Test method for distance of cable fault point_Anhui Honglin Instrument Cable Co., Ltd.

Test method for distance of cable fault point

Author：hlyb Add Time：2016-07-12 10:36:05

The detection of cable fault generally goes through three steps: diagnosis, ranging and fixed point. The test of cable fault is generally divided into two processes: the test of the distance between the fault point of the faulty cable; Test of fault point and fixed point. There are three methods to measure the distance between fault points of faulty cables: loop bridge balance method; Low voltage pulse reflection method; Flashover method.

The circuit bridge balance method is a method of using DC bridge to locate cable faults. It is called bridge method for short. Field personnel have the habit of calling the fault with RF < 100k Ω as low resistance fault, mainly because the traditional bridge method can measure this kind of fault. The electric bridge method has high accuracy for short-distance cable fault location, so it is still in use at present. Based on the uniformity along the cable, the cable length is directly proportional to the cable core resistance, and according to the principle of Wheatstone bridge, the cable short-circuit grounding and the loop resistance on both sides of the fault point are introduced into the DC bridge to measure its ratio. The distance from the measuring end to the fault point can be obtained from the measured ratio and the total length of the cable.

The principle of using the bridge method to locate the single-phase grounding fault of the cable is to jumper the cable conductor of the fault phase and the normal phase of the cable with a conductor not less than the cable section at the other end of the cable. Then connect the cable conductor of the fault phase at one end to the other end of the bridge. When using the bridge method to locate the two-phase short circuit or two-phase short circuit and grounding of the cable, a non fault conductor and the fault conductor need to form a ring together. When the bridge is balanced, the distance of the fault point can be obtained.

Low voltage pulse reflection method. Low voltage pulse reflection method for detecting cable fault is that the pulse generator of the instrument sends a pulse wave to the fault phase of the cable through the lead. The pulse wave propagates along the cable core. When it propagates to the fault point, a pulse signal is reflected because the wave resistance of the cable at the fault point changes, The distance between the test end and the fault point can be calculated by recording the time interval between the transmitted pulse and the reflected pulse at the test end with an oscilloscope.

Low voltage pulse reflection method can be used for open circuit and low resistance faults. The advantage of low voltage pulse reflection method is that the fault waveform measured on site is greatly simplified, and the complex high-voltage impulse flashover waveform is transformed into a short-circuit fault waveform similar to low-voltage pulse method. It reduces the technical and experience requirements for operators, greatly improves the judgment accuracy of on-site faults, and achieves the purpose of rapid and accurate testing of cable faults.

Flashover method. The basic principle of the flashover method is similar to that of the low-voltage pulse method. It uses the principle of reflection at the fault point when the radio wave propagates in the cable, records the round-trip time of the radio wave between the fault points at the test end of the fault cable, and then calculates the location of the cable fault point according to the wave speed. According to statistics, high resistance and flashover faults account for about 90% of the total cable faults. Impulse flashover method shall be used for high resistance fault, and DC flashover method can be used for flashover fault. In practice, high resistance and flashover faults are distinguished by test methods.

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Test method for distance of cable fault point