First Published in EOS/ESD Technology
Aug/ Sept 1991
A Proposed Test
for Floor Materials
Current flooring test procedures and the information
they provide can be misleading. Here's one proposed solution to
the floor-testing problem.
Michael T. Brandt- President, Marketing
Resources Ltd., 305 Euclid Avenue, Sheboyan, WI 53083
Stephen A. Halperin- President,
Stephen Halperin & Associates, P.O. Box 1225, Elmhurst, IL 60126
Floor Testing Series: This is the
first in a series of two articles describing a possible test methodology
for flooring materials. This method involves procedures for testing
electrical resistance, body voltage generation, body voltage decay,
voltage gradient, and voltage suppression.
Part 1 of the series will cover
general test conditions, types of samples, and particular test methods.
These test methods and procedures are for electrical resistance,
body voltage generation and body voltage decay, voltage gradient,
and voltage suppression tests. Part 2 will discuss the observations
and results of these tests and procedures.
The test procedures of this proposed
methodology are designed to specifically evaluate and compare flooring
materials intended for very sensitive electronic manufacturing environments.
They include footwear and human body factors encountered in the
environment. The testing is conducted in a manner similar to the
end use of the products in question.
Floor materials are often evaluated
and selected based on inappropriate electrostatic properties and
inadequate test procedures. Since floor material vendors have been
working with ill-defined measures, they are poorly guided when it
comes to developing and marketing products to meet user requirements.
This puts the vendors at a loss to provide adequate and useful product
Current Evaluation Criteria
There are three traditional approaches
to evaluating floor materials.
1.) Electrical resistance- indicates electrical continuity across
a surface or from surface to ground.
2.) Body voltage generation- as measured according to AATCC/ANSI
3.) Material decay testing- applicable to packaging materials.
These three approaches have their limitations
and drawbacks. Electrical resistance testing does not necessarily
indicate performance properties. The body voltage generation procedure
was developed for less critical computer and consumer environments.
Lastly, the material decay test does not indicate a floor material's
ability to dissipate a charge from an object or person. The evaluation
and discussion of ESD floor materials has often been limited to
very specific materials 8,9 or applications 10
and has often not included the human body in test procedures. 1,2
This paper proposes that a through
characterization of the electrostatic properties of ESD floor materials
should include testing of electrical resistance, body voltage generation,
body voltage decay, voltage gradient, and voltage suppression. Test
methods for measuring these properties have been adapted from eh
previously described approaches and modified to analyze floor materials
used in highly sensitive environments. Where appropriate, the procedures
simulate conditions of use.
Floor Material Samples
The tests evaluate various materials,
such as epoxy floor coatings and resilient floor coverings, with
and without floor finishes. A 4 ft. x 4 ft. sample of each floor
covering was mounted on plywood or pressboard, according to manufacturer's
instructions. If a conductive adhesive was recommended for installation,
that adhesive was used to mount the samples. Floor finishes were
applied on some samples. Ground connections (groundable points)
were also provided on each sample.
A test bed of commercial vinyl-composition
tile, installed on concrete below grade and finished with standard
commercial floor finish, is used as a reference point and is considered
the "uncontrolled" floor material.
Before testing, the floor material
samples were cleaned twice with a solution of alcohol and water.
Floor finishes were wiped with a dry, low-linting cloth. All samples
were air dried and conditioned at least 48 hours at 20 to 24% relative
humidity and 68 to 72 deg. F. The tests were then performed at 20
to 24% RAH and 68 to 72 deg. F
The body voltage generation and decay
tests are conducted using six different types of commonly-used footwear
This footwear includes: 1) shoes with insulative soles (rubber or
other synthetic soles); 2) shoes with insulative soles and conductive
heel strap; 3) tennis-style shoes with static dissipative soles;
4) deck-style shoes with static dissipative soles; 5) dress shoes
with leather soles; 6) Oxford-style shoes with conductive soles.
Prior to testing, resistance to ground plate measurements are made
of the footwear. Shoe samples are prepared by stroking the soles
six times with 100 grit sandpaper, then cleaned twice with a solution
of 70% alcohol and water.
Electrical Resistance Test
Although not a predictor of voltage
generation, resistance indicates a path of electrical continuity
allowing the flow of static charges across the floor surface or
from surface to ground. The test procedure used here adapts the
ASTM F150 and NFPA 99 procedures, but modifies the electrodes and
the applied voltages. In addition to the traditional foil-covered,
5 lb., 2-1/2 in. diameter cylindrical metal electrodes, the procedure
includes a second configuration that substitutes a conductive elastomer
for the foil and non-conductive rubber contact surface. The procedure
supplements the standard ohm-meter prescribed by ASTM F150 and NFPA
99 with a precision Dr. Theidig Milli-TO wide-range ohmmeter.
