The insulation resistance tester is suitable for measuring the resistance value of various insulating materials and the insulation resistance of transformers, motors, cables and electrical equipment to ensure that these equipment, electrical appliances and lines work in a normal state and avoid accidents such as electric shock casualties and equipment damage.
The common problems of insulation resistance tester are as follows:
1. When measuring capacitive load resistance, what is the relationship between the output short-circuit current of insulation resistance tester and the measured data, and why?
The output short-circuit current of the insulation resistance tester can reflect the internal resistance of the high-voltage source.
Many insulation test objects are capacitive loads, such as long cables, motors with more windings, transformers, etc. Therefore, when the measured object has capacitance, at the beginning of the test process, the high voltage source in the insulation resistance tester should charge the capacitor through its internal resistance, and gradually charge the voltage to the output rated high voltage value of the insulation resistance tester. If the capacitance value of the measured object is large, or the internal resistance of the high voltage source is large, the charging process will take longer.
Its length can be determined by the product of R and C load (in seconds), i.e. t = R * C load.
Therefore, during the test, the capacitive load needs to be charged to the test voltage, and the charging speed DV / DT is equal to the ratio of charging current I and load capacitance C. That is DV / dt = I / C.
Therefore, the smaller the internal resistance is, the larger the charging current is, and the faster and more stable the test result is.
2. What is the function of “g” end of instrument? In the test environment of high voltage and high resistance, why is the instrument connected to the “g” terminal?
The “g” end of the instrument is a shielding terminal, which is used to eliminate the influence of moisture and dirt in the test environment on the measurement results. The “g” end of the instrument is to bypass the leakage current on the surface of the tested object, so that the leakage current does not pass through the test circuit of the instrument, eliminating the error caused by the leakage current. When testing the high resistance value, the G end needs to be used.
Generally speaking, the g-terminal can be considered when it is higher than 10g. However, this resistance range is not absolute. It is clean and dry, and the volume of the object to be measured is small, so it can be stable without measuring 500g at the g-end; In wet and dirty environment, lower resistance also needs g terminal. Specifically, if it is found that the result is difficult to be stable when measuring high resistance, the g-terminal can be considered. In addition, it should be noted that the shielding terminal G is not connected to the shielding layer, but connected to the insulator between L and E, or in the multi strand wire, not to other wires under test.
3. Why is it necessary to measure not only the pure resistance, but also the absorption ratio and polarization index when measuring the insulation?
PI is the polarization index, which refers to the comparison of insulation resistance in 10 minutes and 1 minute during insulation test;
DAR is the dielectric absorption ratio, which refers to the comparison between the insulation resistance in one minute and that in 15s;
In the insulation test, the insulation resistance value at a certain time can not fully reflect the quality of the insulation performance of the test object. This is due to the following two reasons: on the one hand, the insulation resistance of the same performance insulation material is small when the volume is large, and large when the volume is small. On the other hand, there are charge absorptivity and polarization processes in insulating materials when high voltage is applied. Therefore, the power system requires that the absorption ratio (r60s to r15s) and polarization index (r10min to r1min) should be measured in the insulation test of main transformer, cable, motor and many other occasions, and the insulation condition can be judged by this data.
4. Why can several batteries of electronic insulation resistance tester produce high DC voltage? This is based on the principle of DC conversion. After the boost circuit processing, the lower supply voltage is raised to a higher output DC voltage. Although the generated high voltage is higher, the output power is smaller (low energy and small current).
Note: even if the power is very small, it is not recommended to touch the test probe, there will still be tingling.
Post time: May-07-2021