Paints, coatings, gaskets, seals and other nonconductive materials can be sufficiently insulating to prevent safe static dissipation. The extent of charge generation current is usually very low, typically no greater than 1×10-4 A; however, on isolated conductors this charge can quickly build up to a very high potential, with voltages in excess of 30 kV not uncommon. Depending on the capacitance of the object, this may result in significant levels of energy available for discharge, well above the MIE of the surrounding flammable atmosphere. Typical MIEs vary according to whether the flammable atmosphere comprises vapor, dust or gas, but many commonly used solvents and other flammable chemicals have MIEs well below 1 mJ (Table 1). If the isolated conductor then comes into proximity with another object at a lower potential, much of this energy could be unleashed via the air gap in the form of an incendive spark. Of course, static ignition of the flammable atmosphere also requires a suitable concentration of fuel (vapor, dust or gas) in the air. For the purposes of safe plant design, though, the very fact there's an identified flammable atmosphere should suggest that this is possible or likely.
Three main international technical standards for static control have been developed and maintained by panels of fire prevention and process safety experts around the globe. NFPA 77 (North America) and Cenelec CLC/TR: 50404 (Europe) both draw attention to a range of hazards, illustrating static control practices for a diverse range of industrial processes. The American Petroleum Institute (API) 2003 standard focuses on hazards more specific to the petroleum industry. The guidelines propose maximum processing rates, recommended charge relaxation times for conductive and nonconductive liquids and, most importantly, the maximum level of resistance recommended for static dissipative circuits.
The standards have a high degree of commonality concerning conductive metal grounding circuits. For such circuits, which encompass the plant equipment at risk of accumulating charge and the route to ground, most standards recommend a maximum resistance of 10 Ω. The rationale is that conductive metal circuits in good condition have a somewhat lower resistance. If a circuit is compromised due to faulty connections caused by long-term degradation, corrosion, damage or operators not following correct procedures, its resistance will exceed 10 Ω. Therefore, this value becomes a good positive benchmark to verify that circuits regularly used for eliminating static hazards are performing their intended safety function effectively, particularly in tough industrial processing environments.
Effective grounding and bonding best remedy the problems associated with isolated conductors. Grounding involves linking the conductive object to a known ground point via a mechanically strong and electrically conducting cable, thereby giving it zero (ground) potential. Bonding (or equipotential bonding) links adjacent conductive objects so as to equalize the potential between them; at some point the linked network also is grounded, meaning everything is at zero potential. For fixed installations such as pipework, storage tanks, etc, this is relatively simple to implement. However, it's more difficult for mobile/portable objects such as drums, IBCs and tankers. Such objects require use of purpose-designed temporary grounding and bonding devices, along with strict procedures to ensure they're always in place prior to starting of the process to prevent any static charge accumulation.