Ground Soil Resistivity is Part 1 of a series of short posts on Soil Resistivity Testing and the common mistakes encountered, with practical advice on how to avoid the Soil Resistivity Testing 10 Common Mistakes.

Soil Resistivity Testing 10 Common Mistakes

What is Ground Soil Resistivity?

'Soil' is the term used in Electrical Earthing Design to define the upper geology, which is of particular interest to us when designing earthing or grounding systems.  Soil is the local geology consisting of layers that are primarily composed of minerals, mixed with at least some organic matter, which differ from their parent materials in their texture, structure, consistency, colour, chemical, biological and other characteristics.

Geologists might refer to soil as the combination of 'drift' and/or bedrock.  The layman might refer to Soil more as 'dirt' or 'ground' (not to be confused with the American term).

Soil is the loose covering of fine rock particles that covers the surface of the earth. Soil is the end product of the influence of the climate, relief (slope), organisms, parent materials (original minerals), and time. (http://en.wikipedia.org/wiki/Soil)

What is Resistivity?

Resistivity is one of the fundamental electrical properties given to a material.  For Earthing System Design, this can relate to any or all of the components used in a power system, e.g. copper, steel, aluminium, lead, etc... so this means EVERYTHING in the system has a 'resistivity' including the 'Soil' in which the Earthing System is buried.

Resistivity is the unit of measure by volume that indicates how much a given material will oppose (resist) the flow of current into it, e.g. its apparent resistivity by volume.

When you consider ... "What is the role of an earth/ground is within an electrical power system?"  It's easy to see why Soil Resistivity and deriving an appropriate, accurate and reliable soil model is of capital importance. This is covered in more detail in part two - Why Measure Soil Resistivity?

What is an Earth?

In Electrical terms, the purpose of an earthing/grounding system is to provide a safe path for the dissipation/flow of Fault Current (an unplanned release of energy), lightning strike, static discharge, EMI (ElectroMagnetic Interference), and control the resulting surface voltage(s) in order to provide safety of the personnel and machinery.  These points enforce the importance of the earthing design in any high voltage (HV) project.

According to BS EN 50522-2010, Earth is defined as … “The conductive mass whose electric potential at any point is conventionally taken as zero.”  Thus, an earth/ground system can be defined as … “A conductor or group of conductors in intimate contact with, and providing an electrical contact to earth.”

Soil Resistivity Testing

Soil Resistivity measurements are a critical foundational piece in the jigsaw of safe earthing system design.  The results form the basis of all safety calculations, therefore, it's of capital importance to get the highest level of accuracy practical as many of the subsequent values of EPR (earth potential rise) and surface voltages are directly proportional to Soil Resistivity.

soil-resistivity-testing-pseudo-section

Definition:  a method of subsurface detection which measures changes in conductivity by passing electrical current through ground soils. This is generally a consequence of moisture content, and in this way, buried features can be detected by differential retention of groundwater.    (hyperdictionary.com, 2013)

Ground Soil resistivity testing techniques have been used for many years for Geological investigations to locate bodies of water and/or other items of interest.  In Electrical Earthing Design, the method is used to characterise the below grade Soil Structure in terms of its electrical properties.

WARNING! Unfortunately, testing the ground soil resistivity is prone to appreciable error from many outside influences.  Understanding how to avoid these errors is broken down into small bite sized chunks in this series.

This is just the first of a series of pieces to show you How to Avoid the Errors of Soil Resistivity Testing.

Please get in touch if you have any queries - we're here to help!

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