An individual floor sample is placed on the test bed and lightly
wiped with a low-linting cloth. For point to point measurements
(Rtt), the electrodes are placed 36 in. apart on the
surface of the test sample (Fig. 1). For electrode to groundable
point measurements (Rtg) , one terminal of the ohmmeter
is connected to the groundable point of the test sample. The other
terminal is connected to an electrode placed on the surface o the
sample (Fig. 2). The groundable point is defined as "the connection
(such as a ground bolt, snap, wire, or conductive adhesive) used
to attach a floor material to an appropriate ground"13.
The test voltage is applied and readings
recorded five seconds after application of the voltage. Measurements
are made at applied voltage of 10V, 100V, and 500V, with both the
foil-covered electrodes and the conductive elastomer-covered electrodes.
Five measurements are taken and the results averaged for each test
condition. The samples are tested in an ungrounded condition; however,
they are temporarily grounded between measurements.
Body Voltage Generation Test
The methodology for evaluating body
voltage generation in a test subject wearing various types of footwear
combines two procedures. The first procedure, an adaptation of the
AATCC/ANSI 134 method, measures the maximum body voltage during
a defined walking cycle. The second procedure measures the minimum
voltage obtained at the end of the walking cycle, when the test
subject has both feet placed on the floor. These two procedures
illustrate the effects of capacitance, which is directly related
to whether or not both feet are on the test surface.
Parallel Plate Capacitance. The formula
for parallel plate capacitance is:
C = k (A/d)
k= Constant (Permittivity)
A= Area of shoe surface
d= Distance between sole of foot and floor surface
At the end of the walking cycle, when
both shoes are on the floor sample, shoe surface are (A) is maximum,
and distance fro the floor (d) is minimum. Therefore, capacitance
(C) is maximum with both feet on the floor and capacitance (C) is
minimum when one or both shoes are removed from the floor surface
during the walking cycle.
The second aspect of this relationship
considers the primary electrostatic relationship of charge, capacitance,
and Q= V/C
editor's note: This may have intented
to be V=Q/C
As capacitance (C) increased, voltage
(V) decreases. Therefore, with both feet on the floor, capacitance
(C) is at maximum and measured voltage (V) will be at a minimum.
With one foot raised from the floor surface, capacitance (C) decreases
and measured voltage (V) increases. Thus, the two procedures provide
the minimum to maximum voltage range expected with the floor and
In addition to these basic voltage
measurements, the procedure includes a statistical analysis
to determine the probability of equaling or exceeding a defined
The procedure requires the following
test instrumentation: charge plate monitor with a voltage range
of 0 to 20,000V DC connected to an XY plotter and a wrist strap
(with a 1-megohm resistor); an electrostatic analyzer and personnel
voltage tester (PVT); and a constant, indicating ground circuit
monitor (Fig 3).
After preconditioning, the sample's groundable point is connected
to the constantly monitored earth ground. The test operator puts
the wrist strap on and connects the discharge end to the charge
plate monitor. The operator walks on the floor sample in front of
the PVT. The step pattern requires forward and backward steps, and
a cross-over step when changing directions. (Fig 4).
Two sets of measurements are obtained
during the procedure. The first measures voltage generation during
the walking cycle. This is defined as the maximum body voltage.
The second, defined as the minimum body voltage, measures the voltage
obtained at the end of the cycle with both feet on the floor sample.
During the walking cycle, the XY plotter
records all operator body voltages as seen through the wrist strap
by the charge plate monitor. At the end of the pattern, when both
feet are flat on the floor sample, the operator touches the PVT
device to record and enter the body voltage into the PVT memory.
A test assistant also marks the XY plotter chart to indicate the
moment of PVT measurement. The process is repeated for a total of
20 cycles, the footwear is changed, and the procedure is repeated
until all footwear and floor material samples have been tested.
The 20 highest peak voltages from the
chart recorder and entered into the PVT analyzer. Averages and standard
deviations (n-1) are calculated for the walking cycle and the end
of cycle data. A normal (Gaussian) probability distribution was
assumed for determining the probability of equaling or exceeding
a defined body voltage. To compare various floor and footwear combinations,
the statistical analysis determines the body voltage above which
there is less that 0.1% probability of being exceeded.
The probability function (f(N)) of
the Gaussian distribution is given as:
and is pictured as the normal bell-shaped
Body Voltage Decay Test
The body voltage decay procedure measures
the time required for a charge to dissipate from the body through
a given floor material and footwear combination. An ESD floor material
should dissipate any accumulated charge from the body before exposure
to sensitive parts can occur.
Body voltage decay measurements require
the following apparatus: charge plate monitor with a voltage range
of 0 to 20,000V DC connected to an XY plotter and a wrist strap
(with a 1-megohm resistor); a constant, indicating ground circuit
monitor; and an insulated, acrylic plate approximately 24 in. x
24 in. (Fig.5).
Procedure. The test subject wears the
test footwear and wrist strap and stands motionless on the insulated
acrylic plate next to the floor sample. The wrist strap is connected
to the change plate monitor. The subject is charged to +/- 5,000V
while standing on the acrylic plate. When the charging voltage is
turned off, the subject steps from the plate to the floor sample.
As the foot makes initial contact with the floor, the decay is tracked
on the XY plotter until the voltage reaches zero or until no further
decay occurs. This procedure is performed six times at +5,000V and
six times at -5,000V for each floor material and footwear combination.
