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The 'Adhesive Test' proved the most challenging for all the clamps tested. A 1mm layer of adhesive was applied to metal conductive strips and all clamps failed at initial clamping. When the clamps were then permitted some "jigging" by hand to dislodge the adhesive, the clamps that passed the coatings test, subsequently passed the adhesive test. Rusted and corroded clamps were also tested for values of resistance. These test results were alarmingly high, even on clean surface tests.
The tests effectively demonstrate that product deposits can severely compromise accepted good earthing methods. Of particular note, welding clamps, alligator clips and copper cables wound around plant equipment showed values of electrical resistance that exceeded generally accepted safe test levels for static electricity.
Grounding and Bonding Cables
Effective Clamps need cables and connections that can stand up to the rigours of industrial use. Due to their mechanical strength multi-stranded steel cables provide much longer lifetime use than copper braids or cables which can easily work harden with constant movement. In manufacturing areas where corrosion is a problem multi-stranded stainless steel cabling is available.
Trailing or taut earthing wires and cables can be a major trip hazard in the work place. Use of brightly coloured highly visible sheathed cable (in accordance with IEC 60446) clearly identifies the cable is for static earthing as opposed to electrical earth use. Hytrel covered cable with proven resistance to abrasion, mechanical wear and most types of chemical corrosion when used in a retractable spiral, keeps earth cable handling simple, and options are available with a special anti-static treatment to even prevent a surface charge accumulating on the cable coating. Regularly inspecting and recording the integrity of bonds to designated earth points is essential. The integrity of bonds can be tested in hazardous areas with Intrinsically Safe portable instruments, which provide a simple reading to ensure the clamp is making effective and positive contact with the plant equipment, and that the bonding cable is properly attached to the local earth point. Regularly inspecting and recording the integrity of bonds to designated earth points is essential. The integrity of bonds can be tested in hazardous areas with Intrinsically Safe portable instruments, which provide a simple reading to ensure the clamp is making effective and positive contact with the plant equipment, and that the bonding cable is properly attached to the local earth point.
Maintenance
Cable Colours
There are no mandated identifying colours for sheathed static earthing cables, however due attention should be paid to IEC60446, and the importance of selecting colours which cannot be confused with electrical "fault current" or circuit protective conductors. In Europe Newson Gale 'Cen-Stat" single core cable is GREEN to distinguish it from the green/yellow used for electrical earthing. Cen-Stat ORANGE cable is used to denote single core static grounding cables in North America.
Conclusion
It is easy to assume that the use of simple clamps will automatically eliminate the risk posed by static electricity. However, the complexity of dissipating static effectively requires careful planning and a sound approach to risk management.
Regulatory & Approval bodies in Europe and in North America emphasise the importance of using specially designed earthing clamps that are both fit for the purposes of dissipating static safely and for use in industrial environments. For ultra-critical applications with extremely sensitive flammable/explosive atmospheres, where a low resistance bond to earth is absolutely vital, self-testing clamps and indicating/interlock systems are recommended. Working to these standards and guidelines is the way to ensure plant and personnel are protected from the ever-present and hidden dangers of static electricity.
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More Information:
If you have any queries related to applying standards compliant methods of earthing and bonding items of plant at risk of static charge accumulation contact:
Mike O'Brien, Product Manager, on +44 (0)115 940 7500 or email michael.obrien@newson-gale.co.uk
Europe / Middle East / Asia
Graham Tyers, President, Newson Gale Inc., +1 732 987 7715 or e-mail graham.tyers@newson-gale.com
U.S. / Canada / South America
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