You finally received your new portable ohm meter with the five-pound probe and someone showed you how to use it. Just drop the two five-pound probes on the floor, connect leads to the meter and hit the test button. That’s all you need to do. Right?
Not even close.
Let’s start with your mission. What are you doing and why are you doing it?
Hopefully your goal is to identify flooring options that meet standards like ANSI/ESD S20.20-2014. To meet standards, you need to utilize standard test methods. When it comes to ESD flooring, the test method you need to follow is ANSI/ESD STM 7.1-2013. Do you have a copy of this test method? If you don’t have a copy, you can order it here on ESDA.org.
Don’t even think about making decisions without first testing the material, following procedures in this test method. It isn’t as simple as measuring samples right out of the box. Let’s go through the entire process step by step:
There are two distinct phases to evaluating and selecting ESD flooring:
- The qualification phase
- the acceptance phase.
In this post, we’ll cover the qualification phase.
Excerpted from ANSI/ESD S20.20-2014
7.3 Product Qualification Plan
A Product Qualification Plan shall be established to ensure that the ESD control items that have been selected meet the requirements in the plan. The test methods and required limits are located in the product qualification columns in Tables 2 and 3. Product qualification is normally conducted during the initial selection of ESD control items. Any of the following methods can be used: product specification review, independent laboratory evaluation or internal laboratory evaluation…
Table 2: Personnel Grounding Requirement
|Technical Requirement||Personal Qualification Test Methods/Required Limits||Compliance Verification Test Methods/Required Limits|
(Both limits must be met)
|ANSI/ESD STM97.1||<1.0 x 109 ohms||ESD TR53|
|1.0 x 109|
|ANSI/ESD STM97.2 Peak||< 100 volts||ESD TR53|
|1.0 x 109|
|Technical Requirement||ESD Item||Product Qualification Test Methods/Required Limits||Compliance Verification Test Methods/Required Limits|
|EPA||Flooring||ANSI/ESD STM7.1||Point to Point|
<1 x 109 ohms
|Point to Groundable Point|
<1 x 109 ohms
– Electrostatic Discharge Association, S20.20-2014
The qualification phase requires you to install 12″ x 24″ samples of the proposed flooring on an insulative support material like rigid plastic sheet stock. You should install the test specimen on the support sheet in the same way the flooring would be installed in your factory. If the flooring uses an adhesive, then you need to secure it with the specified adhesive.
Most ESD flooring is sold in tiles or sheets. You must create one seam in your 12″ x 24″ sample to duplicate the kind of seam that might exist on the floor after it is installed. This will enable your tests to measure resistance values as current moves from one tile or section to the adjacent tile or section.
Last and most important: The installed samples must be conditioned at 12% +/- 3% humidity for at least 72 hours before the tests begin. The tests must be performed inside the 12% +/- 3% environment.
Why must the humidity be so low?
We need to eliminate the possibility of high humidity influencing the test results. Many materials are made with chemicals that gather moisture from high humidity, increasing the conductivity of the flooring material. In most cases, moisture-gathering chemicals become less effective over time or leach out of the material, raising its resistance. Other flooring materials require special waxes that perform well at high humidity and poorly at low humidity. Conditioning the samples at low RH will reveal these deficiencies.
Before starting qualification testing, ask the following questions:
- Does you meter have two voltage settings of 10 volts and 100 volts?
- Does your meter produce these voltage +/- 5% while under load?
- Does your meter measure at least from 1.0 x 103 to 1.0 X 1010 ohms?
If the answer is no to any of these questions, then your meter is not capable of meeting the parameters of a “qualification instrument.”
Let’s assume your meter meets the above required criteria and move on to the testing phase for resistance to ground (RTG).
Resistance to Ground Testing (RTG)
To qualify a flooring product, you will need five 12″ x 24″ samples with two groundable points per sample. The samples should be numbered from 1 to 5. All five samples must be conditioned for the full 72 hours at low humidity. Each sample will be tested 6 times from both ground points per the diagram below. This will yield 60 measurements in total.
Note If you look closely at the diagram below, you’ll note that some of the test points are very close to the ground locations; some are in the opposite corner; and at least 2 tests will probably cross the seam you created when you prepared the samples. These are important details because the purpose of this test is to determine, among other things, if the floor has any shortcomings under different circumstances. One shortcoming might be wide variances across seams vs resistive properties within the same tile.
Point to Point Resistance Testing (RTT)
RTT testing examines the surface of the floor. The qualification phase of RTT testing will tell us if the surface of floor is more or less conductive than the resistance though the thickness of the floor. Like the previous tests, each sample is tested 6 times from 2 fixed probe locations. The fixed probe locations are marked as position A and B in the illustration below.
This test will tell us what might happen if the floor were inadvertently grounded by equipment placed on top of it. In most cases resistance values are a cumulative measurement involving a less conductive surface and a more conductive adhesive or underlayment. However, the surface of some materials is more conductive than the underlayment the material is bonded to. In this scenario, the surface of the floor could form a ground circuit with a piece of nearby energized equipment, with much less resistance than intended. Standards like FAA 019f call out this and other surface tests for this very reason.
We never want an ESD floor to become grounded at a lower electrical resistance than a particular standard prescribes.
To satisfy safety concerns, we need to account for intended and unintended ground connections.
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