Ensuring Accuracy: Why It's Critical to Clean Floors and Probes Before ESD Testing
[14 min read]
Post Summary
This post explains ESD standards and test methods and describes how to test an ESD floor to determine whether or not it complies with industry standards. To be sure test results are accurate, it’s crucial that both the floor and test probes are clean. This article explains why.
If you have any questions or would like to install a free test patch of a StaticWorx ESD floor, please give us a call: 617-923-2000. Or email: [email protected].
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- Testing for Electrical Resistance
- How Do You Know if Your Floor Meets the ESD Standard?
- Reasons For Cleaning Your Floor and Test Probes Before ESD Testing
- Which Values Best Describe an Effective Static-control Flooring Solution?
- Resources
- Terms, ESD Standards, Tests & Test Methods
- StaticWorx Test Equipment
Testing for Electrical Resistance
We measure the electrical resistance of static-control (aka: anti-static, ESD, dissipative, conductive) floors to determine the suitability of the floor/flooring material to dissipate static charges to ground, preventing damage from random electrostatic discharge events.
The resistance of a flooring material provides us with a quantitative result that tells us how well or poorly the material will transport electricity towards ground. Resistance testing also tells us whether or not there is any point in grounding a particular floor.
To meet ANSI S20.20, a floor must measure below 1 billion ohms (1.0 x 10E9)
An electrical resistance test is expressed in a unit of measure called the ohm. The greater the number of ohms, the more difficult it is for electricity to move through the material. When the number of ohms exceeds a certain level, we call that material insulative (non-conductive.) Examples of insulative floors: VCT, standard carpet, LVT, plastic mats and ceramic tile.
Fewer ohms means easier, faster movement of electricity. When ohms measure below a certain level we call that material too conductive. Some examples of floors or surfaces categorized as too conductive are stainless steel, copper sheeting, wet surfaces and carbon loaded floors with too much carbon. The ASTM limit for minimum flooring resistance is 25,000 ohms (2.5 x 10E4)
Test Methods
Both ASTM and ANSI have created test methods for testing the electrical resistance of static- control flooring materials. The two test methods are practically identical. Both methods utilize the same equipment to characterize a flooring material.
Note: Test methods describe the method of performing a test to determine compliance to a standard. Test methods should not be confused with a standard like ANSI/ESD S20.20.
ASTM defines a test method as the following: “Typically includes a concise description of an orderly procedure for determining a property or constituent of a material, an assembly of materials, or a product. The directions for performing the test should include all of the essential details as to apparatus, test specimen, procedure, and calculations needed to achieve satisfactory precision and bias.”
Standards
Standards set specific performance requirements for a given material in question. We use test methods to observe or measure a material’s properties and then reference standards to determine if the material is suitable for use as required by the standard. For example, the ANSI/ESD S20.20 standard tells us that the maximum electrical resistance of an acceptable floor is less than 1 billion ohms. (1 billion is often written as 1.0 x 109 ohms or 1.0 x 10E9 ohms).
ANSI/ESD S20.20 requires us to measure the floor’s resistance using the test method ANSI/ESD STM 7.1.
Note: ASTM and ANSI/ESD are two different organizations. However, ASTM F150 provides the same information as ANSI/ESD STM 7.1.
How Do You Know if Your Floor Meets the ESD Standard?
Testing for Charge Generation
First, it’s critical to understand that static control floors must be tested for more than just electrical resistance. Resistance measurements tell only part of the story. To determine whether a given floor will comply with ANSI/ESD S20.20 we also need to measure a floor’s charge generation properties per ANSI/ESD STM97.2 test method.
Examining multiple properties of a static-control flooring material is similar to evaluating the flammability aspects of materials. In the case of flammability, we look at how much smoke a material will generate and also how fast a flame can spread. Neither tells us the entire story, so we test for both.
There are numerous documented examples of static-control epoxy floors that measure far below one billon ohms and even less than one million ohms per STM7.1. However, some of these multi-layer floors generate several hundred volts on test subjects in ANSI/ESD STM97.2 charge generation tests. For this reason, in 2014, the ESD Association began requiring that floors measure below one billion ohms of resistance per ANSI/ESD STM7.1 and generate less than 100 volts per the ANSI/ESD STM97.2 test method. In other words, on its own a low electrical resistance does not reliably predict whether a floor will get rid of static electricity.
Alone, low electrical resistance does not reliably predict whether a floor will get rid of static electricity.
What factors do determine whether a static control floor will get rid of static?
