A new algorithim to estimate uniform soil resistivity for earthing design calculations

A uniform equivalent soil resistivity is a critical input to earthing design calculations using the formulae provided in international standards but is often not readily available. In the UK, soil resistivity data is available from geological mapping but is in the format of a 2-layer or 3-layer horizontally stratified model. Converting a multi-layer soil model to a uniform equivalent is not a simple task and a method is required to make the process more scientific and consistent. Analysis of the available geological data indicated that a lower layer resistivity (solid geology) was always available. An upper layer resistivity (drift geology) was also available, but this was sometimes split into two sublayers. These could more effectively be represented as a single upper layer without significant loss in accuracy. A simple algorithm was developed to combine the two upper sublayers into one which, together with the bottom layer, provides a 2-layer model for every location. In a 2-layer soil model, the relative influences of the top and bottom layer resistivity on earth electrode resistance depends on the size of the earthing system, the contrast between the layer resistivities and the thickness of the top layer. A series of calculations were carried out using detailed simulation software (CDEGS) for a number of different earthing system arrangements in 2-layer soil models with a range of contrast ratios. The relative influence of each layer resistivity was calculated, and a lookup table produced. Extensive testing was carried out to compare the algorithm results to detailed computer simulation and good correlation was demonstrated. The new soil resistivity algorithm has been embedded in the distribution company earthing design tool and has been well received by the designers who now have a reliable and consistent process based on engineering calculations as opposed to subjective decision-making. The implementation, including the direct importing of geological data into the earthing design tool and automated calculation, has also increased efficiency for this aspect of the earthing design.