How do you calculate voltage variations across a ground grid?

Calculating the voltage variations across a ground grid is a critical step in ensuring the safety and reliability of electrical systems. The process involves a detailed analysis of the soil resistivity, the geometry of the ground grid, and the magnitude of the fault current. The primary goal is to keep the step and touch voltages within safe limits to protect personnel from electric shock hazards during a fault event.

The first step in this calculation is to determine the soil resistivity at the site. This is typically done through a series of measurements using the Wenner four-pin method. The soil resistivity can vary significantly with depth and moisture content, so it is important to take multiple measurements at different locations and depths to obtain an accurate model of the soil.

Once the soil resistivity is known, the next step is to model the ground grid. This includes the layout of the conductors, the depth at which they are buried, and the material they are made of. The geometry of the ground grid has a significant impact on its performance, so it is important to model it as accurately as possible.

With the soil model and the ground grid geometry defined, the next step is to calculate the ground potential rise (GPR). The GPR is the maximum voltage that the ground grid will rise to with respect to remote earth during a fault. This is calculated by multiplying the fault current by the resistance of the ground grid.

Finally, the step and touch voltages can be calculated. The step voltage is the potential difference between two points on the earth's surface separated by a distance of one pace (assumed to be 1 meter). The touch voltage is the potential difference between the GPR and the surface potential at a point where a person might be standing while in contact with a grounded structure. These calculations are typically performed using specialized software such as CDEGS or PSCAD.