1. The floor must contain conductive materials that allow the floor to measure below one billion ohms (1.0 x 10E9) in a resistance test regardless of humidity.
To achieve permanent (as opposed to temporary or surface) conductivity the floor must be comprised of some quantifiable portion of conductive materials, such as carbon, graphite, metal or antimony tin oxide. The material must have enough conductive material content to enable electrical continuity between the floor and ground, to conduct static charges through the material to ground, and to measure less than one billion ohms with an ohm meter.
The floor and test probes must be cleaned prior to performing this test.
2. To meet the S20.20 specification, the floor must prevent charges from accumulating on a person wearing static-control footwear when the person walks on the floor at low humidity.
Charge generation is measured in volts. To achieve low charging, the floor surface must be comprised of a low charging material with conductivity on the surface. This property is critical. A person must generate below 100 volts, at most, in this test.
Some floors marketed as static control are coated with a protective insulative urethane layer to help reduce maintenance. This insulative material generates too much static to meet the charge generation spec. In the case of generation 2 epoxy floors, the top layer of the epoxy relies on a buried conductive layer for its performance. The topcoats on generation 2 epoxy floors are high charge generators; but the top layer is thin enough for resistance readings from an ohm meter test to pass through the surface of the material as if the top surface wasn’t there. This type of conductive floor will generate 100s of volts of static electricity despite what appears to be excellent conductivity. You might equate this to a construction material burning like a wildfire while generating very little smoke.
3. Footwear is a critical component in controlling static electricity on people.
ANSI/ESD S20.20 requires the use of static-control footwear when testing charge generation. The ANSI/ESD S20.20 standard requires [not just any ESD footwear but] the use of the same footwear that will be used in the space. For example, if the client will use foot straps, the charge generation test must be performed with foot straps. The absolute limit is 100 volts.
Reasons For Cleaning Your Floor and Test Probes Before ESD Testing
Cleaning the floor and probes prior to testing helps to ensure that your test results are accurate.
NFPA floor testing probes only weigh 5 pounds. They measure 2.5 inches in diameter (4.9 square inches of surface area). These probes exert less than 1 pound of pressure per square inch. This is insufficient pressure to overcome the slightest amount of debris between the probe and the floor. A dusty floor prevents the bottom of the probe from making adequate electrical contact with the conductive material in the flooring material.
Dust also accumulates on the black rubber surface on the bottom of the probe. This becomes more obvious when you compare resistance tests performed with two 5-pound probes against system resistance tests involving a person standing on the floor, measuring the resistance from their body to the flooring ground connection. See the illustrations below. Essentially, in a system resistance test, the person becomes the probe, albeit a very large probe.
- According to a recent test conducted by an independent consultant, in the point-to-point test the highest measured resistance recorded between two probes was 5.04 x 10E8.
- When the resistance test was performed using one probe and connecting the other probe to ground, called a point-to-ground test, the highest measurement drops to 3.2 x 10E7.
- When the resistance test was performed without probes, and instead through the test subject, a system resistance test, the maximum measurement drops to 1.02 x 10E6.
Each drop in resistance is the result of better contact due to less influence from dust. In the point-to-point test, both probes are on the floor, influenced by dust. In the point-to-ground test, one of the connections is a wired connection to ground; only one probe is influenced by dust. In the system resistance test, we have much higher pressure between the human test subject (with feet acting as probes) and the floor. Additionally, foot straps are textured. This adds pressure to the contact between the heel straps and the floor. Foot straps also provide a larger surface area.
Which Values Best Describe an Effective Static-control Flooring Solution?
To meet the ESD standard described in ANSI/ESD S20.20 and used across the globe, the electrical resistance of your floor must measure below 1.0 x 10E9 (also written 109) or 1 billion ohms. There may be application-specific reasons for a specifier or for your company to narrow your specifications, though to meet S20.20 they must still be below 1.0 x 10E9.
For example, explosives manufacturing facilities prefer conductive floors, reading < 1.0 x 10E6. This is because conductive floors bleed charges to ground more quickly than dissipative flooring. FAA, telecom, PSAPs, call centers and other critical end-user spaces prefer static-dissipative flooring, measuring >1.0 x 10E6, because dissipative floors transport charges at a slower pace and are considered safer for use in spaces where people are in contact with electrified equipment.
But meeting resistance standards is not enough. To meet ANSI/ESD S20.20, your floor must also be low charge generating; in other words it must pass charge resistance tests. To pass S20.20, the floor must generate no more than 100 volts of static on a person walking on the floor, wearing ESD footwear. As with resistance testing, the standards for certain applications differ. For end-user spaces, the floor must generate no more than 500 volts. For Class-0 cleanrooms, 10 volts.
