Study of influence of the main ground properties on grounding device resistivity

The value of resistivity of a grounding device depends on specific electrical resistance of ground, therefore, when designing grounding devices it is necessary to take into account layered structure of ground. The paper presents results of a study of influence of the main ground properties such as ground structure and composition, temperature and humidity, that depend on climatic and weather conditions of terrain and season, ground porosity, presence of salts, alkaline and acid residues, depth of occurrence of groundwater by the value of their specific electrical resistances. On example of ground structure of northern and southern areas of Perm region, effectiveness of a method for calculating grounders over the upper and lower layers of ground is shown. The results of calculations in a form of graphical functions of resistance of a vertical grounding device versus its length in a multilayered ground are given. For northern areas of Perm region the use of simplified techniques in calculation of grounding devices only over the upper layer of ground leads to significant deviations in calculated values from the actual ones. The effect of climatic conditions on specific electrical resistance of ground is analyzed. The results of study of influence of rock porosity on specific electrical resistance of ground are presented in example of such rocks as sand, sandstone and limestone. It is shown that specific electrical resistance varies depending on different values of ground porosity in wide ranges: at high values of ground porosity its specific electrical resistance decreases sharply due to the fact that pores can be filled with liquid while increasing conductivity of ground. Thus, this paper substantiates a thesis about the need to take into account basic properties of ground such as stratification, seasonal climate coefficient and porosity, when designing grounding devices of electrical units to ensure safe electrical conditions. In addition, it is shown that considering heterogeneity of the earth greatly improves accuracy of calculation of grounding devices and reduces cost of their design