NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
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NOYAFA Electronic specializing in testing instruments and cable testers for 16 years
Author: Noyafa–CCTV Tester
Diagnosis of the nature of cable faults The so-called diagnosis of the nature of cable faults refers to determining: whether the fault resistance is high resistance or low resistance; whether it is a flashover or a closed fault; whether it is grounding, short circuit, disconnection, or a combination of them; Diagnosis of the nature of two-phase or three-phase fault cable faults The so-called diagnosis of the nature of cable faults means to determine: whether the fault resistance is high resistance or low resistance; whether it is a flashover or a closed fault; a mixture of; single-phase, two-phase, or three-phase faults. The nature of the fault can be preliminarily judged according to the phenomenon that occurs when the fault occurs. For example, when the running cable fails, if only the ground signal is given, it may be a single-phase grounding fault.
The relay protection overcurrent relay acts and trips, then there may be a two-phase or three-phase short circuit or grounding fault of the cable, or a combined short-circuit and grounding fault. When these faults occur, a short circuit or ground current blown cable will create a broken wire fault. However, the nature of the fault cannot be completely determined through the above judgment, and the insulation resistance must also be measured and the“Continuity test”.
When measuring insulation resistance, use a megohmmeter (for cables below 1kV, use a 500V megohmmeter; for cables above 1kV, use a 2500V megohmmeter) to measure between the cable cores and between the cores Insulation resistance to earth; conduct“Continuity test”When , short-circuit the three-phase end of the cable, and measure the resistance between the core wires at the head end of the cable with a multimeter. The measurement results of a faulty cable are listed in Table 1.2 for reference. According to the measurement results of insulation resistance listed in Table 1.2, it can be analyzed that the fault is two-phase grounding;“Continuity test”As a result, it was determined that the three-phase cable was not disconnected.
The state of this fault point is shown in Figure 1.4. Table 1.2 Measurement of insulation resistance and“Continuity test”Use a megohmmeter to measure insulation resistance (megohm) with a multimeter“Continuity test”AB2500AE2500AB0BC8BE5BC0CA2500CE3CA0 Figure 1.4 Cable line fault state diagram Due to the poor resolution of the megohmmeter, when the indication is zero, it cannot be considered that the fault resistance is zero ohms, and a multimeter should be used to measure the fault The exact value of the resistance to determine if the fault is low resistance. as in§As mentioned in Section 1-3, high resistance and flashover faults can be determined through withstand voltage tests, and the breakdown voltage at the fault point can be found out.
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