Electrode contact can affect a floor resistance reading. ANSI/ESD STM97.1-2025 notes that conductive rubber electrode contacts can warp over time and change measurements. Earlier research illustrates why contact matters: the National Bureau of Standards’ 1959 study, Conductive Flooring for Hospital Operating Rooms, found that electrode contact contributed substantially to the resistance measured in its flooring samples. Changing the electrode could change the reading even though the flooring itself had not changed.
The study provides a detailed examination of how electrode contact and test conditions affect floor resistance readings. That remains useful context for ANSI/ESD STM97.1-2025, which recognizes the effect of warped contacts but identifies no standardized method for checking whether this has occurred. The historical research examined different electrode configurations, rather than rubber-pad warping specifically. It helps explain the measurement problem without establishing a procedure for detecting warped contacts or replacing current testing requirements.
For a facility’s ESD program team, the practical question is whether different resistance readings reflect the flooring or the way it was tested. Electrode contact is one influence to consider when evaluating measurement uncertainty, including whether a shared electrode condition creates correlation between readings. Craftsman Concrete measures floor resistance and reviews the equipment and test conditions to help determine whether further testing or corrective action is needed.
Why the measurement configuration matters
Resistance depends on the test conditions
The study reported that the resistance of the conductive flooring materials depended greatly on the voltage gradient, the type and shape of electrode, and the time and frequency of the applied voltage. A resistance reading therefore describes the floor under a particular set of measurement conditions. Those conditions matter when comparing results.
The study’s guiding principle: simulate the conditions of use
The study’s fundamental principle was that a material whose properties depend on the conditions of measurement should be measured by methods that simulate, as closely as practicable, the conditions under which the material is expected to function.
How the conventional reading was timed
The study described the conventional method of measuring an installed floor’s electrical resistance, such as the method specified in National Fire Protection Association (NFPA) No. 56, as effectively a “steady state” method. Resistance was measured several seconds after the direct voltage was applied.
The time at which the reading was taken was therefore part of the measurement configuration.
What electrode contact adds to a reading
Contact resistance exceeded internal resistance in the tested materials
Electrical resistance occurs at the contact between an electrode and the flooring, as well as within the flooring material itself. For each flooring material tested, the study found that contact resistivity with the NFPA-type electrode was much greater than the material’s internal resistivity.
A two-electrode reading includes the contacts
A two-terminal test using two electrodes includes resistance at the electrode interfaces and resistance through the material between them. It does not isolate the flooring material’s internal or volume resistance.
A difference between two readings therefore cannot automatically be attributed entirely to the flooring material. The electrode contact can also differ between test setups.
What changing the electrode did in the study’s tests
The study measured its flooring samples with different electrodes and substantial changes in the force applied to them. These comparisons used a 500 V bridge at an ambient relative humidity of 30 to 50 percent. The findings are specific to the tested materials and those conditions.
Force on the resilient NFPA electrode
For all materials tested, large changes in force did not appreciably affect the resistance measured with the resilient NFPA electrodes.
The study concluded that these electrodes, which weighed only 5 lb and were reasonably portable, could simulate the much heavier objects likely to be encountered in operating rooms. This finding concerned the resilient electrodes used in the study.
Hard electrodes on hard-surfaced floors
On the tested oxychloride, concrete terrazzo and ceramic tile samples, hard electrodes made much less actual contact with the surface than resilient electrodes.
The resistance measured with hard electrodes was higher by factors as large as 10 or more, and in one case by a factor of 100. These findings concern the hard-surfaced samples tested and do not establish the same effect for resilient floors.
Electrode spacing in the studied samples
Increasing electrode spacing from 1 to 3 ft did not appreciably affect the total measured resistance of the study’s samples. The increase raised the internal resistance by less than 50 percent and did not affect the contact resistance.
The study gave two reasons for the limited effect:
- Contact resistivity was high and independent of electrode spacing.
- The internal resistance between the electrodes depended logarithmically, rather than linearly, on the ratio of electrode diameter to electrode spacing.
The finding is limited to the tested samples and the 1 to 3 ft spacing range. It does not establish that spacing has no effect on other materials or over larger distances.
What this means when comparing floor test results
Before interpreting different readings as a change in flooring performance, compare the electrode type, contact condition, test voltage and reading time.
Craftsman Concrete recommends reviewing these details alongside the applicable testing requirements before deciding whether further testing or corrective action is needed. The historical findings do not justify changing the electrode spacing or equipment prescribed by the current test method.
Frequently Asked Questions
Electrode contact can affect measured resistance, and a shared electrode condition may create correlation between readings. Craftsman Concrete reviews these influences as part of a measurement uncertainty evaluation and advises the facility’s ESD program team on whether equipment checks, further testing or corrective action may be needed.
The 1959 National Bureau of Standards study, Conductive Flooring for Hospital Operating Rooms, found that measured resistance depended on voltage gradient, electrode type and shape, and the time and frequency of the applied voltage. For the tested materials, electrode contact contributed substantially to the reading, so changing the measurement setup could change the result without changing the floor.
No. It includes resistance at the electrode contacts as well as resistance through the flooring material between them. In the 1959 NBS study, contact resistivity with NFPA-type electrodes was much greater than internal resistivity for each flooring material tested.
Hard electrodes made less actual contact than resilient electrodes with the tested oxychloride, concrete terrazzo and ceramic tile samples. Measured resistance was higher by factors as large as 10 or more, and in one case 100.
These comparisons used a 500 V bridge at 30 to 50 percent relative humidity. The factors describe those tested materials and conditions, rather than a general multiplier for other floors.
Not for the study’s samples. Internal resistance increased by less than 50 percent, while contact resistance did not change. Because contact resistance contributed substantially to the reading, the total measured resistance changed little.
The finding is limited to the tested samples and spacing range. Current testing should use the electrode spacing prescribed by the applicable method.