In japan, AC 100V class wiring system, electrical fires might be brought about by the tracking phenomena in the power plug. With the aim of establishing a method to protect the entire wiring system, the tracking phenomena that might occur between socket and power plug have been experimentally reproduced and the electrical aspects have been observed at the beginning of the tracking phenomena. In the early stage of the tracking phenomena, a spark discharge occurs at tiny gap between the blades of the power plug and moisture-absorbed dust. This spark discharge brings out a high frequency spike current near the crest of the source voltage. By continuously monitoring the current, it might be possible to detect the beginning of the tracking phenomena.