EARTHING/GROUNDING SYSTEMS, AN OVERVIEW Earthing/GroundingSystems, anOverview


Earthing systems are installed to protect structures fromthe threats of fault electrical currents. Electricity has the capacity to kill people instantly and it is for this reason that all constructions must place great emphasis on quality in their electrical safety systems.


Earthing systems are engineered to provide a safe, low impedance path to earth/ground for fault currents. Earthing is also used to protect and preserve equipment and assets,which is essential in ensuring the ongoing functionality of a site. Without question the most important cable in any electrical system is the earth cable, which directs fault currents safely away froma location by allowing it to dissipate into the strata of the earth. Electricity, be it in the form of a supply or lightning, is always seeking the most direct and easy path to ground and through astute planning a site can dictate and provide that route. If a location does not have an adequate earthing provision it may lead to electricity finding alternatemethods to ground, this is often through electrical equipment, but in certain circumstances it can be via human beings. Another main objective of earthing electrical safety systems is to establish a common reference potential for the power supply system, building structure, plant steelwork, electrical conduits and instrumentation system. Performance of any earth system is dictated by soil resistivity; stratification; type and size of electrode used; location and depth towhich the electrode is buried;moisture and chemical content of the soil, but these shall be examined in further detail in the following.


What is Resistance? Any effective earth system must take account of a range of differing electrical resistances, but what is resistance? Resistance is the property of a conductor which opposes electric current flowwhen a voltage is applied across two ends. Its unit of measure is the Ohm (Ω) and the commonly used symbol is R. Resistance is the ratio of the applied voltage (V) to the resulting current flow (I) as defined by the well known linear equation fromOhm’s Law:


V=I x R Where: V is the potential difference across the conductor (Volts). I is the current flowing through the conductor in (Amperes). R is the Resistance of the conductor in (Ohms).


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The resistance of a conductor depends on the atomic structure of thematerial. Resistivity is the property of amaterial thatmeasures its ability to conduct electricity.


What is soil resistivity? The resistance performance of an earth electrode is heavily influenced by the resistivity of the soil in to which it is driven. Soil resistivity is the resistance measured between two opposing surfaces of 1m³of homogeneous ground


material, usuallymeasuredinΩm. The evaluation of the resistivity of the local soil determines thebest location,depth andsize of the electrodes in an earth/ground system. Soil resistivity is determined largely by its content of electrolytes, which consist ofmoisture,minerals anddissolvedsalts.Because soil resistivity directly relates tomoisture content and temperature, it is reasonable to assume that the resistance of any earth systemwill vary throughout the different seasons of the year. Since both temperature and moisture content become more stable at greater depths below the surface of the ground, it follows that an earth system to bemost effective at all times should be constructed with the earth rod driven down a considerable depth below the surface. Best results are obtained if the earth rod electrode penetrates the water table.


EARTHING


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