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Omissions in the production process and operating conditions of cables can lead to cable failures, known as hidden dangers of cable faults. Therefore, analyzing the causes of cable faults can reduce the occurrence of cable faults or help quickly identify the fault point after the fault occurs. The causes of cable faults can be roughly classified into the following categories.  

Reasons for cable malfunction

1. Mechanical damage. This type of accident is the most common, and most of these types of faults can cause power outages, so they are relatively easy to identify. Mechanical damage mainly includes direct external force damage, construction damage, and natural damage during normal operation.  

2. The insulation is damp. Cable failures caused by insulation moisture account for about 13% of the failure rate, and are another major cause of cable failures besides mechanical damage. This type of fault generally manifests as a decrease in insulation resistance and an increase in leakage current. The poor sealing process or sealing failure of the middle or terminal of the cable is the main cause of cable insulation moisture.

3. Insulation aging. Long term operation of cable insulation will be subject to the dual effects of electricity and heat, and its physical properties will inevitably change, resulting in a decrease in cable insulation strength or an increase in dielectric loss. If the cable operates under overvoltage or near a heat source for a long time, it may even cause premature aging or local premature aging of the cable.  

4. Overvoltage. Cable insulation has a normal withstand voltage limit, and lightning strikes and other overvoltages can damage cable lines, but this situation is not common.  

5. Overheating. Overloaded operation of cables can cause severe heating and accelerate insulation damage. After oil immersed insulated cables overheat, the insulation may coking and sometimes shatter upon contact.  

6. Product quality defects. Due to the lack of necessary professional technical training, some construction units may encounter problems in production quality and cable material selection, resulting in defects in the quality of the cable body or cable accessories.  

7. Poor design. The structure and form of power cables have been basically determined. However, various accessories of cables are constantly improving, which inevitably leads to defects such as poor waterproofing, inappropriate material selection, and unreasonable processing technology. These defects can also cause cable failures.

Types of cable faults

According to the needs of different departments, the classification of cable faults is also different. Below, we classify cable faults according to their nature, generally divided into low resistance faults, high resistance faults, open circuit faults, disconnected and grounded or short circuit faults, leakage faults, and flashover faults.  

1. Low resistance fault. The definition of low resistance fault is based on the principle of pulse reflection, that is, when the insulation resistance of the cable core wire to ground or between core wires is less than 10Zc (Zc is the characteristic impedance of the cable, generally not exceeding 40 Ω), and the continuity of the conductor is not damaged, it is a low resistance fault, that is, a low resistance grounding or short circuit fault.  

2. High resistance fault. This type of fault is opposite to low resistance faults, where the insulation resistance of the cable core to ground or between the core wires is much lower than the normal value, that is, high resistance grounding or short circuit faults.  

3. Open circuit fault. The insulation performance of the cable core is good, but if the continuity of the core conductor is damaged, the fault is called an open circuit fault.  

4. Disconnected and grounded fault. The fault of a cable with a discontinuous core conductor and grounded through a resistor is a disconnected and grounded fault.  

5. This type of fault is an extreme form of high resistance fault. During the preventive voltage withstand test of cable insulation, the leakage current of the cable increases continuously with the increase of the test voltage, and may even exceed the allowable value of the cable leakage current (at this time, the test voltage has not yet reached or has already reached the earring test voltage). This high impedance fault is a leakage fault. The insulation resistance of this type of fault may be very high, even meeting the qualified standards, therefore it has a certain degree of concealment.  

6. Flashover fault. Flashover fault is another extreme form of high resistance fault. In the insulation prevention and voltage resistance test of cables, when the test voltage rises to a certain value (before or after reaching the earring test voltage), the leakage current of the cable will suddenly increase and quickly cause flashover breakdown. This high resistance fault is called flashover fault. The insulation resistance of flashover faults is extremely high and generally meets the qualified standards. Cables with this type of fault nature may not even experience flashover breakdown in the short term at lower voltages, and their electrical performance is good, making them a major hidden danger for cable faults.  

In summary, by summarizing the causes of cable faults, we can analyze the development direction of cable distance measurement, which is beneficial for us to research intelligent cable testers that are easy to operate, measure accurately, and have small blind spots.