1. External damage. From the analysis of the operation in recent years, especially in the high-speed development of Shanghai Pudong, quite a number of cable failures are caused by mechanical damage. 2, Insulation damp. This is also common, usually in the case of a cable joint in a vertical or pipe. For example: cable joint production unqualified and in humid weather conditions do joint, will make the joint water or mixed with water vapor, long time R in the electric field to form a branch, gradually damage the insulation strength of the cable caused by failure. 3, chemical corrosion. Cable buried directly in the area of acid and alkali, often lead to the cable armor, PB skin or the outer layer of corrosion, the protective layer due to long-term chemical corrosion or electrolytic corrosion, resulting in failure of the protective layer, insulation reduction, will also lead to cable failure. Chemical: The corrosion of the unit cable is very serious 4, long-term overload operation. Overload operation, due to the heat effect of the current, the load current through the cable will inevitably lead to fever, at the same time, the charge of the skin effect and the steel armor of the eddy current loss, dielectric loss can also produce additional heat, so that the cable temperature rise. 5, cable connector failure. The cable joint is the weakest link in the cable line, the cable joint fault caused by the direct negligence of the personnel (bad construction) often occurs. Construction personnel in the production of cable joint process, if there is a joint pressure is not tight, insufficient heating and other reasons, will lead to the cable head insulation lower, resulting in an accident. 6. Environment and temperature. The external environment and heat source of the cable will also cause the high temperature of the cable, the insulation breakdown and even the explosion.