To reduce the instances of static discharge, manufacturers rely on two relatively low-tech and low-cost lines of defense: static-dissipative (SD) flooring and footwear used in conjunction with wrist straps fitted with 1-MW resistors. Unfortunately, this combination may not be a practical solution in certain manufacturing environments.
While wrist straps are an effective way to combat static electricity, their usage can be limited by the tasks performed by employees. Indeed, workers responsible for packaging or moving equipment between work areas find that wrist straps impede their mobility.
In virtually all environments, the combination of SD flooring and footwear is an effective way to combat static discharge. However, the flooring and footwear system must form a conduction path from the person’s body to the ground with a resistance no higher than 109 W.
What’s in a Shoe?
In essence, SD shoes reduce the accumulation of excess static electricity. The footwear drains static charges from the operator’s body directly through the soles to the floor material, then to ground. The concept is to drain the charge off the body fast enough to prevent static buildup, yet slow enough to avoid being conductive in nature.
When it comes to SD footwear, there is no one-size-fits-all solution. Some shoes offer slip-resistant soles and others feature steel toes; some shoes provide both safety features. Styles range from dress shoes to tennis shoes, and quality varies from hand-crafted footwear using top-of-the-line leather to SD slippers with canvas tops.
In short, select shoes that are best for your situation. Regardless, all SD shoes should be durable and comfortable, with proper insole cushioning and arch support.
Electrical Resistance
The National Safety Council has accredited the American National Standard Institute’s (ANSI) standards relative to SD footwear (ANSI Z41-1991 SD Type 1 standards). These standards require resistance in the range of 106 to 108 W (1 to 100 MW)—the range of SD footwear.
According to ANSI’s Method of Test for Electrical Resistance of Shoes, footwear should be tested in a controlled environment at 70°F and 50% humidity for 24 hours or more prior to measurement.
The resistance of a pair of shoes to ground is measured with a test setup that has sufficient current limitation to prevent shock should the wearer present a low-resistance path to ground (Figure 1). During the test, the wearer stands on a stainless steel ground plate that accommodates the entire heel and sole of both shoes. The test setup supplies a DC voltage of 50 V. The fixed 50-V potential is provided by a power supply that is current-limited to 5 mA for shock protection should the voltmeter or reference resistor (RE) fail.
The voltmeter displays three significant digits or more on all measurements. The voltmeter and RE have a combined paralleled resistance (RP) of 1 MW nominal. Hand contact is made to a conducting rod that produces good body contact. The equation to calculate the shoe resistance (R) is:
R = 50-V RP
V
Periodic checks ensure that the footwear continues to meet your requirements. Wall-mounted footwear checkers placed at the entrance to the production area enable operators to verify their footwear resistance to the ground. In production areas with automatic doors, this footwear checker can be built into the door control, providing access only if the footwear resistance is within a set limit.
Proper care of SD footwear will ensure years of wear. Shoes should never be worn outside the production area and must be cleaned with soap and water to remove the accumulation of substances that could change resistivity.
About the Author
Ken Dille is the director of research at Red Wing Shoe. He participated in the development of the footwear standards issued by the ESD Association for the Protection of Electrostatic Discharge Susceptible Items—Footwear Resistive Characterization (ANSI Z41-1991). Red Wing Shoe, 314 Main St., Red Wing, MN 55066, (612) 388-8211.
Copyright 1998 Nelson Publishing Inc.
September 1998