The Static Statistics | On a reported annual basis, the US accounts for an average of 280* reported industrial static related incidents while the UK and Europe account for an average of 50 and 350 incidents** respectively. Datasource * NFPA (USA), ** HSE (UK).
Grounding (also known as Earthing) clamps connected via cables to identified ground points are the established and proven method of preventing electrostatic charge accumulating on movable or fixed items of plant in flammable and explosive atmospheres.
With some operations requiring hundreds of ground connections to be made and broken every day it is essential that a good ground contact is made each and every time. The effectiveness, reliability and durability of any grounding clamp and associated cabling is therefore key to keeping process operations safe from the dangers of a static discharge.
In the coatings, resins, adhesives, paints, solvents, explosive or combustible powders and related industries, it is common that process plant, associated containers, drums and IBC’s can build up layers of product or rust, or have surface coatings present. These layers can form an unpredictable insulating barrier that can easily defeat certain designs of clamps and other “in-house” methods of making ground connections.
Hazardous area certified (Factory Mutual / ATEX) static grounding clamps provide extra guarantees over alligator clips and welding clamps.
The importance of effective clamp design and its suitability for use in flammable atmospheres has not gone unnoticed by regulatory and approval bodies around the globe. Approved ATEX, grounding clamps must meet specific criteria to be certified as suitable for use in hazardous areas. For example, a grounding clamp made from aluminium must be coated with material that will not contribute to mechanical sparking under normal operating conditions if it is to be used in a Zone 0 or 20.
There are also limitations placed on the amount of plastic that may be used in the clamp body as this may enable surface accumulation of static charge, as well as the obvious problems of durability, resistance to chemical attack, and thermal stability. Clamps are also assessed for sources of potentially stored energy and their ability to cause a spark if the energy is released in the hazardous area. One major energy source in grounding clamps is the spring. This has the potential to generate a mechanical spark through contact with other objects if its escapes the body of the clamp. Therefore clamps are tested for their structural robustness to ensure any stored energy is reliably contained within the clamp.
Combined with structural robustness testing, US approval bodies such as FM Global assess several other design criteria regarded as being essential for static grounding clamps. For use in hazardous locations, the electrical resistance across the clamp, including contacts and clamp body must not exceed 1 Ohm when attached to plant equipment. Additional tests ensure the clamp must be suitable for use in normal industrial conditions. The clamp must pass separation force testing, minimum-clamping force testing and vibration testing at varying frequencies to ensure that approved clamps guarantee positive and stable contact with mobile or portable plant equipment.
Typical markings to be found on an ATEX and/or FM approved clamps.
Newson Gale Studies
Engineers at Newson Gale have studied the effect of product accumulation, rust build up and protective coatings on the ability of grounding clamps to dissipate static effectively. Lab tests, designed to reflect real world operating conditions, have been conducted to investigate the impact layers of protective coatings and adhesives can have on the ability of clamps to establish positive contact with strips of conductive metal. Based on earthing clamp approval requirements, the benchmark clamp resistance test was set at 1 Ohm.
The tests showed some surprising results. Most notably, in the ‘Coatings Test’ even the thinnest layers (400 μm) provided a wide range of clamp resistance readings that varied based on clamp design. The test indicated the highest levels of clamp resistance (upwards of 1 x 108 Ohm) were exhibited in clamps with varying combinations of high surface area contact with poor to good spring pressure. The clamps that exhibited consistent positive values (less than 1 Ohm) combined low surface area contact with good spring pressure. Low surface area contact, achieved via sharpened tips (typically manufactured from Tungsten Carbide or Stainless Steel) supported by good spring pressure, enabled penetration of the entire range of test coatings.