n 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.
This creates multiple problems encompassing product liability, economic loss, failure to perform and in compliance with industry standards.
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
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:
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.
“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.”
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.
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.
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.
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.
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
1.0 x 106 at 10 volts.
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.
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.”
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.
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.
- “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.