Overvoltage reduction in the TN system of a building

To protect the equipment in a building struck directly by lightning, surge protective devices (SPDs) are installed between the N-phase and ground lines at the power utility substation of a TT system. In this case, when operating the SPD, the wiring system of the low-voltage distribution line will be a TN system. Although the overvoltage between these lines at the power utility substation can be limited during SPD operation, an overvoltage between these lines is induced at each floor distribution board connected thereafter. Here, we analyzed this overvoltage in the VSTL electromagnetic analysis software. The overvoltage was found to largely depend on the loop impedance of the wirings as the lightning current flowed from the structure to the protective ground line. The most effective solution was separating the ground lines from the power supply lines connected to the distribution boards of each floor with another electric pipe shaft.