Voltage Gradient Test
Voltage gradient measurements indicate
the practical dissipative characteristics of a floor material. Originally
developed by G. Baumgartner to evaluate ESD worksurfaces, the procedure
has been modified for use on floor materials. The method measures
the voltage "seen" by an object resting on the surface
when a nearby second object is charged to 2,000V. This measurement
indicates whether dissipation is across the material's surface,
through its bulk to ground, or both. If the path is across the surface,
tote boxes, carts, or other objects on the floor can be exposed
to the charge as it dissipates.
An NFPA metal electrode is placed near the material's installed
groundable point and attached to a metal plate isolated from ground.
The plate voltage is measured with a non-contacting, precision electrostatic
voltmeter connected to an XY plotter. A second electrode is placed
on a diagonal line from one (grounded) corner to the opposite corner.
Attached to this second electrode is a current-limited 2KV is applied
to the electrode. The maximum voltage indicated on the electrostatic
voltmeter is recorded (Fig. 6).
measurement is made with the sensing electrode 5 in. from the material's
groundable point and the powered electrode 5 in. from the sensing
electrode. A second measurement is made with the sensing electrode
12 in. from the material's groundable point and the powered electrode
36 in. from the sensing electrode (Fig. 7). Six measurements at
+2,000V and six at -2,000V are made for each electrode position.
The test is repeated for each groundable point on the test sample.
Voltage Suppression Test
The test procedure for voltage suppression
was developed by Dr. Joe Crowley in conjunction with preliminary
work for the EOS/ESD Association Draft Standard 4 on worksurfaces.
The test indicates a material's ability to drain a charge form a
such as a person wearing specialized footwear. As the object contacts
an ESD floor material, that charge should fully drain.
Procedure. A round, aluminum test plate
(1/8 in. thick x 6 in. diameter with an attached insulated handle)
is connected to the plate of a charge plate monitor. An XY plotter
is also connected to the charge plate monitor to record the test
voltages. With the floor sample grounded, the test plate is charged
to +/- 5,000 V and lowered onto the sample's surface for approximately
two seconds. The test plate is carefully tilted at an angle, then
lifted free of the worksurface (Fig. 8). Any residual voltage remaining
on the aluminum plate is recorded by the plotter. Any charge generated
by lifting the plate free of the surface is also recorded. Six +5,000V
and six -5,000 V test cycles are conducted at different positions
on the sample.
Evaluation of ESD floor surfaces should
be performed on parameters other than electrical resistance. The
test methodology proposed here allows evaluation across a wide range
of electrostatic properties: static generation, static decay, voltage
suppression, voltage gradient, and electrical resistance. The procedures
are designed to specifically evaluate and compare flooring materials
which are intended for very sensitive electronic manufacturing environments.
They include both footwear and the human body which are encountered
in the environment. The testing is also conducted in a manner similar
to the end use of the product in question. Combining the performance
characteristics of the materials with appropriate statistical analysis
can lean to a prediction of performance in the field and can lead
to better selection o the most appropriate floor material and footwear
combinations for the specified application.
1. ASTM F150, "Standard Test Method
for Electrical Resistance of Conductive Resilient Flooring."
2. AATCC/ANSI 134, "Electrostatic Propensity of Carpets."
3. Chase, E.W., and Unger, B.A., "Triboelectric Charging of
Personnel from Walking on Tile Floors," 1986 EOS/ESD Symposium
4. Crowley, Joseph M., and Halperin, Stephen A., "Resistance
Testing of Static Dissipative Worksurfaces," 1988 EOS/ESD Symposium
5. Crowley, Joseph M., "Floor Covering Resistances Measured
with Persons and Electrodes," and "Resistance Testing
of Carpets and Other Floor Coverings," test reports prepared
for the EOS/ESD Association Standards Subcommittee on Floor Materials,
6. EOS/ESD Association, "Standard for Protection of Electrostatic
Discharge Susceptible Items: Worksurfaces."
7. Halperin, Stephen A., "How to Select Flooring," EOS/ESD
Technology, February/March 1988.
8. Kolyer, John J., and Cullop, Dale M., "Methodology for Evaluation
of Static-Limiting Floor Finishes," 1986 EROS/ESD Symposium
9. Kolyer, John M., Watson, Donald E., Anderson, William E., and
Cullop, Dale M., "Controlling Voltage on Personnel," 1989
EOS/ESD Symposium Proceedings.
10. Lingousky, J. E., and Holt, V. E., "Analysis of Electrostatic
Charge Propensity of Floor Finishes," 1983 EOS/ESD Symposium
11. NFPA 99, "Health Care Facilities."
12. Shah, B. M., Martinex, P. L., and Unger, B. A., "Test Methods
to Characterize Tribolectric Properties of Materials," 1988
EOS/ESD Symposium Proceedings.
13. EOS/ESD Draft Standard 7.1 (Proposed), "Floor Materials-Resistive
Characterization of Materials," June 7, 1990.