Data shown in the article give evidence that car fires refer to extremely serious accidents, therefore the problem of car fire safety enhancement is very topical. In this connection development of measures for car fire prevention is of great importance. One of preventive measures is fire investigation. In the article it is stated that them majority of conclusions of expert fire engineers regarding technical reasons for car fires has presumable (probable) nature which does not allow developing and implementing exact engineering solutions. This maybe explained by many reasons including absence of scientifically based methods. In particular, the problem of identifying primary, secondary short circuit or current overload in fusing copper conductor is not clearly solved, even though attempts to solve this problem using instrumental methods were made by criminalists starting from the 50s of the last century. The article gives results of studying fracture surfaces of copper conductors exposed to high-temperature, secondary short circuit and current overload. To conduct study we used raster electronic microscope JSM-6390LV. It has been proved that on the fractures surface of copper conductors specific features for such as high temperature, secondary short circuit, current overload may occur in form of cell-type relief, microporosity, appearance ball-shaped microfusion around microcracks, steps on the fracture surface. Identification of such features allows identifying reason for destruction of copper conductor in case of fire and significantly simplifying determination of fire reason. It is found that the determined reasons are stable and not subject to changes in natural conditions of storing thermally damaged car. Data given in the article may be used by specialists in expert investigation of 12 V electric circuit copper conductors taken from fire locations, in identification of damage nature and, finally, in determination of car fire root cause.