Insulation Resistance Tester FAQs

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, etc., to ensure that these equipment, electrical appliances and lines work in normal conditions and avoid accidents such as electric shock casualties and equipment damage.

Common problems with insulation resistance testers include the following:

1. When measuring the resistance of a capacitive load, what is the relationship between the short-circuit current output by the insulation resistance tester and the measured data, and why?

The magnitude of the short-circuit current output by the insulation resistance tester can reflect the magnitude of the internal resistance of the high-voltage output source inside the megohmmeter.

Many insulation test objects are capacitive loads, such as long cables, motors with many windings, transformers, etc. Therefore, when the object under test has capacitance, at the beginning of the test process, the high-voltage source in the insulation resistance tester will charge the capacitance through its internal resistance, and gradually charge the voltage to the rated high-voltage output value of the insulation resistance tester. If the capacitance value of the object under test 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 Rinner and Cload (in seconds), that is, T=Rinner*Cload.

Therefore, when testing, such a capacitive load needs to be charged 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, the greater the charging current, and the faster and more stable the test results will be.

2. What is the function of the "G" terminal of the instrument? In a high-voltage and high-resistance test environment, why is it required to connect the instrument to the "G" terminal?

The "G" terminal of the instrument is a shielding terminal. The function of the shielding terminal is to eliminate the influence of moisture and dirt in the test environment on the measurement results. The "G" terminal of the instrument bypasses the leakage current on the surface of the tested product, so that the leakage current does not pass through the test circuit of the instrument, and eliminates the error caused by the leakage current. The G terminal is needed when testing high resistance values.

Generally speaking, if the resistance is higher than 10G, you can consider using the G terminal. However, this resistance range is not absolute. When the environment is clean and dry and the object to be measured is small, 500G can be measured stably without the G terminal; while in a humid and dirty environment, a lower resistance value also requires the G terminal. Specifically, if you find that the result is difficult to stabilize when measuring a higher resistance value, you can consider using the G terminal. In addition, please note that the shield terminal G is not connected to the shield layer, but to the insulator between L and E or to other wires in the multi-strand wire that are not being measured.

3. Why is it that when measuring insulation, not only the simple resistance value is required, but also the absorption ratio and polarization index are required? What is the significance?

PI is the polarization index, which refers to the comparison between the insulation resistance value at 10 minutes and the insulation resistance value at 1 minute during insulation testing;

DAR is the dielectric absorption ratio, which refers to the comparison between the insulation resistance value of 1 minute and the insulation resistance value of 15 seconds during insulation test;

In insulation testing, the insulation resistance value at a certain moment cannot fully reflect the quality of the test product's insulation performance. This is due to the following two reasons. On the one hand, for insulation materials with the same performance, the insulation resistance is small when the volume is large, and the insulation resistance is large when the volume is small. On the other hand, insulation materials will have an absorption ratio process and a polarization process for charges after high voltage is applied. Therefore, the power system requires that the absorption ratio - that is, the ratio of R60s and R15s, and the polarization index - that is, the ratio of R10min and R1min - should be measured in insulation tests on many occasions such as main transformers, cables, and motors, and these data are used to determine the quality of the insulation condition.

4. Why can the electronic insulation resistance tester generate a high DC high voltage when powered by several batteries? This is based on the principle of DC conversion. After the boost circuit is processed, the lower supply voltage is increased to a higher output DC voltage. Although the generated high voltage is higher, the output power is smaller (low energy and low current).

Note: Even at very low power, it is not recommended to touch the test probe, as there will still be a tingling sensation.