Common methods used to locate faults in power cables
Fault location in power cables generally proceeds in four steps: fault nature diagnosis, fault distance measurement, path detection, and fault pinpointing. Methods for fault distance measurement mainly include: first, the bridge method, which mainly includes the traditional DC bridge method, voltage drop comparison method, and DC resistance method; second, the low-voltage pulse method, also known as the radar method, which involves injecting a low-voltage pulse signal into the cable from one end through an instrument. When the pulse encounters a fault point with mismatched wave impedance, it reflects and returns to the measuring instrument. This method has advantages such as simple operation and high testing accuracy, and is mainly used for testing open circuits and low-resistance faults, but cannot test high-resistance faults and flashover faults; third, the pulse voltage method, which requires applying a DC high-voltage signal to the faulty cable; fourth, the pulse current method, which also requires applying a DC high-voltage signal to the faulty cable; fifth, the secondary pulse method.
Main causes of faults in street light low-voltage cables
1. External force damage
Due to the non-standard and arbitrary nature of road construction and other pipeline construction, street light cables are often cut or damaged. Moreover, most construction units, after digging out street light cables, do not notify the street light management department and privately re-bury them. The cable damage caused by this, after a period of operation, will lead to the complete destruction of the cable at that location.
2. Long-term overload operation of cables
Due to design deficiencies, "shortchanging" of cable sizes, unbalanced three-phase loads, and arbitrary load increases, some cables operate under overload for long periods, causing the cable temperature to rise. Especially in hot summer, the temperature rise often leads to breakdown at the weaker points and joints of the cable first. This is why the cable failure rate is high in summer.
3. Poor cable construction quality
Rough construction and illegal dragging of cables can damage the cable sheath. After several months or even years, moisture ingress and reduced insulation can lead to complete breakdown at the damaged site, causing a fault. In addition, cable joints are the weak link in cable lines. If construction personnel do not crimp tightly, wrap properly, or shrink well during the production of cable joints, it can lead to reduced insulation at the cable head and trigger faults.
Common types of faults in street light low-voltage cables
1. According to the burial method, they can be divided into open faults that leak to the soil and closed faults that do not leak to the soil.
2. According to the fault location, they are divided into faults in the cable body, inside the lamp pole, under the lamp pole, in inspection wells, and at joints.
According to the nature of cable faults, they are divided into:
(1) Open circuit: This is caused by long-term high current burning or external force digging. The phase line or neutral line is broken, and sometimes power can be supplied normally but some street lights do not light up. This type of fault is common in street light cable faults and is relatively easy to resolve.
(2) Short circuit: Often manifested as a short circuit between the neutral line and phase line, or between phase lines. This is caused by instantaneous high current sintering, and normal power supply is not possible at this time. If the outer sheath is damaged, the short circuit fault is easier to resolve, but if the sheath is not damaged, it is more difficult to resolve.
(3) Poor insulation: Power can be supplied for a short time, but the current in the circuit is abnormal. After a period, the circuit breaker protection trips, which we call a "soft fault". This is caused by damage to the cable sheath or aging of insulation, and this type of fault is a difficulty in our testing.
