Probability of damage of apparatus powered by an HV/LV transformer due to lightning to a structure protected by a lightning protection system

The paper deals with the probability of damage of circuits and apparatus connected to an HV overhead power line through an HV/LV transformer due to direct lightning to a structure protected by a Lightning Protection System (LPS) where such circuits are installed. An approach is proposed to evaluate such probability and guidance is given for dimensioning a surge protective device (SPD) able to protect LV apparatus against lightning surges with a given probability. Two conditions are analyzed, namely where the earthing system of LPS may be assumed as equipotential or not equipotential.