... device (hereinafter called SPD) is suitably used in the IT, TT, TN-C, TN-S, TN-C-S and etc power supply system of AC ... . As per the conditions of IEC61643-1:1998-02 standard, Class I surge protective device, it is category B surge protective ... , low residual voltage. Installation Category B surge protective device (SPD), used in the connection between equal potential electrodes while thunder occur ...