The cable faults can be summarized into three categories: grounding, short circuit and disconnection. The fault types are mainly as follows:
1. Flashover fault. The cable is in a good insulation state at low voltage, and there is no fault. As long as the voltage value rises to a certain range, or a certain voltage continues to rise after a period of time, the insulator will be broken down instantly, causing flashover fault.
2. One phase core wire is broken or multi-phase wire is broken. In the continuous test of cable conductor, the insulation resistance of each conductor of the cable is consistent with the relevant provisions, but one or more phases cannot be continuous in the inspection, which means that one phase core wire is broken or more phases are broken.
3. One or two cores of three core cable are grounded. Discontinuities in one or two cores of the three core cable shall be measured with an insulation megger, and then the insulation resistance of one or two cores to the ground shall be telemetered. If the insulation resistance between cores is much lower than the normal value, it is called high resistance grounding fault if the insulation resistance is higher than 1000 Ω; On the contrary, it is a low resistance grounding fault. These two faults are called disconnection and grounding fault.
4. The three-phase core wire is short circuited. The size of grounding resistance during short circuit is the basis for judging the short circuit fault of three-phase core wires of cables. There are two kinds of short circuit faults: low resistance short circuit fault and high resistance short circuit fault. When the three-phase core wire is short circuited, the grounding resistance below 1000 Ω is a low resistance short circuit fault, on the contrary, it is a high resistance short circuit fault.
Cause Analysis of Common Cable Faults
1. External force damage. External force damage is the most common cause of cable failure. After the cable is damaged by external force, a large area of power failure will occur. For example, in the process of underground pipeline construction, the cable is pulled off due to the large traction force of construction machinery; The cable insulation layer and shielding layer are damaged due to excessive bending of the cable; Excessive cutting and too deep knife mark during cable stripping. These direct external factors will cause certain damage to the cable.
2. The insulation is damped. The poor cable manufacturing and production process will lead to the rupture of the protective layer of the cable; Insufficient sealing of cable terminal joints; The cable protective sleeve is punctured or corroded by objects during cable use. These are the main reasons why the cable insulation is affected with moisture. At this time, the insulation resistance decreases and the current increases, causing power failure problems.
3. Chemical corrosion. Long term current action will make cable insulation generate a lot of heat. If the cable insulation work is in a bad chemical environment for a long time, its physical properties will be changed, and the cable insulation will age or even lose its effect, which will lead to power failure.
4. Long term overload operation. If the power cable is in the high current operation environment for a long time, if there are impurities or aging in the line insulation layer, plus the impact of external factors such as lightning on the over-voltage, the overload operation will generate a lot of heat, which will easily lead to power cable failure.
5. Quality of cables and cable accessories. Cables and related accessories are two important cable materials, and their quality problems have a direct impact on the safe operation of power cables. The production of cable and its accessories and three ends of cable is easy to cause quality problems, for example, the cable will be affected by moisture due to loose sealing during transportation and storage; The insulation pipe is rough, uneven in thickness, and there are bubbles in the pipe; The three ends of prefabricated cable cannot be peeled accurately; The designer and producer did not manufacture the cable joint according to the requirements. In addition, improper material selection and poor waterproof property in cable product design will also cause cable quality problems.