When dust accumulates in the space between a cord plug and an electric outlet, and part is wetted by moisture or dew condensed on a wall or a window, small leakage current flows between electrodes of a cord plug. The insulating materials of the cord plug and the outlet are carbonized gradually by this current, which sometimes results in a fire accident. This phenomenon called a tracking fire causes 64 accidents in the area of Tokyo Fire Defense Agency and 500-600 all over japan in 2005. Under such a circumstance，establishment of a prevention system of tracking fire is strongly required.
As countermeasures against tracking fire，a cover installed at the foot of electrodes of a plug or a reinforced electrical insulation at electrodes is applied. They may be effective to reduce leakage current under contaminated and wetted condition and delay occurrence or progress of the tracking phenomenon，but are not essential measures to prevent fire accidents.
Some studies have also been carried out on the tracking phenomenon of a cord plug，but they do not seem satisfactory from practical point of view.
It is indispensable to investigate leakage current characteristic in order to understand the mechanism of tracking progress and to establish fire prevention system as a goal. In this paper，results of a series of fundamental experiments are reported，where a novel system developed enables measurement of small leakage current.