（1） Mechanical damage accounts for a large proportion of AAC Cable accidents. There are three main causes of mechanical damage.
(1) it is directly damaged by external force. If the cable is damaged by accident in the process of urban construction, transportation, or excavation, piling, lifting and handling of underground pipeline engineering.
(2) damage during cable installation: for example, the cable is damaged due to excessive traction, or the insulation or shielding layer is damaged due to too small cable bending.
(3) damage caused by other factors, such as expansion of insulating glue in joint or terminal and cracking of shell or nearby cable sheath; Without proper cable expansion arc, the cable sheath installed on the pipe orifice or bracket is damaged due to thermal mechanical force; Excessive tension caused by land subsidence, breaking the joint or conductor; The breakage of terminal porcelain bushing; Due to the vibration of large mechanical parts or running vehicles nearby, the lead bag is damaged.
(2) damp insulation
(1) because of the joint box or terminal box structure is not sealed or poor installation, resulting in water.
(2) poor cable manufacturing: there are small holes or cracks in the metal sheath of the ACSR Cable.
(3) the metal sheath is pierced or corroded by foreign objects.
in the fault repair work, the damp insulation must be removed. At the same time, check whether there is moisture in the conductor and remove it.
cable insulation has been operating under the action of electricity and heat for a long time, and its physical properties change, which leads to the decrease of insulation strength or the increase of dielectric loss, and finally causes insulation breakdown, which is called insulation aging. If the operation time exceeds the designed service life of the cable, the aging of the insulation is called normal aging, which belongs to improper operation. If the similar situation occurs in a short period of time, it is regarded as premature aging of the insulation.
it is rare to damage cable equipment due to lightning stroke or other impulse overvoltage. Generally, 3 ~ 4 times of instantaneous overvoltage will not affect the cable with good insulation. However, there are still some cases that the cable line is broken down when it is struck by lightning, such as the insulation of the vertical part of the terminal is dry, which may be broken down when it is struck by lightning.
there are many factors for cable overheating. Cable overload operation (over cable ampacity or under abnormal operation mode) will cause cable overheating; In areas with dense cables, poor ventilation of cable trench and tunnel, cables passing through dry ducts, etc., insulation damage may be accelerated due to overheating of cables. The vertical part of oil paper cable will lose insulating oil quickly due to overheating, resulting in dry insulation and even coking of insulating paper. Overload can also accelerate the recrystallization of lead clad grains, resulting in lead clad fatigue damage. In the oil paper cable line, when the shell of the joint filled with a large amount of asphalt insulating glue is welded with copper sheet, due to the large thermal expansion coefficient of the asphalt insulating glue itself, it is difficult to avoid the overload. The expansion of asphalt insulating glue will cause the copper shell to crack, or lead sheath of a section of cable at the lead sealing joint to crack.
Due to the fire or the influence of other adjacent cable faults and other external overheating damage, most of them can be determined from the burning phenomenon of the protective layer outside the cable, which is easier to identify.
(VI) material defects
this is a congenital defect of the cable. There are mainly mechanical damage of lead sheath, impurities in lead sheath, and joints in pressing lead. These are not easy to see outside the armor, and are often found in the process of accident repair. The end of the plastic cable is not sealed. After a large amount of moisture enters into the conductor, the insulation will gradually form “water branch” under the action of voltage, leading to insulation breakdown.
it belongs to congenital defects of cable accessories, such as sand holes in cast iron, insufficient mechanical strength of porcelain parts, rough and loose processing of assembly parts, non-conforming specification of waterproof rubber ring, rubber aging, etc. If the installation unit has a sound acceptance system, it can basically find these disadvantages and plug the loopholes.
(7) poor design
such as the joint and terminal waterproof design is not careful, the selection of materials is improper, the process requirements are not strict, the mechanical strength is poor and so on. For new equipment, new materials and new technology, trial operation should be arranged first, and then gradually popularized according to the maturity of operation experience, so that it is easy to correct even if problems occur.