ensure a minimum amount of insulation resistance. By contrast, the ESD industry requires simply that conductive floors measure below 1.0 x 106 at 10 volts.
Again, resistance measurements alone should not be used to determine the safety of a particular floor. There are multiple reasons for this that are beyond the scope of this article. However, as an experiment, we solicited a third-party lab to apply both AC and DC voltages to various ESD floors and measure the resulting current at the floor-ground connection. The results of this testing are shown in Table 2.
| Carpet Tiles with Black Backing – 2.5 x 104 – 1.0 x 108 | |
| AC Volts Volts ac |
AC Amperes mili Amps ac |
| 4 | 1 |
| 11.5 | 3 |
| 18 | 5 |
| 30.5 | 10 |
| 52.3 | 20 |
| 117 | 50 |
| EC Rubber Tiles – 2.5 x 104 – 1.0 x 106 | |
| AC Volts Volts ac |
AC Amperes mili Amps ac |
| 31 | 0.1 |
| 40 | 0.4 |
| 66 | 2 |
| 80 | 4 |
| 93 | 5 |
| 120 | 7.6 |
| Static Dissipative Carpet Tiles – 106 – 109 | |
| AC Volts Volts ac |
AC Amperes mili Amps ac |
| 5 | <0.1 |
| 10 | <0.1 |
| 25 | <0.1 |
| 50 | <0.1 |
| 100 | <0.1 |
| 120 | <0.1 |
Table 2: Results of testing applying AC and DC voltages to various floor types
As the chart illustrates, some conductive floors appear to enable significantly more electrical current than others. The amount of current is not accurately predicted mathematically by using electrical resistance measured with an ohm meter. In part this is due to the construction of conductive floors, whether they are comprised of composite layers, if they are fully conductive on the surface or constructed of the same material throughout the thickness of the material.
However, the experiment clearly illustrates what we already know: a floor with an inherent resistance over 1,000,000 ohms is less likely than a very conductive floor to enable a dangerous leakage current. This fact drives recommendations for using dissipative flooring in data centers, flight towers, dispatch operations, and areas where energized equipment is used. Whereas we need to control static generation and charge decay to an extremely low threshold in electronics manufacturing, we do not need the same level of performance in end-user spaces like data centers, etc. While the electronics in these end-user spaces can be damaged by electrostatic discharge, they’re less sensitive than components in manufacturing and handling facilities.
According to an ASHRAE white paper, the data center industry views 500 volts as an upper threshold compared with the 100 volt upper limit for meeting ANSI/ESD S20.20 in electronics manufacturing.
The Semantics Problem
The ESDA has produced a glossary of terms. Three newly proposed terms referencing flooring include flooring systems, conductive flooring systems, and dissipative flooring systems. But terms like dissipative and conductive are frequently misunderstood and misapplied. In some cases, the misapplication leads to problems in the field. In many cases, specifiers don’t know which electrical range is the correct one for their client’s specific industry. In other cases, specifications are copied from previous static-control projects even though the application may be entirely different.
For example, per DOD 4145-26-M, DOD explosives-handling applications require conductive floors as defined by resistance testing at 500 volts. Per ANSI/ESD STM 7.1, the same floor tested at 10 volts might actually measure in the very low part of the static-dissipative range. As previously noted, resistance is predicated by the applied voltage.
“To avoid any confusion and future liability due to misunderstandings about conductivity and test method, we recommend that explosives handling specifications always be cowritten by the end-user and the specifier.”
Let’s look at the definition of a dissipative flooring system. A static-dissipative flooring system, measured with a full combination of components, including surface material, adhesive, grounding mechanism, substrate and any other material in the system, is considered static dissipative as long as the system has a resistance greater than or equal to 1.0 x 106 ohms and less than 1.0 x 109 ohms.
This sounds like a comprehensive definition with no room for misunderstanding. However, if an installer laminated the highly conductive bronze tiles (mentioned in McKesson’s 1926 article) with a static-dissipative adhesive, it would appear in a typical ANSI/ESD STM 7.1 resistance to ground field test that the bronze floor was not conductive, but, in fact, static dissipative. How?
Because we would be grounding bronze through a series resistor network. The dissipative adhesive, not the bronze surface, would be the groundable point, and the adhesive would represent a false indication of the resistance to ground if the dissipative ground were bypassed due to an inadvertent connection to ground. Relying upon a less conductive surface as the groundable point below a more conductive surface is an imprudent concept for multiple reasons.
