What to Know Before Buying a Static Control
(ESD) Carpet                               Read the article in PDF format

When selecting ESD (often incorrectly called low kV or computer grade) carpet tile, facilities managers and specifiers often forget to factor real-life environmental issues into the equation. Such seemingly minor oversights can quickly become costly mistakes when people walking on the floor fail to comply with personnel grounding procedures or when normal wear-and-tear causes the floor to lose its static control properties, rendering it useless—or worse, turning it into a static generator.

Understanding Charge Generation

Static electricity is generated by contact and separation. Any time two materials interact then separate, an exchange of electrons occurs between the two materials. If one or both materials are either nonconductive or insulated from ground, one or both retain a static charge. If a person or object gains electrons, it becomes negatively charged; if it loses electrons it becomes positively charged. This electron transfer is known as “triboelectric charging.” Certain materials demonstrate strong, repeatable tendencies and always charge to a particular polarity (positive or negative) when frictioned by other materials.

When people walk across a floor, their shoe soles interact with and separate from the floor surface and either strip or deposit electrons. This phenomenon is known as “walking body voltage.” When a body voltage test detects a negative static charge on a person, he or she has accumulated a negative charge from the floor. A positive charge results from a similar scenario involving different material compositions.

Walking body voltage or “triboelectric compatibility” is not determined by measuring the conductive properties of a material. Conductivity involves a flooring material’s ability to discharge stationary and moving conductors. The conductivity of an ESD floor influences charge accumulation only when moving persons wear special conductive footwear. In the absence of conductive footwear, the conductivity—or resistance to ground—of a floor has little or no impact on static charge accumulation on people.

A fault tolerant floor grounding system can be achieved by installing ESD flooring materials with a combination of antistatic and conductive properties.

Test Flooring Based On Real World Conditions

In a recent study comparing ESD Carpet Tiles, PVCbacked carpet tile from two manufacturers demonstrated excessive tribocharging, while ESD carpet tile with conductive thermoplastic backing, on the other hand, significantly decreased walking body voltage— even when heel straps were not used. These results call into question the validity of choosing conductive flooring based solely on controls, specifications and procedures that will not be duplicated in the actual static sensitive environment.

When evaluating ESD flooring, buyers should consider whether the performance of the floor will rely upon the use of heel straps or other types of conductive footwear. In labs, server rooms, call centers and technical environments where extremely sensitive electronic equipment is commonly handled or operated, some form of ESD mitigation is necessary—yet in these environments it can be all but impossible to mandate or monitor employee compliance in the use of heel straps or other special static control footwear. Often, these environments are populated by people wearing ordinary static generating footwear; this situation challenges the capabilities of many static control materials and adds complexity to the process of selecting the right floor – particularly for labs, data centers and computer rooms.

In cases where special footwear compliance may be difficult or even impossible, buyers would be smart to purchase flooring based upon meeting not three but four electrical testing parameters: Resistance to ground (should be below 1,000,000 ohms); total system resistance (should be below 35,000,000 ohms); static voltage measurements in combination with a person with conductive footwear (should be less than 50 volts or .05 kV); and static voltage measurements in combination with a person wearing uncontrolled footwear (should be below .5 kV). Static control performance parameters should be examined closely to avoid the misapplication of low Kv computer grade flooring like Dataguard or SDT into mission critical spaces where an ESD event could alter, interrupt or cripple a computer reliant operation. A rule of thumb that will help avoid buying the wrong material is to stay away from any flooring if the specification states an upper electrical resistance range over 10 million ohms (< 1.0 X 107); for example a specification describing electrical properties between 2.5 X 104 and 1.0 X 109 should not be used because the electrical range is too broad and the upper limit too high.

Most ESD Flooring Performs Differently In the Installed Environment

Some processes used in carpet manufacturing operations temporarily alter the finished flooring surface. For instance, antistatic coatings are sprayed on yarn and fibers, reducing static during the actual making of carpet in the carpet mill. Antistatic coatings used to assist manufacturing are temporary; they are only used to enhance the handling of yarns that would otherwise hold too much static to be processed on high speed textile machinery; after carpet is installed, steam cleaning or traffic from normal usage quickly removes these coatings. An antistatic coating may remain on the carpet just long enough to prevent significant walking body voltages during lab testing and initial floor certification. A lab report showing low kV antistatic properties may be flawed because the tested samples were not washed before antistatic testing was performed. A few months after installation, that same carpet will lose its temporary antistatic properties and generate thousands of volts on mobile personnel.

Accelerated Life Testing Can Reveal Product Weaknesses

Damage from rolling chairs is one of, if not the biggest problem facilities face after their ESD carpeting is installed. Rolling chair casters destroy or “mash” the carbon fibers in conductive carpeting, rendering a once conductive product an insulator. When subjected to a chair caster test, two out of three ESD carpet designs show total electrical failure. At present, only one manufacturer guarantees ESD performance after chair cast exposure; When buying antistatic or conductive carpet, keep in mind  that a lifetime-time electrical warranty cannot be interpreted as immunity to caster rolling damage. The warranty means nothing unless it specifically states that it is lifetime even when the carpet is exposed to abuse from chair casters. For these reasons, a chair caster test should always be part of the evaluation process. The chair caster test, used by carpet mills to evaluate the surface wear and appearance of yarn systems, simulates the abuse that results from one to two years of task chairs rolling over a carpet’s surface in a normal lab or assembly environment. A properly conducted test should expose floors with conductive yarn systems to at least 100,000 cycles. Recent tests revealed significant weaknesses in all carpet tile designs with PVC backing. In fact, after the chair caster test, only one conductive carpet design met the recommended requirements of ANSI/ESD S20.20. A lack of knowledge about conductivity and charge generation creates an atmosphere of confusion for buyers, particularly for those who do not specialize in ESD prevention or hold an electrical engineering degree. This confusion is compounded by misconceptions about what constitutes proper testing procedures. Despite notions to the contrary, no one test provides a complete or accurate picture of a floor’s ability to eliminate static charges. When selecting an ESD carpet, buyers should evaluate the carpet’s electrical properties, by testing as described in ANSI/ESD S20.20, as well as the environmental conditions in which it will be used. To ensure ESD protection over the life of the product, buyers and specifiers are well-advised to investigate the carpet’s long-term performance and should always ask to see the results of a rolling chair caster test.

A fault tolerant floor grounding system can be achieved by installing ESD flooring materials with a combination of antistatic and conductive properties; Where compliance with grounding protocols may be an issue, buyers would be smart to purchase flooring based upon meeting not three but four electrical testing parameters: Resistance to ground; total system resistance; voltage measurement in combination with a person with conductive footwear; and voltage measurement in combination with a person wearing uncontrolled footwear; Some processes used in carpet manufacturing operations temporarily alter the finished flooring surface, causing detrimental changes in the electrical performance of the carpet after installation; A chair caster test should always be part of the evaluation process. When subjected to a chair caster test, two out of three ESD carpet designs show total electrical failure. At present, no manufacturer promotes performance after chair cast exposure; a lifetime-time electrical warranty cannot be interpreted as immunity to caster rolling damage. The warranty must specifically state that chair caster damage will not affect electrical performance.

Dave Long has been solving static problems in companies worldwide for more than twenty-five years. His articles have appeared in numerous trade and technical journals and he frequently leads box-lunch seminars for architects and designers. Dave can be reached by email at dave@Staticworx® .com.