Soil resistivity testing with a 4-electrode Wenner array
Soil resistivity testing measures the capacity of the ground to pass an electrical current. This property is utilised in the electrical power industry for assessing the design requirements of earthing systems. Our company provides soil resistivity testing services to assist the design and installation of new generators, substations and engineering infrastructure.
Standard soil resistivity testing procedure involves four equidistant electrodes set-out in a fixed configuration array, as illustrated in the diagram below. A low frequency electrical current is applied across the outer electrodes and the voltage is measured between the inner potential electrodes. The resulting soil resistance reading is subsequently converted by standard equations into a resistivity reading. This value represents the average resistivity of the ground between the potential electrodes. Depth readings are acquired by electrical soundings using an expanding electrode array which is centred on the same point. The depth penetration achieved by individual resistivity readings increases in direct proportion to the electrode spacing of the array.
Soil resistivity test equipment configuration and current paths
Earthing systems provide a safe outlet between an electrical circuit and the ground. The systems are used for the dissipation of electrical faults, grounding lightning strikes and maintaining the correct operation of electrical equipment. The design of an earthing system requires detailed knowledge of the resistivity of the ground in the site area. This is measured as a function of depth at a series of locations around the site, using an expanding four electrode Wenner array (standard BS EN 50522). This procedure is known as soil resistivity or earth resistance testing.
The correct measurement of soil resistivity is particularly important in high resistivity ground, where electrical currents are not able to dissipate readily. In such ground conditions achieving a good earth can be problematic, with information on ground resistivity required to much greater depths for the successful installation of an earthing system .
Inversion software is used to remove the effects of electrode geometry from the data set and produce a model of the resistivity layers at the test location. Correlation with background geological data interactively refines the model and ensures that it relates to actual ground conditions on the site.
Earth rod testing by the fall of potential method
Soil resistivity equipment is also used on new earthing installations, to verify the system has sufficent capacity to dissipate fault related current and lightning strikes. Known as earth-rod testing, this method measures the electrical resistance of an earth rod once it has been emplaced in-position in the ground.
A specially adapted testing procedure termed the Fall-of-Potential (standard IEEE 81), uses two electrodes and a resistivity meter all connected to the installed earth rod. The current electrode is inserted in a fixed position, outside the direct influence of the earthing rod. Readings are then taken with the potential electrode spaced at set intervals inbetween the earth rod and the current electrode. The results are plotted graphically to determine the resistance of the earth rod and to assess if the earthing system requires additional earth rods.
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