Soil Resistivity Testing 

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. SUMO provide soil resistivity testing services to assist in the design and installation of power generation plant, substations and engineering infrastructure.

Soil resistivity testing configuration with a 4-electrode Wenner array

Soil resistivity testing configuration with a 4-electrode Wenner array

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 measured between the inner potential electrodes. The resulting soil resistance reading is subsequently converted by standard equations into a resistivity reading, which represents the average resistivity of the ground between the potential electrodes. Depth readings are acquired by electrical soundings using an expanding electrode array centred on the same point. The depth penetration achieved increases in direct proportion to the electrode spacing of the array.

Soil resistivity test equipment configuration and current paths.

Soil resistivity test equipment configuration and current paths.

Soil Resistivity Testing & Earthing Design

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. Their design requires detailed knowledge of the resistivity of the ground in the site area. This is measured as a function of depth at locations around the site, using an expanding Wenner array (standard BS EN 50522),  termed soil resistivity or earth resistance testing.

The correct measurement of soil resistivity is particularly important in high resistivity ground, where electrical currents do not 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

Earth rod testing by the fall of potential method

Earth System Testing

Soil resistivity equipment is also used on new earthing installations, to verify the system has sufficient 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 in between 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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