The fault finding of low-voltage cables is not much different from the fault diagnosis of high-voltage cables. The principles, methods and techniques are exactly the same. Only when the flashover test and fixed-point test are carried out, the impulse voltage can be properly controlled to avoid damage to the overall insulation level of the cable due to the high impulse voltage.
 

Difficulties in the fault distance of low-voltage cables are usually caused by the following reasons:

1. Three-head cable laying process is not standardized. The main reason is that the ground wire or neutral wire is not handled properly, resulting in unsatisfactory test waveforms. Specifically, a. the steel armor of the intermediate joint is discontinuous; B, steel armor is not grounded.

2. Test wiring error. Multi-finger, non-shielded and non-shielded multi-core cables will cause undesirable waveforms, and even reflected waves will not cause rough measurement failure. The specific performance is as follows: a, no grounding in the non-test phase; B, test phase grounding; C, test line and shielded wire reverse connection; D, whether the shielded wire is connected with the grounding phase or steel stern; E, the shielded wire is directly connected to the grounding column unrelated to the cable under test.

It can be seen that when we regulate the operation of cable construction, maintenance and repair, the situation will be much better. A rough measurement of the fault distance will reduce the uncertainty. In addition, the location of the fault is a problem because it has many key factors. This is mainly due to the fixed-point failure of the low-voltage cable, the voltage impact is usually not high, the power is not large, and the construction of long-point mode countries is not perfect. Therefore, the error points of the mechanical vibration and the electromagnetic wave are relatively weak. In addition to detecting the sensitivity of equipment, it is also more difficult to intervene in environmental monitoring, technical and empirical testing, and urban work fixed points than working under high pressure. Fortunately, unlike the test equipment, voice and magnetic positioning technology are used, which greatly alleviates the location problem.

The fault point of low voltage cable is difficult to locate. The following aspects should be improved: first, to improve the discharge state of the fault point; second, to select and use appropriate test equipment; third, to avoid the interference of environmental factors; after that, we should improve our testing skills and technology, strengthen learning, conscientiously sum up, continuously accumulate cable fault testing experience, and communicate with other peers as much as possible, which is very important for cable fault testing.

Common low voltage cables are as follows:

(1) Low-voltage power cables are generally divided into PVC insulated (plastic) power cables, cross-linked polyethylene insulated power cables, rubber insulated power cables, and oil-filled and oil-impregnated paper insulated power cables. According to the type and nature of its work, it can be divided into general power cables, overhead power cables, mine underground power cables, submarine power cables, fire-resistant (fire-resistant) and flame-retardant power cables.

(2) Several commonly used low-voltage cables and their characteristics: Copper core power cables: At present, domestic low-voltage power cables are twisted into various core cables. This structure of the cable anti-interference ability and anti-lightning ability is poor. The unbalanced three-phase impedance and zero-sequence impedance of the cable make it difficult for the line protection device to operate reliably. Cross-linked polyethylene insulated power cable: referred to as cross-linked polyethylene cable, which uses chemical or physical methods to change the molecules of polyethylene plastic, the insulation material of the cable, from a linear structure to a three-dimensional network structure, that is, thermoplastic polyethylene to thermosettingCrosslinked polyethylene plastic, which greatly improves the heat resistance and service life of the cable, and still maintains its excellent electrical properties. PVC insulated PVC sheathed power cable: The long-term working temperature of PVC insulated PVC sheathed low-voltage power cable shall not exceed 70 ℃, and the high temperature of the cable conductor shall not exceed 160 ℃. The duration of short circuit shall not exceed 5s, and the very low temperature of construction and laying shall not be lower than 0 ℃.