NFPA 77, "Recommended Practice on Static Electricity 2007 Edition," suggests specific types of clamps and devices for grounding and bonding portable or mobile plant, drums and containers; these generally have to employ hardwearing sharp contacts and positive spring pressure, and be universally adaptable to a wide range of plant objects. If such units are properly specified and used, in most cases you can be sure of effective static control through grounding and bonding. In all situations, it's also important to periodically test the control measures used, checking clamp/contact/cable condition and the all-important connection back to the ultimate grounding point. Intrinsically safe instruments are required for working "live" in a hazardous area.
Even when the appropriate static safety equipment has been specified, those responsible for operations within hazardous areas must address some additional concerns. In operational terms attaching a grounding clamp to a plant object is always a "physical" action. Even if diligently following company recommended safety procedures, an operator can never know whether the clamp has made good enough contact with the object to safely dissipate any static generated before it can accumulate to dangerous levels. Lots of conductive objects capable of accumulating high static charges also have insulating layers — e.g., paint or a coating or even product build-up — on their surfaces that may prevent this low resistance contact. Many common grounding and bonding clamps show very high resistance readings when clamped onto conductive objects with insulating surfaces. The problem can be even worse if a plant uses standard welding clamps or lightweight alligator clips in place of purpose-designed devices.
To solve these problems, use intrinsically safe, continuously self-testing grounding clamps (Figure 1), as recommended in NFPA 77. An operator employs these in exactly the same way as conventional grounding clamps.
These devices employ certified, active electronic monitoring circuits powered by a low energy battery. The circuit only is completed when the clamp achieves a low resistance contact onto the object to be grounded; the operator receives visual confirmation of this via a light/indicator (usually a pulsing LED). The self-testing grounding clamp also monitors cable condition back to the designed ground point, and won't give the visual go-ahead if the cable has worked loose or is broken. Clamps for use in hazardous locations should carry the appropriate certification or approval mark, e.g., from Factory Mutual (FM) or the Canadian Standards Association (CSA).
To move to an even higher level of security, NFPA 77 recommends ground verification and interlock systems to provide not only visual verification to the operator but also interlock switching contacts that may be linked to process pumps, valves, alarm/shutdown/control systems, etc. Such interlocks can preclude process startup until the conductive object has been safely grounded; if at any time during the operation the condition changes (due to a clamp falling off or wire breaking), the system automatically shuts down the process. Systems employing interlocks also can prevent accidents caused by operators approaching plant objects already carrying accumulated static charges, as in these cases static electricity won't be generated until the process is initiated.
Static ground verification and interlock systems generally are line-powered. They employ approved intrinsically safe barriers to limit the monitoring circuit to safe levels but still must have proper hazardous location and safety certification. These systems typically handle ultra-safety-critical applications like loading/unloading tanker trucks and rail cars with low conductivity flammable liquids (Figure 2), IBCs, fluid bed dryers, mixers, transfer equipment and special process machines. They also are useful wherever it's highly likely that static charge will accumulate in very low MIE flammable atmospheres.