The causes of cable faults can be roughly classified into the following categories:
1 Mechanical damage
The cable fault caused by mechanical damage accounts for a large proportion of cable accidents.
Some mechanical damage is so minor that it doesn't cause a malfunction at the time, but it can take months or even years for the damage site to develop a malfunction.
The main causes of cable mechanical damage are:
(1) Damage during installation: Accidentally injuring the cable during installation, straining the cable due to excessive mechanical traction, or damaging the cable due to excessive bending of the cable.
(2) Direct external damage: urban construction shall be carried out on or near the cable after installation, so that the cable will be directly damaged by external forces.
(3) Lead (aluminum) cladding damage of underground cables will be caused by vibration or impact load of running vehicles.
(4) Damage caused by natural phenomena: such as expansion of insulation glue in intermediate joint or terminal head and expansion of shell or cable sheath;
The sheath of the cable installed on the pipe mouth or bracket is scraped due to the natural travel of the cable;
Break intermediate joint or conductor due to excessive tension caused by land settlement.
Damp insulation
Damp insulation may cause trouble.
The main causes of cable dampness are:
(1) Inlet water is caused by unsealed structure or poor installation of the joint box or terminal box.
(2) The cable is badly made, and the metal sheath has holes or cracks.
(3) The metal sheath is punctured or corroded and perforated by external objects.
Insulation aging and deterioration
The air gap inside the cable insulation medium produces dissociation under the action of electric field, which makes the insulation drop.
When the insulating medium is ionized, ozone, nitric acid and other chemical products are produced in the air gap, corroding the insulating layer.
The moisture in the insulation layer causes the insulation fiber to hydrolyze, resulting in the insulation drop.
Excessive heat can cause deterioration of insulation layer.
The air gap inside the cable generates electrical dissociation, causing local overheating and carbonizing the insulation layer.
Cable overload is an important factor in cable overheating.
Cables installed in cable dense areas, cables in cable trenches and cable tunnels with poor ventilation, cables in drying pipes, and parts of cables close to thermal pipes can accelerate insulation damage due to overheating.
4 overvoltage
Atmospheric and internal overvoltage action, so that the cable insulation layer breakdown, the formation of fault, breakdown point is generally there are material defects.
5. Poor design and manufacturing process
The fault of cable head will be caused by the poor design of water-proof and electric field distribution of intermediate joint and terminal head, improper material selection, poor process and non-production according to regulations.
6 Material defects
Material defects are mainly manifested in three aspects.
One is the cable manufacturing problems, lead (aluminum) sheath left by the defects;
In the process of wrapping the insulation layer, the paper insulation layer has some defects such as fold, crack, crack and overlap gap.
Second, the cable accessories on the manufacturing defects, such as cast iron with sand holes, porcelain pieces of the mechanical strength is not enough, other parts do not meet the specifications or assembly is not sealed;
The third is the poor maintenance and management of insulation materials, resulting in cable insulation layer affected by moisture, dirt and aging.
Corrosion of sheath
Due to the influence of underground acid and alkali corrosion and stray current, pitting, cracking or perforation occur on the lead sheath corrosion of cables, causing faults.
Insulation loss of cable
During the laying of oil-immersed paper insulated cables, the trench is uneven or the outdoor head on the electric pole is located. As a result of the fluctuation and great difference of height and height, the insulating performance of the high cable is reduced due to the insulating oil flowing to the low place, leading to the occurrence of faults.