The detection of cable faults generally goes through three steps: diagnosis, distance measurement and localization. Cable fault testing is generally divided into two processes: that is, the fault cable fault point distance test; fault point fixed point test. Fault cable fault point distance test that ranging method has three kinds: loop bridge balance method; low voltage pulse reflection method; flashover method.
Loop bridge balance method is the use of DC bridges on the cable fault distance measurement method, referred to as the bridge method, the field personnel have Rf <100kΩ faults known as low-resistance fault habit, mainly because the traditional bridge method can measure such faults. The bridge method is quite accurate for the ranging of short-distance cable faults, so it is still in use. Based on the cable is uniform along the cable, the cable length and the cable core resistance is proportional to the cable, and according to the principle of the Whisden bridge, the cable short-circuit grounding, fault point on both sides of the loop resistance into the DC bridge, measuring the ratio. From the measured ratio and the full length of the cable, the distance from the measurement end to the fault point can be obtained.
The use of bridge method of single-phase ground fault cable ranging principle is the first at the other end of the cable, the fault phase of the cable and the normal phase of the cable conductor with not less than the cable cross-section of the wire across the connection. Then at one end of the fault phase of the cable conductor connected to the other end of the bridge on the son. Using the bridge method of cable two-phase short-circuit or two-phase short-circuit and grounding, fault ranging, you need to have a non-faulty conductor and faulty conductor together to form a ring, when the bridge is balanced to get the distance to the point of failure.
Low voltage pulse reflection method. Low-voltage pulse reflection method of detecting cable faults by the instrument's pulse generator sends out a pulse wave, through the lead to the fault phase of the pulse wave sent to the cable, the pulse wave propagates along the core of the cable, when propagated to the fault point, due to the fault point of the cable resistance to change, and thus there is a pulse signal is reflected back, with an oscilloscope in the test end of the record from the sending pulse and the reflection of the pulse between the time interval, you can Calculate the distance between the test end and the fault point.
Open-circuit and low-resistance faults can be used low-voltage pulse reflection method, low-voltage pulse reflection method is advanced in the field to make the measured fault waveforms have been greatly simplified, the complex high-voltage impact flashover waveforms into a very easy to read similar to the low-voltage pulse method of the short-circuit fault waveforms. Reduces the technical requirements of the operator and experience requirements, greatly improving the accuracy of the judgment of the field faults, to achieve the purpose of rapid and accurate testing of cable faults.
Flashover method. The basic principle of the flashover method is similar to the low-voltage pulse method, is the use of electric wave propagation in the cable at the point of failure to produce the principle of reflection, write down the electric wave at the test end of the fault cable between the point of failure between the round-trip time, and then according to the wave speed to calculate the location of the cable fault point. According to statistics, high resistance and flashover faults account for about 90% of the total number of cable faults. High resistance faults should be impact flashover method, and flashover faults can be tested by DC flashover method. The actual field is through the test method to distinguish between high resistance and flashover faults.