Soil Resistivity testing why measure? is Part 2 in a series of short posts on Soil Resistivity Testing and the common mistakes encountered, with practical advice on how to avoid Soil Resistivity Testing 10 Common Mistakes.
Soil Resistivity Testing 10 Common Mistakes
This is a series of short posts on Soil Resistivity Testing and the common mistakes encountered, with practical advice on how to avoid Soil Resistivity Testing 10 Common Mistakes.
Soil Resistivity Testing Why Measure Soil Resistivity?
The ‘Soil’ itself forms the very medium that any fault current will flow through. Therefore, understanding the electrical properties of the soil in which an earthing/grounding system is deployed is absolutely vital. Soil Resistivity constitutes the foundation for any electrical earthing design and cannot be understated. It is a primary component to all Electrical Safety Calculations, such as, the permissible touch voltage calculations, the EPR (Earth Potential Rise), Hot Zone calculations, and individual system elements.
Soil Resistivity and the soil structure can have a massive effect on the complexity of the earth/ground grid design. The soil acts as part of the return path for the fault current to source; higher soil resistivity will lead to a higher earth grid impedance value(s) leading to a higher Earth System grid voltage rise (Earth Rise Potential), which in turn gives rise to higher surface voltages across site. Surface voltages, if not controlled with appropriately positioned grading conductor design, could lead to an unsafe condition.
Every Electrical Power System design relies on understanding all the component parts (impedance’s) and Electrical Earthing Design is no different. The ground or soil is another component part in a circuit design… HOWEVER, whereas all the other constituent parts behaviour’s are well understood, e.g. the cables, transformers, generators, VT’s, CVT’s, etc. The soil on the other-hand, is a ‘natural’ semi-conductor, which will have a unique set of electrical properties and be subject to infinite variability from site to site.
This set of unique properties are analysed and characterised into what is known as a ‘Soil Model’. This soil resistivity model can be made up of many layers going down to depths of many 10’s of meters in order to understand how much of the energy from a fault (an unplanned release of energy) will a) pass through into the soil, and b) for each layer, how much will enter and travel through each respective layer.
Example Soil Model – CDEGS RESAP
The amount of energy passing through each layer has a particular relevance when considering ‘Surface Voltages’ and the step and touch potential (voltage) safety calculations. For example, if the site has a LOW-ON-HIGH resistivity model, then more of the fault energy will prefer to travel in the upper layer, whereas, a HIGH-ON-LOW would encounter the reverse, where more of the Earth Return Current will prefer to travel in the lower layer. Both the example scenarios are important to understand when designing earthing arrangements to provide safety from the system.
These points alone support the importance of Soil Resistivity Testing. Click for more information on how to avoid the common mistakes, or simply get in touch – we’re here to help!