The insulation layer of power cable is made up of paper, oil, hemp, rubber, plastic, asphalt and other combustible materials. Therefore, the cable has the possibility to burst into flame. The causes are:

(1) Insulation damage causes short circuit fault. The protective lead cover of power cables is damaged during laying or the cable insulation is damaged by mechanical damage during operation, which causes the breakdown of the insulation between the cables or the lead cover. The resulting arc causes the fire of the insulating materials and the outer protective cover of the cables.

(2) The cable is overloaded for a long time. Long overload operation, the operating temperature of the cable insulation material exceeds the maximum allowable temperature of normal heating, so that the cable insulation aging and drying, this phenomenon of insulation aging and drying, usually occurs in the whole cable. Due to the cable insulation aging and drying, the insulation materials lose or reduce the insulation and mechanical properties, so it is easy to break down the fire combustion, or even along the whole length of the cable several simultaneous combustion fire.

(3) Oil - immersed cables leak due to height difference. When the height difference of oil-immersed cable is large, the cable may run oil. As a result of the flow, the upper part of the cable is dried up due to the loss of oil, and the thermal resistance of this part of the cable increases, which makes the paper insulation coking and breakdown ahead of time. In addition, due to the oil flowing down the top, make room at the top end of the cable and generate negative pressure, making the cable easy to absorb moisture and make the end of the cable damp. Due to the accumulation of oil under the cable, there is a great static pressure, which causes the cable head to leak oil. Damp and leaking cables increase the chance of a faulty fire.

(4) Insulation breakdown of intermediate connector box. The middle joint of the cable connector box is not tightly pressed, not solidly welded, or the material of the connector is not properly selected. When making the middle connector of cable, the quality of the insulating agent in the middle connector box does not meet the requirements. During the filling of insulating agent, there are air holes in the box memory, and the cable box seals are damaged, which leak into the moisture. All the above factors can cause the insulation breakdown, form a short circuit, and cause the cable to explode and fire.

(5) Cable end burning. The surface of the cable end is polluted by damp, the porcelain bushing of the cable end is broken and the distance between the lead wires is too small, which leads to flask-fire, causing the surface insulation of the cable end and the lead wire insulation burning.

(6) External fire source and heat source cause cable fire. Such as oil system fire spread, oil circuit breaker explosion fire spread, boiler pulverizing system or coal conveying system pulverized coal spontaneous combustion, high temperature steam pipe baking, acid and alkali chemical corrosion, electric welding sparks and other kinds of fire, can cause cable fire.