What Factors Affect Resistance
For ESD Shoes Being Tested While Worn?
by: Steve Fowler, Fowler Associates, Inc.
This article is the result of questions
about how some people when testing a specific style shoe may "fail"
and others may "pass." After several questions about shoes failing
"low" the following comments were added to the article
which appeared previously.
An ESD shoe can be only as good as
the amount of water in the shoe. This water comes from sweat as
well as other sources. When a shoe fails on low resistance after
continuos wear after having passed previously, this usually means
the shoe is saturated with either water from walking in a wet area
or from extreme sweat. Just as electrical hazard shoes should not
be worn in wet conditions prior to work on hazardous voltages, ESD
shoes should not be used for work situations after such exposure
until they have dried. WATER IS CONDUCTIVE - especially salty
Testing shoes while being worn is at
best difficult. One fact that must be remembered when shoes are
tested in a store or van environment is that the person wearing
the shoes is very important to the results. This is even more so
than when a person tests his worn shoes on a daily basis. People
dump a least a pint of water in the form of sweat into their shoes
daily. If a shoe is designed not to be dangerously conductive taking
this fact into account, then the resistance when it is dry on a
dry skin person wearing fresh polyester socks will of course be
very different than expected for normal use.
When a shoe is tested on a person who
stands on a metal plate and touches a metal button (or similar tests)
many factors are present which affect the results. A few of these
factors involve just the shoe resistance especially in its dry state.
These factors are:
1. Shoe outersole resistance and
its contact with the measuring plate
2. Shoe insole and mid sole resistance
with sufficient sweat involved and dry
3. Shoe sock liner resistance with
sufficient sweat involved and dry
4. Shoe size -The larger the
shoe the more area of contact with the metal plate and the foot
which gives typically lower overall resistances for a given shoe
design and manufacturing.
5. Sock resistance due to fabric
and level of sweat involved - If a person wears waterproof socks
they will totally insulate the foot from the shoe. While this seems
ridiculous, it is very similar to a person wearing a fresh pair
of thick polyester socks early in the morning before a good sweat
layer has formed. Cotton socks at one time were very common. Now
even the white "crew" style socks can not be assumed to be cotton.
For good contact with the shoe, the socks must be taken into account.
In fact for ESD work, cotton socks should be required. For
laboratory testing purposes, only cotton socks are allowed, they
are worn for a minimum of 2 hours prior to the beginning of tests
and moisturizing lotion is used. These precautions make sure the
testing persons are uniform in their contact resistance with the
6. Foot resistance due to skin thickness
and level of sweat involved - The dead layer of skin on people
is not a fixed thickness or moisture content. This leads to gross
differences in the contact resistance of the skin especially on
the foot with any electrode system such as an ESD shoe. Older persons
have thicker and drier skin on their feet than young people. When
using ESD shoes people with a high foot resistance should use a
moisturizing lotion on their feet before they wear shoes, which
require a specified conductance to ground during a test.
7. Body resistance - Once the
skin contact resistance is made uniform by moisture content, the
body resistance of the person involved in the tests is less important.
In fact the body resistances of most people once the dead layer
of skin has been moisturized are very close and fairly low compared
to the other resistances involved.
8. Hand resistance due to skin thickness
and level of sweat or moisture involved - Just as the skin of
the foot plays a very important part in the testing of shoe resistance
with a person involved, the hand is just as important. Some people
have extremely high skin resistance especially on their hands and
fingers. Again, the older the person the more the resistance of
the hands. For testing purposes all technicians use moisturizing
lotion on their hands prior to performing the tests.
9. Voltage and accuracy of the shoe
tester device -The test method by the ESD Association for testing
a person/shoe/floor system uses 50 volts as the test voltage. ESD
9.1 Shoe test method uses 100 volts as a test voltage. Many shoe
testers, which are used in the field, have voltages ranging from
9 volts to 30 volts. When all the above factors are considered to
affect the outcome of the resistance tests, then the addition of
a variable of applied voltage makes the test non-uniform and non-repeatable.
This voltage will not be a problem if the above factors are made
to conform to the suggestions made here.
These factors must
be considered for any shoe program to have the credibility necessary
to assure a good ESD Control System.
from Ingrid Cohen, Marketing Manager, Static Solutions, Inc.
One imput, some
hand lotions are filled with things like parabens and other emollients
that will INCREASE resistance or be harmful. Check parabens in any
search engine. Some of other lotions use salt as a thickner. Ouch!
Clorine will cause soldering problems by increasing HCL generation
in higher humidities.