Procedures For The Design, Analysis And Auditing Of Static Control Flooring/Footwear Systems
Stephen L. Fowler
Fowler Associates, 3551 Moore-Duncan Hwy, Moore, SC 29369
Tel: 864-574-6415, FAX: 864-576-4992, Email:
William G. Klein
K&S Laboratories, 2026 Bay Rd., Stoughton, MA 02072
Tel: 617-341-8331, FAX: 617-341-8331, Email:
Larry Fromm
Hewlett Packard, 1501 Page Mill Rd., Palo Alto, CA 94304
Abstract
It is the purpose of this paper to show that the electrostatic performance of footwear/flooring systems, defined as the electrostatic potential of personnel arising out of the use of these systems, can be predicted with adequate precision based on component resistance data alone, and further to present resistance testing methodologies which are at once more relevant and more reproducible than most in common usage today. Conclusions, which are to a considerable extent a matter of opinion though based on hard data, suggest that inappropriately defined criteria and overly stringent specification are significant problems today to users, suppliers, and auditors.
Figure 1: Equivalent Circuit
Figure (1) shows a highly simplified "equivalent" circuit7 of a person walking on a floor surface. It is presented here not as a model from which to make calculations, but as a demonstration tool to indicate the complexity of the general problem and as a basis for useful further simplification in the special case of interest here, a high degree of static control resulting in very low body voltages. The static potential on the individual is the result of the interfacial EMF's due to triboelectrification, the surface neutralization at the foot/floor interface where the foot is down, and the flow of current through the same interface in response to a body potential to ground.
While it is conventional to consider this flow, and therefore the body resistance to ground, to be the main controlling factor in limiting body voltage this is strictly true only for body charges originating from sources other than the shoe and floor. In order to segregate the effects of dissipation to ground and minimization of surface accumulation of charge, body voltages under dynamic conditions were measured with the various flooring/footwear combinations in a normal manner and also with the body insulated from ground by nonconductive shoe inserts. Maximum body voltages will be quantitatively characterized by shoe sole resistance, floor surface resistance, and body to ground resistance. It will be shown that, for the resistance levels used in static controlled systems, the surface resistances of the sole and floor are the main controlling factors. Since our interest here is more than academic, it will be necessary to define and justify our test methodologies for both body voltage and resistances.
Background and Introduction
It is generally accepted that the resistance and triboelectric properties of footwear and flooring materials together constitute the main parts of the system which limit the electrostatic body voltage of a person walking on the floor. Charge decay time may also be inferred from resistance data. Unfortunately, the use of resistance to predict or define quantitatively the electrostatic performance of flooring/footwear systems has been fraught with many problems. The problems are not trivial because the industry-wide failure to establish appropriate standards for the measurement of components and the performance of systems and the consequent use of various methodologies yielding significantly different values have often led to serious difficulties. These difficulties have resulted, for example, in both failed installations and expensive claims of failure not based on "true" criteria of performance. It is not possible to set up universally useful standardized criteria for product design, field performance, or auditing procedures without a significant degree of basic knowledge and an agreement within the ESD community as to the specific, quantifiable objectives toward which these criteria are aimed. With different manufacturers and different users playing by different rules, chaos has resulted.
Past work (2,3) has addressed some of the difficulties and has suggested solutions in defining relevant criteria for the design and evaluation of static controlled flooring, footwear, and flooring/footwear systems. It has not, however, had any noticeable effect on the way that floors and shoes are specified and tested, nor has there been any systematic effort to reconcile the troublesome differences. It is well understood that the rule of thumb for personnel voltages of a hard grounded worker are as follows: at 100 Meg W the personnel voltages may be over 100 Volts ; at 10 Meg W personnel voltages will usually be less than 100 Volts; at 1 Meg W the expected personnel voltages are in the 10 Volt range. Flooring and footwear systems are more complex than a simple wriststrap grounded situation. The current work expands on past work, introduces new methodologies which are both reproducible and relevant, and verifies the assertions made with quantitative data from both the laboratory and operative factory installations. A critical examination will be made as to how some currently used test methodologies fit into this picture, specifically ESD 7.1, ASTM F-150, NFPA 99, ESD 9.1 and IEC 1340-4-1. It is hoped that this work will be sufficiently intriguing to lead to a concerted effort to develop and promulgate a performance oriented set of specifications and more universally accepted evaluation and auditing procedures.
This work is aimed equally at those whose good fortune it is to have the freedom to write specifications for new installations and those tortured souls who must make critical choices and compromises in the utilization, modification, or scrapping of existing non-conforming flooring.
Basic System Analysis
In order to intelligently approach the issue of specifying and evaluating the resistance properties of footwear and flooring as they affect body voltages, it is necessary to have some reasonable idea as to how these properties act and interact together with other pertinent system parameters. This is a highly complex problem for which there is no easy solution. Briefly, some common deficiencies of often cited analyses consist of:
- the concept of linear, lumped parameters when they are in fact neither linear nor lumped;
- neglect or inadequate definition of important variables; and
- the use of a steady state approach to a dynamic event.
The list could go on. An example of this sort of specious reasoning is the popular representation of the human walker as a parallel plate capacitor with constant charge. Although it is tempting to use such a configuration, which sort of looks like a foot on the floor and is simple electrically, it suffers from all three deficiencies mentioned above and has virtually no analytical value. It is well beyond both the scope and purpose of this paper to delve deeply into this issue. Rather, we shall simply attempt to put the problem into a proper perspective so that the simplifications used to deal with the area of very low personnel static will be both credible and useful.
