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The recommendation by all of the standards of monitoring up to 10 ohms is designed to compensate for the effect industrial environments can have on the capacity of equipment to dissipate static well before it has the potential to be a health and safety risk.
VESX45 static earthing clamp fitted with tungsten carbide teeth Some examples include:
- Inadequate mechanical penetration of electrostatically charged equipment (more info on this in ETTG 16).
- Degradation of circuits and connections resulting from chemical, mechanical or environmental attack.
- Infrequent or inadequate servicing of equipment.
- Human errors in following the correct static control procedures.
The recommendation of utilising <10 ohm circuits should not be confused with the resistance of 1 x 106 ohms often referred to throughout the standards. This is the generally accepted value of resistance capable of dissipating static. This theoretical value is based on the relatively small size of currents that generate static charges in relation to the very high voltages they are capable of inducing. In a basic V = RI equation this makes sense, but when real world effects are taken into account, the standards recommend static dissipative circuits of 10 ohms or less.
To ensure complete protection from incendive spark discharges in operations that require frequent processing of hazardous materials the standards recommend continuous monitoring of bonding/earthing circuits to 10 ohms or less. This ensures that a proper bond or earth connection is established, preferably, before the process is initiated, guaranteeing that an incendive spark will not be discharged throughout the process.
Another area that can be can be confusing when specifying fit for purpose static control equipment is identifying the difference between hazardous area electrical protection approvals (ATEX, FM, UL, CSA) and equipment designed to control electrostatic ignitions. Approval classifications should not be confused with specifying systems that demonstrate Best Practice compliance in the area of static control. Hazardous area certification only provides a method of protection that guarantee electrical faults are prevented from igniting flammable atmospheres. This has nothing to do with preventing the occurrence of incendive spark discharges from industrial processes. In the same way gas analysers perform functions that detect gases before they become a health and safety risk, static control products should perform functions that ensure spark discharges are prevented well before they can become a hazard.
When specifying static control equipment, hazardous area operators should seek out equipment suppliers that can provide static control products that reinforce compliance with the various Best Practice standards for static control. Hazardous area operators can record this information in their Safety Report for review by the local Health and Safety inspector or corporate body responsible for occupational safety. Should there be an unfortunate incident, investigators will be in a position to rule out static electricity as an ignition source.
The standards for controlling static electricity as an ignition source in hazardous areas are available for purchase on each of the association's websites.
www.nfpa.org | www.cenelec.eu | www.api.org
* BS5958-1:1991 was superseded by CLC/TR:50404
*** Static Earthing = Static Grounding
** Road Tanker = Tank Truck
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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
Click here to download pdf - A4 version.