Contents I. Introduction
It has been reported that the use of insulator such as wood to support medium-voltage lines increases the dielectric strength of distribution lines and is effective in reducing lightning damage [1]. However, concrete poles that have weak dielectric strength are commonly used as supports for distribution lines in Japan, and ground fault accidents caused by lightning are often occurred. By the way, since the general distribution substations of medium-voltage distribution lines in Japan are composed of ungrounded systems. Therefore, if it is a 1-phase ground fault, which often results in spontaneous are quenching[2][3][4]. For this reason, in many cases, medium-voltage lines have lightning countermeasures in which only two phases are protected by lightning protection devices, while the remaining one phase is allowed to ground fault[5].
