1. The faults of underground power cables are complex and changeable, and 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 so minor that it does not cause a failure at the time, and it takes months or even years for the damage to develop into a failure. The main causes of mechanical damage to cables are:

(1) Damaged during installation. The cable is accidentally bumped during installation; the cable is pulled by excessive mechanical traction; the cable is damaged by excessive bending.

(2) Directly damaged by external force. Carry out civil construction on or near the installed cable path, so that the cable is directly damaged by external force.

(3) The vibration or impact load of the driving vehicle will also cause the lead (aluminum) package of the underground cable to crack.

(4) Damage caused by natural phenomena. For example, the inner insulating rubber of the intermediate joint or the terminal head expands and bursts the outer casing or cable sheath; the cable sheath installed on the nozzle or bracket is scratched; due to excessive tension caused by land settlement, the intermediate joint or conductor is pulled off.

2. Insulation damp

After the insulation is damp, it will cause the cable withstand voltage to drop and cause failure. The main reasons for cables getting wet are:

(1) Water ingress due to unsealed structure or poor installation of joint 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 will cause the cable withstand voltage to drop and cause failure. The main reasons for cable aging are:

(1) The slag or air gap inside the cable medium is freed and hydrolyzed under the action of the electric field.

(2) The cable is overloaded or the ventilation of the cable is poor, causing local overheating.

(3) The insulation of the oil-impregnated paper insulated cable is lost.

(4) The power cable is used beyond the time limit.

4. Overvoltage

Overvoltage can cause electrical breakdown of defective cable insulation, causing cable failure. The main reasons are: atmospheric overvoltage (such as lightning strike); internal overvoltage (such as operating overvoltage).

5. Poor design and craftsmanship

Poor design and manufacturing process of the cable head and the middle can also cause cable failure. The main reasons are: the design of the electric field distribution is not thorough; the material is improperly selected.