You’ve invested in ESD flooring to protect your valuable electronic components. To be sure the floor meets industry specifications and to ensure its performance, always test your floor after it’s been installed. And be sure tests are done properly, on a clean floor with clean test equipment.
Resources
Test Methods
Measurements on uncleaned floor per report:
Point to point resistance:
- Highest 5.04 x 10E8 = 504,000,000 ohms
- Lowest 3.06 x 10E6 = 3,600,000 ohms
Point to ground resistance:
- Highest 3.26 x 10E7= 32,600,000 ohms
- Lowest: 4.60 x 10E5 = 460,000 ohms
System resistance measurements per report:
Highest 1.02 x 10E6 ohms
Lowest 8.38 x 10E5 ohms
To be sure you’re testing the electrical resistance of your ESD floor properly, use a ProStat PRS 812B digital ohm meter (or equivalent test kit.) This instrument complies with the following:
The meter shall be capable of making measurements from 1.0 x 10E3 ohms to 1.0 x 10E10 ohms.
The meter shall have a circuit voltage of 100 volts (± 5%) while under load for measurements of:
- 10E6 ohms and above and 10 volts (± 5%) while under load for measurements of 1.0 x 10E4 ohms to less than 1.0 x 10E6 ohms.
The meter shall have an open circuit or under load:
- voltage of 10 volts (± 5%) for measurements from 1.0 x 10E3 ohms to less than 1.0 x 10E4
Excerpts from Test Methods
ASTM and ANSI both agree that the floor should be cleaned prior to acceptance testing. (See below from both test methods.)
From ASTM F150-06 (2018)
Standard Test Method for
Electrical Resistance of Conductive and Static Dissipative
Resilient Flooring
8.2 Installed Testing-Lightly wipe the area to be tested with a lint-free cloth to remove any foreign material prior to placing of the electrodes. The surfaces of the electrodes, prior to placing. should be cleaned with a minimum 70 % isopropanol-water solution using a clean low linting cloth.
Allow to dry. Follow the manufacturer’s recommendation as to the time after installation prior to testing. Prior to the initial installed test the floor should be cleaned per the manufacturer’s recommendation and be given sufficient time to dry completely.
From ANSI/ESD STM7.1 Test Method
For the Protection of Electrostatic
Discharge Susceptible Items
Flooring Systems
Resistive Characterization
6.2.1.2 As part of acceptance testing, new floors, mats, and floor finishes shall be cleaned per the manufacturer’s recommendations before testing for resistance.
6.2.1.3 Perform tests at ambient humidity.
6.2.1.5 Place the resistance measurement electrode on the surface of the flooring system being tested, a minimum of 1 meter from the grounding connection. If the sample has a seam, do not place the electrode on the seam. If the test position falls on a seam, move the electrode slightly off the seam, further from the ground connection. Perform the following steps.
- Set the voltage to 1 0 volts
- Energize the measurement equipment. If the indicated resistance is less than 1.0 x 10E6 ohms, record the value after 5 seconds and continue with the next specimen or next step of the procedure. If the indicated resistance is equal to or greater than 1.0 x 10E6 ohms, de-energize the equipment.
- Energize the measurement equipment at 100 volts. Record the resistance after 15 seconds or as defined by the meter electrification period and continue with the next specimen or next step of the procedure.
NOTE: If switching the test voltage to 100 volts results in a resistance reading of less than 1.0 x 10E6 ohms, then the reading made with the 100-volt test voltage is to be used.
Terms, ESD Standards, Tests & Test Methods
- Megohm meter: A megohm meter is used to test electrical resistance of materials including static-control floors. Per ANSI, the meter must be capable of measuring at least as low as 1000 ohms (1.0 x 10E3) and as high as 10,000,000,000 ohms (1.0 x 10E10)
- Ohm: unit of measure for electrical resistance. More ohms = more resistance
- Megohm: one million ohms.
- 2.5 x 10E4 = 25,000 (minimum electrical resistance of a floor per ASTM F150)
- 1.0 x 10E6 = one million = 1 megohm
- 1.0 x 10E9 = one billion = 100 megohms
- 1.0 x 10E7 = 10 million
- 1.0 x 10E9 = 1 billion ohms (upper limit under ANSI guidelines)
- ANSI/ESD S20.20: Outlines an ESD control program for limiting electrostatic discharges that harm electronics and electric parts.
Test Methods
- ANSI/ESD STM7.1: ANSI test method for measuring electrical resistance. To meet ANSI/ESD S20.20, the material must measure less than one billion ohms.
