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FAQ Of Insulation Resistance Tester

Time:2020-07-28 Number:5

       Insulation resistance tester, also known as megohmmeter, is suitable for insulation test in the maintenance, repair, test and verification of various electrical equipment. GDZX sorted out the insulation resistance tester common questions and answers summary, for your reference.
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Q1: 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?

A1: the output short-circuit current of insulation resistance tester can reflect the internal resistance of high-voltage source. 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. Obviously, if the capacitance value of the test 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). Please note that the current charging the capacitor and the current flowing through the insulation resistance of the tested object flow into the insulation resistance tester together during the test. The current measured by the insulation resistance tester includes not only the component on the insulation resistance, but also the component of the capacitor charging current. At this time, the measured resistance will be smaller.

Q2: when testing insulation, it is required to measure not only the pure resistance, but also the absorption ratio and polarization index. What is the reason?

A2: in the insulation test, the insulation resistance value at a certain time can not fully reflect the insulation performance of the test object. This is due to the following two reasons: on the one hand, the insulation resistance of the insulation material with the same performance is small when it is large, but it is large when the volume is small.

       On the other hand, the process of charge absorption ratio and polarization exists in the insulating materials after applying high voltage. Therefore, the power system requires that in the insulation test of main transformer, cable and motor, the absorption ratio, that is, the ratio of R60s to R15s, and the polarization index, that is, the ratio of R10min to R1min, should be measured in the insulation test of main transformer, cable, motor and so on.

Q3: in the test environment of high voltage and high resistance, it is required to connect the instrument with "G" terminal. Why?

A3: when the higher rated voltage is applied at both ends of the tested object and the insulation resistance is high, the surface of the tested object is wet, and the leakage caused by pollution is large, and the indication error is large. The "G" end of the instrument is the current bypass of the leakage surface of the tested object, so that the leakage current does not pass through the test circuit of the instrument, and the error caused by the leakage current is eliminated.

Q4: when measuring the rated output DC high voltage at both ends of "L" and "E" of some types of insulation instruments, use the pointer type multimeter DCV to measure the voltage at L and E. Why does the voltage drop a lot, but the digital multimeter does not?

A4: use ordinary pointer type multimeter to measure the rated DC voltage of insulation resistance tester "L" and "E" directly. The measured result is much smaller than the nominal rated voltage value (out of the error range), while the digital multimeter will not. This is because the internal resistance of pointer multimeter is smaller than that of digital multimeter. The internal resistance of pointer multimeter is small, and the output voltage of L-E terminal of insulation resistance tester decreases a lot, which is not the output voltage of normal operation. However, it is wrong to measure the output voltage of the insulation resistance tester directly with a multimeter. It should be measured with an electrostatic high-voltage meter with a large internal resistance impedance or a voltage divider with sufficient load resistance. 

Q5: can we use a megohmmeter to directly measure the charged object under test? What is the effect of the result and why?

A5: for personal safety and normal test, in principle, it is not allowed to measure the charged test object. If the charged object is to be measured, the instrument will not be damaged (in a short period of time), but the test result is inaccurate. After being charged, the tested object will be connected with other test objects, so the results obtained can not truly reflect the actual data, but with other tests The resistance value in parallel or series.

Q6: why do several batteries of electronic insulation resistance tester generate high DC voltage?

A6: according to the principle of DC conversion, the lower supply voltage is raised to a higher output DC voltage after being processed by the boost circuit. Although the high voltage generated is high, the output power is small. (for example, several batteries of electric baton can generate tens of thousands of volts of high voltage)

Q7: when measuring insulation resistance with insulation resistance tester, what factors will cause inaccurate measurement data? Why?

A7:

① Insufficient battery voltage. Battery voltage under voltage is too low, resulting in the circuit can not work normally, so the readings measured are not accurate.

② The test wiring is not correct. Wrong connection of "L", "G", "E" three terminals, or connecting "G" and "L" to both ends of the tested object.

