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Fowler Associates Labs

 

 

Static Fire Stories Articles & Technical Papers Current News

What Factors Affect Resistance For ESD Shoes Being Tested While Worn?
by: Steve Fowler, Fowler Associates, Inc.
sfowler@sfowler.com............... http://www.sfowler.com

UPDATED: 2/06/03

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 water.

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 shoe.

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.

UPDATE: Comments 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.

 

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