Tungsten Carbide Tips (Cen-Stat™ X45)
The ‘Adhesives Test’ proved the most challenging for all the clamps tested. A 1 mm layer of adhesive was applied to metal strips and all clamps failed at initial connection. 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 demonstrated that protective coatings and product deposits can severely compromise clamps/clips that are regularly used for static earthing protection. Of particular note, welding clamps, alligator clips and copper cables wound around plant equipment showed values of electrical resistance that exceeded accepted safe test levels for static electricity.
From the range of externally sourced clamps tested 64% failed the Coatings Test. There was a 100% failure rate for standard welding clips on the Adhesives Test.
Earthing and Bonding Cables
Effective earthing 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 over 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 should be used. In accordance with IEC/TS 60079-32-1 “Explosive Atmospheres: Electrostatic hazards, guidance” the maximum benchmark resistance through such cables must 10 ohms or less. Hence the primary concern with such cables is their mechanical strength and robustness – not their current carrying capacity.
Trailing or taut static earthing wires and cables can be a major trip hazard in the work place. Use of brightly coloured high visibility cable coatings (in accordance with IEC 60446) clearly identifies the cable is for static earthing protection as opposed to electrical fault protection.
There are no mandated identifying colours for dedicated static earthing cables, however due attention should be paid to IEC60446, and the importance of selecting colours which cannot be confused with electrical fault and lightning protection circuits. In Europe ‘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 earthing cables in North America.
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.
Regulatory and approval bodies in Europe and North America emphasise the importance of using specially designed static earthing clamps that are fit for the purposes of dissipating electrostatic charges safely and are robust enough for use in industrial environments. FM approved clamps are especially significant as they have been subjected to mechanical and electrical tests that validate their use as a static earthing clamp. For ultra-critical applications with extremely sensitive flammable/explosive atmospheres, where a low resistance bond to earth is absolutely vital, self-testing clamps and inter-lockable earthing systems with visual indication are recommended.
Mike O’Brien, Managing Director for Newson Gale
If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.