A static safety audit also requires us to consider the people working in the hazardous area. If the human body is insulated from contact with the ground, either by nonconducting shoes, insulating floors or both, static charge can start to accumulate as the person walks along the floor. After just a few steps, a fairly high potential may develop (especially in low-humidity atmospheres); if the person then approaches a conductive material at either ground or a lower electrical potential, a spark may occur. It's generally thought such sparks can reach energies as high as 30 mJ, sufficient to ignite almost all flammable vapors and gases, and many sensitive combustible powder (dust) atmospheres. These sparks can be avoided by using ground wrist-straps for sedentary workers, or static dissipative (SD) safety shoes for personnel who need to move around to carry out their tasks. In the case of the latter, it's also necessary for floor materials to be sufficiently conducting to allow a "ground leakage" path to safely dissipate any static charge as each foot contacts the ground.
In the U.S., SD footwear is manufactured according to ASTM F2413-05, which specifies maximum and minimum levels for ground leakage via the shoe. Leakage should be low enough to allow static charges to relax to ground but high enough to provide some protection against electric shock. SD footwear can be checked for ongoing effectiveness using test stations (Figure 3). These types of devices help prevent incorrect shoes being used in static-sensitive environments.
The dangers of static electricity in hazardous areas demand a rigorous approach to plant, process and personnel safety. As the speed and scale of modern manufacturing and distribution techniques increase, and the range of materials used and processed grows, such an approach to safety becomes even more important. So, gain a greater understanding by reviewing NFPA 77 and other industry-specific codes of practice that provide practical guidance for specific situations.
GRAHAM TYERS is president of Newson Gale, Inc., Jackson, N.J. Email him at firstname.lastname@example.org.