Earthing design. This topic explains objective of earthing design, requirements for earthing mat design, determination of earth grid current , and earth potential. In general, the calculation methodology is as per IEEE:80 or IS 3043. The first objective is that least ground grid resistance less than 1-5 ohms is achieved so that fault current flows only through earth mat to ground. Second, the calculated Ground Potential Rise, GPR shall be less than tolerable touch potential. In case GPR is more than tolerable touch potential, the calculated step potential should be less than the tolerable value. Additionally, the calculated touch potential should be less than the tolerable value. Earthing design can be carried out either by manual calculations or advanced software tools like ETAP, CYMGRD. In this screen, inputs required for earthing mat design calculation are shown. They are system fault level, duration of fault, soil resistivity value, length and breadth of switch yard, earthing conductor material, conductor, electrode size and corrosion elements, number of electrodes, current division factor. Determination of number of vertical electrodes. The vertical electrode generally is used to provide a dedicated earthing for equipment neutral earthing system. Generally, two dedicated electrodes are provided for neutral earthing system so that if there is any failure, the other electrode will remain intact. Two dedicated vertical ground electrodes are provided for the neutrals of transformer, reactor. Single dedicated electrode is provided for surge arrester, capacitor voltage transformer. Additional electrode if necessary, used for corner point of the ground grid, peripheral portion of the ground grid. As you know from the short circuit analysis, when there is a fault between live conductor and earth, the current will flow through earth. In short, we can say that the path which includes earth will have current flowing through earth. The point at which the current is injected, the earth potential is higher and it gradually decreases to zero to a point which is away from the SS. This point is mathematically called as zero potential or remote earth. Let us take an analogy. In a standstill water, if a stone is dropped, then the wave is produced in all 360 degrees. The wave spreads until energy is available. Similarly, the point at which the current is injected, the potential is higher than any other point in the vicinity and it gradually decays to zero. This is also called as Ground Potential Rise or GPR, or Earth Potential Rise, EPR. In case of a ground fault, fault current flows through soil to system neutral. Let us assume system neutral is grounded at a remote station. Hence, fault current flows through soil to remote station, giving rise in ground potential. GPR depends on the part of ground fault current which flows into or out of local station from R to remote station respectively. Let us consider a scenario. How to determine the ground fault current for design of earth grid. As you would be knowing from the subject that all the substations are interconnected between them. Let us take the following example for our understanding. There are two substations, namely substation A, SS_A and substitution B, SS_B. Further, we can consider that SS_A is at a local end where you are interested in design of earthing system. While SS_B is a remote SS which is already existing. To determine the earth grid current, three cases need to be analyzed as shown on the screen. In the first case, if the fault occurs within the local substation, the current will flow through the metallic return earth conductor to system neutral grounded within local SS. Therefore, there will not be any current dissipated into earth and no significant ground potential rise. In the second case, if the fault occurs within the local substation, where the local SS neutral is not grounded, then the fault current would be entirely contributed from remoter which would flow through the local SS earth mat into the ground. Therefore, there will be current through the local SS earth. This in turn will lead to rise of its earth potential with respect to remote earth. In the third case, you will be able to notice that the fault current has got two components. The local SS current will return through the metallic return earth conductor to system neutral grounded within local SS. While a component of the current fed from the remote SS will have to return only through the local SS earth. This in turn, will lead to rise in earth potential of local SS with respect to remote earth. Other than the earth grid current and earth potential rise, factors such as step potential and touch potential need to be considered. Step potential. Having known the GPR during the earth fault condition, if somebody walks into substation or moves within switchyard, then they will experience electric potential difference between their two feet. The distance between two feet is considered as one meter for analysis of step potential as per standard. Touch potential, like step potential, if somebody touches the body of a grounded object during fault conditions while standing on the ground with legs together, they will experience an electric potential difference between the hand and feet. The distance between feet grounded object is considered as one meter for analysis of touch potential. Here is a quick recap of what you've learned this far. The vertical electrode generally is used to provide a dedicated earthing for equipment neutral earthing system. Generally, two dedicated electrodes are provided for neutral earthing system so that if there is any failure, the other electrode will remain intact. Two dedicated vertical ground electrodes are provided for the neutrals of transformer, reactor. Single dedicated electrode is provided for surge arrester, capacitor voltage transformer. Additional electrode if necessary, used for corner point of the ground grid, peripheral portion of the ground grid. The distance between feet and any grounded structure is considered as one meter for analysis of touch potential and the distance between two feet is considered as one meter for analysis of a step potential. The point at which the current is injected, the earth potential is higher and it gradually decreases to zero to a point which is away from the SS. This point is mathematically called as a zero potential, or remote earth. In case of a ground fault, fault current flows through soil to system neutral. If system neutral is grounded at remote station, fault current flows through soil to remote station, giving rise in ground potential. In the event of a fault, only a portion of the full fault current flows through the earth grid. Hence, only that portion of fault current should be considered to prevent overdesign of the grounding system.
