NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
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NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
Author: Noyafa–CCTV Tester
According to the test method, cable faults can be divided into the following categories: ①. Broken wire fault (open circuit) - The cable has one or more conductors broken or the metal sheath (armour) broken. Simple disconnection is not common, and is generally accompanied by the phenomenon of grounding through resistance. According to the test method, cable faults can be divided into the following categories: ①. Broken wire fault (open circuit) - The cable has one or more conductors broken or the metal sheath (armour) broken.
Simple disconnection is not common, and is generally accompanied by the phenomenon of grounding through resistance. The distance measurement of simple disconnection fault adopts the low-voltage pulse method. The fixed point adopts the acousto-magnetic synchronization method (treated as a flashover fault).
The audio signal method can also be used for the fixed point of the disconnection fault with a short distance. The fixed point of the disconnection fault through the resistance grounding adopts the acousto-magnetic synchronization method. ②. Short-circuit fault (dead ground) - the insulation resistance of one or more cores of the cable to ground or the insulation resistance between the cores is less than 10Ω.
The distance measurement of such faults adopts the low-voltage pulse method, and the bridge method can be used to verify if necessary. The fixed point adopts the audio signal method or the step voltage method. ③. Low resistance fault—The insulation resistance of one or more cores of the cable to ground or the insulation resistance between the cores is greater than 10Ω, below 200Ω.
The distance of this kind of fault adopts the low-voltage pulse method (the wave impedance of the cable is generally 200Ω, the fault point where the insulation resistance is lower than the wave impedance can receive the low-voltage pulse reflection waveform), if necessary, the arc reflection method, the pulse current method or the bridge method can be used for verification. The fixed point adopts the acoustic and magnetic synchronization method, and when there is no sound at the fault point, consider the audio signal method or the step voltage method. ④. High resistance fault - the insulation resistance of one or more cores of the cable to ground or the insulation resistance between the cores is greater than 200Ω.
This kind of fault location adopts the fox reflection method (second pulse method), and if necessary, the pulse current method is used for verification. The fixed point adopts the acousto-magnetic synchronization method. ⑤. Flashover fault - the insulation resistance of one or more cores of the cable to ground or the insulation resistance between the cores is very high, but when the cable is subjected to the withstand voltage test, when the voltage is added to a certain value, the insulation breakdown occurs suddenly. Phenomenon.
This type of failure generally occurs in preventive testing and is not very common. This type of fault turns into a high-resistance fault after a few breakdown discharges. The arc reflection method is used to locate such faults, and if necessary, the pulse current method is used for verification.
The fixed point adopts the acousto-magnetic synchronization method. ⑥. Special faults of cable main insulation i. Water or damp at the fault point—When the arc reflection method or the pulse current method is used for distance measurement of this kind of fault, the water will absorb most of the energy of the high-voltage pulse, causing the fault point to fail to break down and discharge. In this case, the burn-through source can be used to heat the fault point to evaporate the water, and then the arc reflection method or the pulse current method can be used to measure the distance.
ii. Long-distance missing sheath and copper shield at fault point iii. Long low-voltage cables with a high number of intermediate connectors—The reflected signal of the discharge pulse at the fault point of the cable will be added to the reflected signal of a large number of impedance mismatch points, making it difficult to distinguish the fault point. The above three faults can be located by the bridge method. ⑦. Sheath fault of single core high voltage cable—Poor insulation occurs between the metal sheath of the cable and the ground.
Ranging is carried out using the bridge method. Use the step voltage legal point. According to statistics, high resistance and flashover faults account for more than 95% of the total cable faults.
In fact, there is also a closed fault, which mostly occurs in cable joints or terminal heads, especially in oil-immersed cable heads. When this type of fault occurs, sometimes the insulation breaks down under a certain test voltage, and the breakdown phenomenon disappears completely when the insulation is restored. This type of fault is called a closed fault, and it is difficult to find because the fault cannot be reproduced.
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