1. Underground power cable faults are complex and changeable. The causes of power cable faults can be roughly classified into the following categories.
1. Mechanical damage
Cable failures caused by mechanical damage account for a large proportion of cable accidents. Some mechanical damage is very minor and does not cause failures at the time. It takes months or even years for the damage to develop into a failure. The main causes of mechanical damage to cables are:
(1) Damage during installation. The cable is accidentally damaged during installation; the cable is pulled by excessive mechanical traction; the cable is damaged by excessive bending.
(2) Direct damage by external forces. Civil construction is carried out on or near the cable path after installation, causing the cable to be directly damaged by external forces.
(3) The vibration or impact load of moving vehicles can also cause the lead (aluminum) sheath of underground cables to crack.
(4) Damage caused by natural phenomena, such as the expansion of the inner insulating rubber of the intermediate joint or terminal head, which may crack the outer shell or cable sheath; the outer skin of the cable installed on the pipe mouth or bracket may be scratched; excessive tension caused by land subsidence may break the intermediate joint or conductor.
2. Insulation is damp
When the insulation is damp, the cable's withstand voltage will drop and cause failure. The main reasons for the cable to be damp are:
(1) Water ingress due to unsealed structure or poor installation of the junction box or terminal box.
(2) The cable is poorly manufactured and the metal sheath has small holes or cracks.
(3) The metal sheath is punctured or corroded by foreign objects.
3. Insulation aging and deterioration
Insulation aging can cause the cable's withstand voltage to drop and cause failure. The main reasons for cable aging are:
(1) The slag or air gap inside the cable medium becomes free and hydrolyzed under the action of the electric field.
(2) The cable is overloaded or the cable ventilation is poor, causing local overheating.
(3) Insulation loss of oil-impregnated paper insulated cables.
(4) The power cable is used beyond the time limit.
4. Overvoltage
Overvoltage can cause electrical breakdown of defective cable insulation, leading to cable failure. The main causes are: atmospheric overvoltage (such as lightning strike); internal overvoltage (such as operating overvoltage).
5. Poor design and workmanship
Poor design and manufacturing process of cable heads and intermediate parts can also cause cable failures. The main reasons are: inadequate design of electric field distribution; improper material selection; poor process and failure to comply with regulations.
