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Check cable quality, insulation status and various tests on cable lines. Conventional tests include leakage current and insulation tests.
1. Insulation resistance test
When measuring the insulation resistance of cables, it is necessary to measure the insulation resistance between the outer skin and core wires or between the outer skin and core wires. The basic principle of measuring insulation resistance is similar to that of measuring ordinary resistance. It is also to add a relatively stable DC voltage to both ends of the test sample, and then express the relationship between the current and time flowing through the test sample by measuring instrument. Thus, the curve of insulation resistance of cable sample with time can be obtained by conversion, and a specific time can be obtained on the curve. Insulation resistance value. For large capacity cables, the concept of absorption ratio is usually more illustrative, so it is used to replace the results of insulation resistance measurement. In practical engineering measurement, megaohmmeter is usually used as measuring equipment. The wiring diagram is shown in Figure 1.1. Among them, E represents ground terminal, L represents line terminal, G represents shielding terminal. When measuring insulation resistance, it is necessary to connect the sample between the circuit terminal and ground terminal. The measured insulation resistance includes both volume resistance and surface resistance. Usually, the surface of the sample can be connected to the shielding terminal to avoid the influence of surface resistance on the measurement results.
2. Leakage current test
The basic principle of leakage current test is to find the insulation defect in cable by measuring the current flowing through the cable under DC voltage. Fig. 1.2 shows the basic principle of leakage current test. In general, the DC withstand voltage test and leakage current test of cable insulation are carried out simultaneously. In leakage current test, appropriate micro-ampere meters need to be installed on the high voltage side of the sample. The basic principle of the test is almost the same as that of the insulation resistance test. The difference is that the high voltage rectifier is used as the DC power supply in the test, and the microampere is used as the indicator of the current in the test. The index of good insulation is reflected by leakage current displayed by micro-ammeter.