Resistivity for power companies

Installation of earthing rods

Earthing is vital for protecting people and animals from electrical currents and also to maintain proper function of electrical systems. Earthing simply connects an electrical circuit with the ground allowing current to flow to the earth, preventing electric shock. The BGS has been working with the power industry to provide tools to assist in the planning of electrical earthing installations. Earthing is used at all substations and transformers and thus is of great interest to the power industry.

The BGS has developed a suite of tools and datasets for the earthing industry, which:

  • advise on possible earthing array installations given the resistive properties of the ground
  • indicate which type of earthing installation system is possible given the physical properties of the ground
  • indicate whether current ground conditions for earthing will be adequate given future climate and seasonal variation

Selection of earthing installation

The BGS has created a tool for the power industry that generates site-specific prognoses indicating the type of earthing required for 11 kV transformers. The prognoses consider both the resistivity of the ground and also its physical properties (i.e. strength) as relevant for the installation of conductor cabling and earthing rods. The latter is important because in many geological deposits, the ground is too strong to allow the driving of earth rods to an adequate depth.

The output generated for use by the industry included a mobile app that could be used in the field, indicating whether installations should be single rod, multiple rod, horizontal trench or whether specialist techniques were required to penetrate the ground.

Adequacy of earthing given climate and seasonal variation

Example output from the earthing decision support system

The power industry runs a significant number of electrical earthing installations. Whilst the performance of these arrays is tested at the time of installation, further testing of the assets may not pick up changes or natural variance in the ground conditions that influence the earthing performance. In dry conditions, the resistivity of the ground is increased and earthing is therefore less effective. Such dry conditions may occur due to seasonal variation, or in the longer term due to climate change, so it is useful for the power industry to be aware of those installations that are more likely to be affected by drying-out conditions.

In order to understand where these areas are located, the BGS created a new map that estimated soil and geology sensitivity to climate change, incorporating resistivity values for cold and wet, and warm and dry conditions. This type of map saves significant resources by allowing the power industry to prioritise visits to earthing installations at most risk, rather than visiting all locations.

National maps

The BGS's earthing work relies on two key national maps: resistivity and strength.

BGS resistivity dataset

Electrical resistivity is an intrinsic property of a material, measured as its resistance to current per unit length for a uniform cross section. The BGS derives electrical resistivity values for soils using an effective medium methodology (Berg, 2007). This methodology uses a number of parameters (e.g. clay content, porosity, saturation and water conductivity) that are either known for the lithologies of many geological units or can be reasonably estimated using knowledge of similar materials. This data is sourced from the Soil Parent Material Model, the National Geotechnical Properties Database and the geophysical laboratories database, and field soundings are utilised as a guide. For each lithology on the geological map, the Berg algorithm is computed with likely parameter values, and a minimum and maximum value resistivity is derived thereof.

BGS strength dataset

The strength of a rock depends on the composition, density and strength of components within the rock and how these components are bound together. The BGS strength dataset is a national spatial model of engineering soil strength. The methodology attributes strength values (minimum, maximum and typical), estimated from data in the National Geotechnical Properties Database, to each of the lithology codes used in BGS Geology 50k.

Contact

Contact Russell Lawley for more information.