As the rate of electronicization of automobiles increases, the use of insulation boards for electrical equipment has increased simultaneously. The insulation board used in automobiles has been demanded for its stability as well as miniaturization. When the insulation board for electric equipment is used in a harsh environment, it can cause a fire by forming a carbonized conductive path due to wetting. This paper analyzed the progress and characteristics of the carbonized conductive paths formed on the insulation board. And the electric field analysis was also performed by stages of the formation of a carbonized conductive path using the simulation program. As a result, two carbonization paths were found and theoretical analysis was performed. The results of the above study can be used for predicting the risk of insulation boards.