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The basic principle used by the cable fault tester to test cable faults is the “echo test†principle, which is based on the transmission and reflection of electric waves in the cable.
(1) Pulse method test principle: The pulse method test is to generate a pulse wave for the instrument. This pulse wave is added to the cable. When the pulse wave meets the point where the characteristic impedance of the cable changes, an echo signal will be generated (according to Transmission line principle) This instrument collects and displays these two signals (emission wave and echo) at the test end of the cable, and calculates the distance between the fault point and the test terminal based on the time difference between these two waves. Therefore, this test method is not affected by the cable laying rules and is only related to the transmission speed of the electric wave in this cable.
(2) High-voltage flashover test principle: In the case of a high-impedance fault for a cable, an external device is used to apply high voltage to the cable. When the faulty cable cannot withstand the applied high voltage, the breakdown point will produce a breakdown arc. Breakdown arc will produce a return wave, so that we can use the tester to record the entire breakdown process at the test end using the waveform of the acquisition, through the breakdown process to analyze and calculate the distance of the breakdown point from the test end. The same test method is not affected by the cable laying condition, and only relates to the transmission speed of the electric wave in this cable.
Cable fault test procedure
The following procedure should be followed:
(1) Multimeter, shake meter or cable pre-test results to determine the nature of the cable fault.
(2) Select the appropriate test method according to the nature of the cable fault:
Low-impedance faults and open-circuit faults are pulsed, but of course low-impedance faults can also be flash-over. Leakage high-impedance faults adopt flashover method (redundancy L method). Some flashback high-impedance faults can also use direct flash method, but usually do not do so. Flashover high-impedance faults adopt direct flash method, and they can also adopt two methods. The methods are tested to improve the accuracy of the comparison.
(3) Select the working ground point and draw two ground lines from the selected ground point, one to the ground of the instrument and the other to the ground of the high voltage equipment. The principle of selection of the working ground is to ensure that the test phase is formed as a closed loop so that the instrument can obtain the correct test waveform. It should also be noted that the working ground must be connected systematically with the human body, the instrument ground, the high voltage equipment ground, and the measured cable ground. To ensure personal safety and equipment and equipment. If the cable is a phase-to-phase fault, the working ground must be one of the phase lines and the ground line must be connected to the system ground. For lead-free bushings, the majority of faults in high-voltage flashover tests are ground or space discharges. Theoretically speaking, a relatively closed loop is also formed by a fault or space, but in practice, the equivalent impedance in the loop is quite large, and the correct test waveform cannot be obtained on the instrument. If you encounter this kind of situation in the test, you can solve it by directly fixing the point of the suspected fault. Regardless of the ground, the grounding point must be reliable.
(4) Use the pulse method to test the full length of the cable to calibrate the transmission speed of the cable under test.
(5) Roughly measure the fault points according to the selected test method. If one test method is not ideal, compare and test with other methods.
(6) After the rough test is completed, the fault point is fixed.
(7) After the test is completed, experience summary and error analysis can be conducted if necessary.
February 04, 2024