Primary and Secondary earthing points.

The earth electrode, more commonly known as the “earthing point”, can be any metal object ranging from a network of earthing rods protruding from concrete to standard installations such as building structures and pipes entering the ground.

The composition of the ground or soil surrounding the electrode is normally the component that presents the most resistance to the flow of static electricity. Different soils have varying levels of resistance due to soil type resistivities and moisture content which can vary dramatically throughout the seasons. For example, soils with high moisture content levels can have very low values of resistance to a True earth ground, but in winter, this water and moisture can freeze, dramatically increasing the resistance between the electrode and True earth, which could rise to a level that would impede the flow of static electricity. In effect, the soil can be described as a resistor in a circuit and we want to know if this resistance is low enough to safely and reliably channel the static charges from the tanker to earth.

Tanker Application

Vacuum truck transferring product out of storage tank.

All sites with classified hazardous areas will have electrical fault and lightning protection systems that are connected to structures that will have been tested by engineers and will be defined as “designated” earthing points. These points can also be used to earth plant equipment and vehicles at risk of static charge accumulation. These “primary” earthing points should be regularly tested to ensure they will not only function as reliable paths to earth for stray currents and lightning strikes, but also protect against the accumulation of static electricity.

When looking at static electricity as distinct and separate from the hazards of lightning strikes and stray currents, higher values of resistance to a True earth ground are permitted. Although the hazardous voltages associated with static electricity are very high, when compared with the currents resulting from lightning strikes and electrical faults, the magnitude of static charging currents is very low.

Because the magnitude of static charging currents is low, “secondary” earthing points like pipes running beneath the ground, beams of building structures, storage tanks and temporary earthing rods can be tested by the MGV to determine if they have a resistance to a True earth ground that will easily channel static electricity off the tanker.

These are structures that will not be tested to verify their suitability for fault current protection and lightning protection, however, because of their inherent and permanent contact below the surface of the ground, they can be tested to determine if they have resistance values to True earth that would permit the safe transfer of static electricity.

Conclusion

Until now vacuum tanker service providers, and the customers of vacuum tanker services, have not had access to the levels of static earthing protection available to companies, who, for many years, have been operating tanker loading gantries with dedicated interlocking static earth monitoring systems. Additionally bulk transporters may be delivering combustible product to sites with out of date or non-existent static earthing protection measures.

With the Earth-Rite MGV system vacuum tanker service providers and bulk transporters can now match the levels of control and safety that have been available to loading gantry operators ensuring their employees, tankers, customer employees and customer properties are fully protected from the ignition hazards associated with static electricity.

If the validity of primary earthing points is not fully known, or secondary earthing points must be used, they should be tested by a system like the MGV prior to their use. A verified resistance will safely allow the rapid transfer of static charging currents to True earth, ensuring the tanker, hoses and any other equipment used in the transfer process are protected from incendive static spark discharges within a potentially combustible atmosphere.

Mike O'Brien

Author Details:
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.


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