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Frequently Asked Questions about Insulation Resistance Teste

Announcer:Merike ElectronicsViews:2020-10-14

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 normal conditions to avoid electric shock, casualties and equipment damage.
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 the insulation resistance tester and the measured data, and why?
 
The size of the output short-circuit current of the insulation resistance tester can reflect the size of the internal resistance of the high-voltage source inside the megger.
 
Many insulation tests target capacitive loads, such as longer cables, motors with more windings, and transformers. Therefore, when the measured target has capacitance, at the beginning of the test process, the high-voltage source in the insulation resistance tester must charge the capacitor through its internal resistance, and gradually charge the voltage to the additional high-voltage output of the insulation resistance tester. . If the capacitance value of the measured target 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 inner and C load (unit: second), that is, T=R inner*C load.
 
Therefore, during the test, it is necessary to charge such a capacitive load to the test voltage, and the charging speed dV/dt is equal to the ratio of the charging current I to the load capacitance C. That is, dV/dt=I/C.
 
Therefore, the smaller the internal resistance and the greater the charging current, the faster the test results will be stable.
 
2. What is the function of the "G" side of the appearance? In a high-voltage and high-resistance test environment, why is it required to connect the "G" terminal externally?
 
The “G” end of the surface is a shielding terminal. The function of the shielding terminal is to remove the influence of humidity and dirt in the test environment on the measurement results. The external "G" terminal bypasses the leakage current of the tested product, so that the leakage current does not pass through the external test circuit, and eliminates the error caused by the leakage current. The G terminal is used when testing high resistance.
 
Generally speaking, the G terminal can be considered for higher than 10G. However, this resistance range is not certain. When it is clean and dry and the volume of the test object is small, it can be stable without measuring 500G at the G end. In humid and dirty environments, a lower resistance value also requires the G end. Specifically, if you find that the results are difficult to stabilize when measuring higher resistance, you can consider using the G terminal. Also note that the shielding terminal G is not connected to the shielding layer, but to the insulator between L and E or to the multi-stranded wire, not to the other wires under test.
 
3. Why is it not only required to measure the pure resistance value when measuring insulation, but also to measure the absorption ratio and polarization index. What's the point?
PI is the polarization index, which refers to the comparison between the insulation resistance of 10 minutes and the insulation resistance of 1 minute during the insulation test;
 
DAR is the dielectric absorption ratio, which refers to the comparison between the insulation resistance of 1 minute and the insulation resistance of 15s during the insulation test;
 
In the insulation test, the insulation resistance value at a certain moment cannot fully reflect the insulation function of the test sample. This is due to the following two reasons. On the one hand, the insulation resistance of the same function of the insulation material is small when the volume is large. , The insulation resistance appears when the volume is small. On the other hand, the insulating material has the process of the absorption ratio and the polarization process of the charge after the high voltage is applied. Therefore, the power system requires the measurement of the absorption ratio-the ratio of R60s and R15s, and the polarization index-the ratio of R10min and R1min in the insulation test of main transformers, cables, motors and many other occasions, and use this data to determine the insulation Good or bad.
 
 
4. Why can the electronic insulation resistance tester produce higher DC high voltage when powered by several batteries? This is based on the principle of DC conversion. The lower power supply voltage is raised to a higher output DC voltage through the boost circuit processing. The high voltage generated is higher but the output power is small (low energy and small current).
 
Note: Even if the power is very small, it is not recommended to personally touch the test probe, there will still be a tingling sensation.

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