11 - An introduction to the analysis of short-circuit earth return current and rise of earth potential

Abstract

In this chapter, we present the effect of electric shock on the human body caused by ac currents flowing through it and we discuss the nature of body resistance to the flow of ac currents. We describe the sources of potential electric shock, namely step, touch, mesh and transferred potentials. We then describe a substation earthing or grounding electrode system, its functions and equivalent geometric resistance to remote earth. We then present the earthing network of overhead lines and cables. We analyse in detail the earth fault current distribution in overhead lines, earth wires, towers and earth, as well as in cables and substations. We present the concept and derive equations of screening factors for overhead lines and cables. Earth return currents are then analysed in substations and overhead line towers and rise of earth potential are calculated. Finally, we introduce earth or soil resistivity, how it is affected by environmental factors such as temperature, moisture and salt content, and present popular and widely used earth or soil resistivity measurement methods. Equivalent models of one- and two-layer earth or soil are presented.