Earthing clamps connected via cables to identified earthing points are the established and proven method of preventing electrostatic discharges from movable or fixed items of plant in flammable atmospheres.
With some operations requiring hundreds of connections to be made and broken every day it is essential that a good electrical connection is made each and every time. The effectiveness, reliability and durability of any static earthing clamp and associated cabling is fundamental to keeping process operations safe from the dangers of an incendive static spark discharge.
It is common for process plant, associated containers, drums and IBC’s to 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 like alligator clips and other “in-house” methods of static earthing protection.
Hazardous area certified (ATEX / Factory Mutual) earthing 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, static earthing clamps must meet specific criteria to be certified as suitable for use in hazardous areas. For example, earthing clamps made from aluminium must be anodized to prevent mechanical sparking under normal operating conditions if they are to be used in a Zone 0 or Zone 20 atmosphere. There are also limitations placed on the amount of plastic that may be used in the clamp body as this may enable the accumulation of static charge. Under an ATEX assessment earthing clamps are also assessed for sources of stored energy and their ability to cause a spark if the energy is released in the hazardous area. One potential source of energy in earthing clamps is the spring which has the potential to generate a mechanical spark through contact with other objects if its escapes the body of the clamp. Therefore clamps are assessed for their structural robustness to ensure any stored energy is reliably contained within the clamp.
US approval bodies such as FM Global assess several other design criteria regarded as critical for static earthing clamp performance.
For use in hazardous locations, the electrical resistance through the clamp, including through the contacts and the 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. To achieve FM approval earthing clamps must undergo, and pass, the criteria set by the following tests:
- Separation force testing: to ensure clamps are not easily or accidentally displaced during operations.
- Clamp pressure testing: to ensure the clamp contacts can penetrate connection inhibitors like rust, coatings and product deposits and make a positive connection to equipment requiring static earthing protection.
- Vibration testing: at varying frequencies to ensure that approved clamps guarantee positive and stable contact with vibrating and portable plant equipment.
- Have a maximum resistance through the clamp body of 1 ohm.
Typical markings to be found on an ATEX and/or FM approved clamps
Newson Gale Studies
Lab tests, designed to reflect real world operating conditions, were 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 metal. Based on Factory Mutual static 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 100 meg-ohm) were exhibited in clamps with varying combinations of high surface area contact and poor to good spring pressure – e.g. alligator clips.
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 teeth (typically manufactured from tungsten carbide or stainless steel) supported by good spring pressure, enabled penetration of the entire range of test coatings.