This may seem like a ridiculous example, except for the fact that many concrete on-grade substrates retain a high concentration of water due to the local water table. Water saturates adhesives, lowering the conductivity of the system, and changes the path to ground. This scenario occurs so often that flooring installers test concrete per ASTM 2170 for moisture, in part, to determine how vapor content and emissions in the substrate might negatively affect the adhesive.
What if this floor system were installed in a space where energized systems were resting on the floor while operating at 480 volts, three-phase. Obviously, any electro-mechanical system resting on the floor would become the groundable contact point and bypass the series resistor (dissipative adhesive) below the bronze tiles.
Another misstatement is the claim that “Flooring meets or exceeds ANSI/ESD S20.20.” The first error is the failure to recognize that flooring is only one component of a system within a program that must comply with all aspects of a standard, which typically includes many items unrelated to the flooring itself. For example, ESD flooring, whether conductive or dissipative, is often mistaken as having only to ground people and prevent charge generation on people wearing ESD footwear.
This is not the case. Most users of ESD flooring rely on the floor to ground and prevent charges on people, carts, shelves, benches, and chairs. Due to surface hardness or spacing of conductive surface particles, a particular design conductive floor may do an excellent job of grounding and charge prevention on personnel but fail at grounding mobile carts and shelving. If a circuit board manufacturer expects the floor to provide a path to ground for workstations and carts and the floor fails in this task, it cannot be described as meeting S20.20, whether or not the root cause of failure is the drag chain on the cart, the contact area of the conductive casters, or the arrangement of conductive layers or conductive particles embedded into the flooring.
If we remove the question of which standards are better or more valid or more clear, we are left with the most important question: Why would one write a specification for a specific industry and fail to mention the standard for that industry? Now we are back to the beginning: semantics, incorrect standards cited for a specific industry, and a general lack of understanding about electricity and static-control flooring.
What happens when an industry or entity like the FAA publishes a frequently updated 500-page grounding standard and specifiers, installers or facilities managers neglect to follow the standard? This question may be one for the product liability attorneys, but over the course of several discussions, liability attorneys tell me that meeting standards is a “minimum expectation.” In the case of ESD flooring and electricity, this means privileging safety equal to or greater than potential performance enhancements from increased conductivity.
In the construction trade, there is an old saying, “electricity always follows the path of least resistance.” The saying is only partially true. Electricity flows through all paths – intended and unintended. We must keep this in mind when we verify the resistance of installed ESD vinyl or carpet tiles.
If we only follow test method ANSI/ESD STM 7.1, we might overlook an unintended path to ground. STM 7.1 only requires testing the resistance of floor tiles to the ground connection specified by the manufacturer. But what if that ground connection relies on resistors or high resistance adhesive as part of its path to ground, even though the equipment racks on top of certain floor tiles are also grounding the floor?
For this reason, always test the resistance connections between the surface of tiles directly under equipment, and the connection to either the equipment racks or the pedestals of the equipment sitting on the surface. This is a case of prudently exceeding standards and test methods when those standards emphatically warn that they are not intended for evaluating safety.
The bottom line? To be safe and to protect yourself or company from liability, be sure you know what the terms mean and follow the standards specific to the industry. If you’re not sure, do your homework, ask questions or enlist an expert to help.
Endnotes
- “How Can We Eliminate Static From Operating Rooms to Avoid Accidents with Anaesthetics?,” E.I. McKesson, published in the British Journal of Anaesthesia, April 1926.
Available at https://academic.oup.com/bja/article/3/4/178/271645. - Note that proposed changes in ANSI/ESD STM7.1 would address the need to mitigate the hard line between the conductive and dissipative range.
- According to FAA-STD-019f, “conductive ESD control materials shall not be used for ESD control work surfaces, tabletop mats, floor mats, flooring, or carpeting where the risk of personnel contact with energized electrical or electronic equipment exists.” FAA-STD-019f, Lightning and Surge Protection, Grounding, Bonding, and Shielding Requirements for Facilities and Electronic Equipment, Federal Aviation Administration, published October 18, 2017.
- “Static electricity and floor resistance,” posting to the IBM Knowledge Center website, https://www.ibm.com/support/knowledgecenter/en/SSWLYD/p7eek_staticelectricity_standard.html.