- ANSI/ESD STM97.1: ANSI test method for measuring system resistance, or the total resistance of a person standing on a floor wearing ESD footwear. To meet S20.20, the system must measure less than one billion ohms.
- ANSI/ESD STM97.2: ANSI test method for measuring static charge generation on a person walking on a static-control floor while wearing static-control footwear. The result is measured in volts. To meet S20.20, the measurement must be less than 100 volts.
- ASTM F150: ASTM test method for measuring electrical resistance.
StaticWorx Test Equipment
Analog Surface Resistance Test Kit
StaticWorx Analog Surface Resistance Test Kit is a portable battery-powered instrument designed to measure resistance point-to-point (RTT) and surface to ground (RTG). The meter is equipped with an automatic test voltage selector. The test voltage will switch from 10V to 100V should the measured resistance exceed 1 x 10E5 ohms.
StaticWorx Analog Surface Resistance Test Kit is a portable battery-powered instrument designed to measure resistance point-to-point (RTT) and surface to ground (RTG). The meter is equipped with an automatic test voltage selector. The test voltage will switch from 10V to 100V should the measured resistance exceed 1 x 10E5 ohms.
Walking Test Kit System
Our walking test kit system is a unique electrostatic data analysis device for use with the Fieldmeter/Charge Plate Monitor Set. It records, plots, analyzes and automatically constructs reports of body voltage generation, electrostatic decay, voltage retention, ionizer performance and other static measuring functions. Its analytical features document and automatically calculate projected levels of typical Human Body (HBM) voltages. It helps determine the risk of equaling or exceeding damaging or hazardous HBM discharge voltages in static sensitive facilities.
Our walking test kit system is a unique electrostatic data analysis device for use with the Fieldmeter/Charge Plate Monitor Set. It records, plots, analyzes and automatically constructs reports of body voltage generation, electrostatic decay, voltage retention, ionizer performance and other static measuring functions. Its analytical features document and automatically calculate projected levels of typical Human Body (HBM) voltages. It helps determine the risk of equaling or exceeding damaging or hazardous HBM discharge voltages in static sensitive facilities.
Portable Walking Test Kit System
Our walking test kit system is a unique electrostatic data analysis device for use with the Fieldmeter/Charge Plate Monitor Set. It records, plots, analyzes and automatically constructs reports of body voltage generation, electrostatic decay, voltage retention, ionizer performance and other static measuring functions. Its analytical features document and automatically calculate projected levels of typical Human Body (HBM) voltages. It helps determine the risk of equaling or exceeding damaging or hazardous HBM discharge voltages in static sensitive facilities.
Our walking test kit system is a unique electrostatic data analysis device for use with the Fieldmeter/Charge Plate Monitor Set. It records, plots, analyzes and automatically constructs reports of body voltage generation, electrostatic decay, voltage retention, ionizer performance and other static measuring functions. Its analytical features document and automatically calculate projected levels of typical Human Body (HBM) voltages. It helps determine the risk of equaling or exceeding damaging or hazardous HBM discharge voltages in static sensitive facilities.
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Learning Center Articles
- ESD Basics
- Installation & Maintenance
- Selecting & Specifying an ESD Floor
- Technical Information
- 7 Common Mistakes Selecting an ESD floor
- A Guide to ESD Flooring Selection
- Avoid Costly Failures: What You Need to Know When Specifying ESD Flooring
- Choosing ESD Flooring for:
- ESD Footwear: What Is It and When Is It Necessary?
- ESD Footwear for Electronics Manufacturing and Handling Applications
- Facility Managers’ Guide to Selecting ESD Flooring
- The Need for Due Diligence in Specifying Static-Free Flooring
- Standard of Care for Specifying Floors in Mission-Critical Spaces
- Understanding the Hidden Costs of ESD Flooring
- The Case Against Overly Conductive Flooring
- Conductive vs Dissipative
- Electrical Resistance
- Electrical Resistance in Mission-Critical Spaces
- Ensuring Accuracy: Why It’s Critical to Clean Floors and Probes Before ESD Testing
- ESD Standards and Test Methods
- Resistance, Resistivity, and Real World Application
- Walking Body Voltage
StaticWorx high-performance static-control floors protect electronic components, explosives, and high-speed computers from damage caused by static electricity. ESD flooring is part of a system. Choices should always be based on objective, researched evidence. When you partner with us, we look at all possible items that may need to integrate with the floor, and, focusing on your goals and objectives, help you find the right floor for your application.