Test Methodologies

A. Measurement of Body Potential
Since body potential is taken in this work to be the criterion of electrostatic performance in a floor/shoe system, it is imperative that there be an understanding of the way that it is generated, measured, and reported. First, the measuring equipment must possess certain basic properties. It must not constitute in itself a significant electrical part of the body/footwear/floor system and its frequency response must be sufficient to capture the voltage pattern without distortion. Both of these conditions were satisfied by the equipment used in this work. It is of interest to note here that some test runs were performed with bandwidths of both 5Hz and 100KHz with no discernible difference in peak value readings. Peak values were determined in two ways, from a new versatile peak hold LED array instrument and by observation of an oscilloscope trace. For the body of the work, a 100KHz bandwidth was used.

The manner in which the test voltage is generated is important. Figure 2 is a sketch of the means.

A person holding a probe walks on a test surface wearing test shoes and his peak voltage is recorded. This procedure has been rather generally used for a large number of years. Except for the peak voltage criterion, it is the same as that required by AATCC TM-13412, which calls for a damped response, and it is substantially the same as a Work in Process standard of the ESD Association. Basically, both say the same thing: if you want to find out what happens when you walk on a floor, walk on it and see.

The differences among various methodologies presented in the literature reside in the prescribed method of walking for test purposes. While this is not usually a significant factor, it can be. In this work we have adopted a brisk natural gait for tests made in the field and a brisk, short, slightly high step for laboratory work where one can hardly walk naturally on a three or four foot square test specimen. This lab step produces, as observed on an oscilloscope, the highest voltages short of doing anything really wild.

These procedures clearly exaggerate the danger of any real human body discharge, at least statistically, as they involve high generation motions and define peak values of very short duration which, in any likely real life situation, would not be seen by a sensitive device.

B. Shoe Resistances
Three types of resistance measurement have been used to characterize the resistance properties of shoes. The ESD Association has a method S9.18 which involves lining a shoe with foil, loading it with metal shot, and measuring the resistance to a base plate from the foil. We have found that this method does not adequately represent the body resistance from shoe to ground in normal wear. We have not used that method in this work. Previous work (2,3) has demonstrated the measurement of body resistance to ground using an actual body. This is also the general methodology of the ANSI Z-419 standard and is one type of evaluation we use here. The third method is one we also use - the measurement of shoe sole surface resistance. This has been described in other work (3) but has been slightly modified by the use of a convenient step-on floor test unit which gives rapid, reproducible results and is suitable for on site testing. It may also be hand held as shown. This procedure is diagrammed in Figure 3.