A built-in power lightning protection device should be used in the internal power supply of the electrical equipment which functions as the final line of defense, so as to completely eliminate the tiny transient overvoltage. The power lightning arrester used at this level requires a maximum impact capacity of 20KA per phase or less, and the required limit voltage should be less than 1000V. It is necessary to provide class-III protection for some particularly important or sensitive electronic devices, as well as to protect the electrical equipment from transient overvoltage produced within the system.