Author: Noyafa–CCTV Tester
Reasons for cable faults Knowing the causes of cable faults is very important to quickly identify the fault point in order to reduce cable damage. The causes of cable faults can be roughly classified into the following categories: 1. Mechanical damage Causes of cable failures caused by mechanical damages It is very important to know the reasons for cable failures to reduce cable damage and quickly determine the fault point. The causes of cable faults can be roughly classified into the following categories: 1. Mechanical damage Cable faults 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, but it takes months or even years for the damaged site to develop into a failure. The main reasons for the mechanical damage of the cable are as follows: ① Damage during installation: the cable is accidentally bumped during installation, the cable is pulled due to excessive mechanical traction, or the cable is damaged due to excessive bending; ② Direct external force damage: in After installation, urban construction is carried out on the cable path or near the cable, so that the cable is damaged by direct external force; ③The vibration or impact load of the driving vehicle will cause the lead (aluminum) package of the underground cable to crack; ④Damage caused by natural phenomena: Such as the expansion of the insulating glue in the intermediate joint or the terminal head to burst the outer casing or cable sheath; the cable sheath installed on the nozzle or the bracket is scratched due to the natural stroke of the cable; due to excessive tension caused by land settlement, the intermediate joint is pulled or broken. conductor. 2. The insulation is damp and the insulation is damp, causing failure.
The main reasons that cause the cable to get wet are: ① Water ingress due to the unsealed structure or poor installation of the 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 stabbed by foreign objects 2. Or corrosion perforation; Insulation aging and deterioration The internal air gap of the insulation medium of the cable is freed under the action of the electric field, which reduces the insulation. When the insulating medium is ionized, chemical products such as ozone and nitric acid are produced in the air gap, which corrodes the insulation; the moisture in the insulation causes the insulation fibers to hydrolyze, causing the insulation to decline. Overheating will cause insulation deterioration.
The electrical ionization in the air gap inside the cable causes local overheating and carbonization of the insulation. Cable overload is a very important factor for cable overheating. Cables installed in cable-intensive areas, cable trenches and cable tunnels and other poorly ventilated places, cables worn in drying pipes, and parts of cables close to heat pipes will be overheated and will cause accelerated insulation damage.
4. The overvoltage atmosphere and the internal overvoltage cause the cable insulation to break down and cause a fault. The breakdown point is generally defective. 5. Poor design and manufacturing process The waterproof and electric field distribution of intermediate joints and terminal heads are not carefully designed, improper selection of materials, poor workmanship, and non-compliance with regulations will cause cable head failure. 6. Material defects Material defects are mainly manifested in three aspects.
One is the problem of cable manufacturing, the defects left by the lead (aluminum) sheath; in the process of wrapping the insulation, defects such as wrinkles, cracks, breaks and overlapping gaps appear on the paper insulation; the second is the defects in the manufacture of cable accessories , such as trachoma in iron castings, insufficient mechanical strength of porcelain parts, other parts that do not meet specifications or are not sealed during assembly, etc.; third, poor maintenance and management of insulating materials, causing cable insulation to be damp, dirty and aging. 7. Corrosion of the sheath Due to the influence of underground acid and alkali corrosion and stray current, the lead sheath of the cable is corroded and pitted, cracked or perforated, resulting in failure. 8. The insulation of the cable is lost. When the oil-impregnated paper insulated cable is laid, the trench is uneven, or the outdoor head is on the pole. Due to the ups and downs, the height difference is very different, and the insulating oil at the high place flows to the lower place, which reduces the insulation performance of the cable at the high place. cause failure.
When analysing the causes of cable failures and finding the point of failure, it is extremely important to pay special attention to understanding the cable laying, failure and repair. Pay attention to the recording work during the installation and laying of cables and fault repair. The record should mainly include the following contents: ① Line name and starting and ending points.
②The time when the fault occurred. ③ The location of the fault and the process of elimination. ④Cable specification: such as voltage level, type, conductor cross-section, insulation method, manufacturer name and purchase date, etc.
⑤ Installation record: such as installation date and climate, design type, insulation type, heat treatment temperature and precise location of each butt joint and tee joint. ⑥ Cable burial situation: such as the size of the cable bending radius, the direction of the path, whether there is abnormal laying depth or special protection measures, such as steel plates, pipe penetrations and pipes, etc.; the names of technicians and technicians in the cable laying (This is also often one of the sources of important clues). ⑦ Surrounding environment: such as the ground conditions near the fault, whether there are new excavation, piling or pipe laying projects, whether there is acid or alkali in the soil, whether there are small stones, and whether there is a chemical factory in the nearby area Wait.
⑧Operating conditions: such as cable line load and temperature. ⑨ Calibration situation: including the test voltage, time, leakage current and insulation resistance values, historical records. There are not many cable failures caused by manufacturing defects. It is very helpful to analyze and understand the possible causes of cable failures to find cable failure points.
For example, if the fault distance of the cable is known through distance measurement, and at the corresponding position, if it is found that urban construction has been carried out recently, it can be suspected that the cable under test was damaged during the construction process and caused the fault, which usually does not require a lot of cost. If you work hard, you can quickly locate the fault.
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