③ The "G" terminal is not connected. Due to the error caused by the current leakage of the tested object due to pollution, humidity and other factors, it is necessary to connect the "G" terminal wire to prevent the error caused by leakage current.

④ Too much interference. If the tested object is subject to excessive environmental electromagnetic interference, the meter reading will jump. Or the pointer shakes. The result is inaccurate reading.

⑤ Artificial reading error. When measuring with the pointer type insulation resistance tester, the indication value is not accurate due to the artificial angle error or scale error.

⑥ Instrument error. The error of the instrument itself is too large and needs to be corrected again.

Q8: when measuring capacitive load (such as main transformer), the pointer shows that the resistance value suddenly drops in a certain section (not the slow and small swing in the normal test interval), and swings back and forth quickly. Why?

A8: this phenomenon is mainly caused by discharge and ignition in some part of the test system. When the capacitive test object is charged to a certain voltage, the above phenomenon will occur if the internal test line of the instrument or any part of the tested object has breakdown discharge and ignition. The discrimination method was as follows:

① If the instrument test stand is not connected to the test line, turn on the power supply and high voltage to see if there is ignition in the instrument (if there is a spark, you can hear the sound of discharge ignition).

② Connect the L, G and E test lines, and do not connect the tested object. The end clamp of L test line is suspended and the high voltage is turned on to see whether there is ignition phenomenon on the test wire. In case of ignition, check: a) whether the core wire (L end) of L and G test line is too close to the exposed wire (g end) to produce arc striking. b) The L-end core wire plug and the shielding ring of the test stand or the test clamp are not in good contact with the tested object, resulting in ignition. c) The open circuit of false welding between test line, plug and clamp results in gap discharge.

③ Connect the tested object and check whether there is discharge and ignition near the contact point between the terminal clamp and the test object.

④ Remove the above reasons, connect the tested object and turn on the high voltage. If the above phenomenon still exists in the instrument, it indicates that the insulation breakdown of the tested object causes partial discharge or arcing.

Q9: Why are the values measured by different insulation resistance testers different?

A9: due to the non ideal voltage source of the test power supply of the high voltage insulation resistance tester, the internal resistance RI is different, the series resistance rm of the measuring circuit is different, the dynamic measurement accuracy is different, and the field measurement operation is unreasonable or wrong, the measurement results of different types of insulation resistance tester for the same tested object will be different. In the actual measurement, the particularity of insulation test conditions of insulation resistance tester should be considered to reduce the possible measurement errors as far as possible:

① When measuring the same sample with different types of insulation meters, the same voltage level and wiring method should be adopted. For example, in measuring the insulation of high-voltage winding of power transformer, when the L-terminal button of insulation resistance tester is always connected to the outgoing end of winding, there is a direct method of connecting E-terminal button with low-voltage winding and shell, and G-terminal button hanging in the air; E-terminal is connected to the low-voltage winding, and the G-terminal is connected to the shell (low potential shielding); the G-terminal button is connected to the surface of the bushing of the high-voltage winding, and the E-terminal button is connected to the low-voltage winding first, and then connected to or not connected to the shell (high potential shielding). The E-terminal is connected to the shell, and the G-terminal is connected to the low-voltage winding. For insulation resistance tester with different structure and system, the potential of G-terminal button is different, so the placement position of G-terminal button on the surface of bushing should be changed accordingly.

② Different types of insulation resistance tester have different range and indication calibration method, different calibration resolution and different measurement accuracy level, which will lead to the difference between indication values. In order to ensure the accurate measurement of power equipment, it is necessary to avoid the use of low accuracy and inconvenient megger.

③ Most of the samples contain capacitive components and have dielectric polarization phenomenon. Even if the test conditions are the same, it is difficult to obtain ideal data repeatability.

④ During the measurement, the temperature of insulating medium and oil temperature shall be consistent with the ambient temperature, and the difference is generally ± 5%.

⑤ The measured values should be read as soon as possible within the allowable time difference of a specific time period. In order to make the measurement error not more than ± 5%, the allowable error of reading R60s time is ± 3S, while the reading time of r15s should not be different by ± 1s.

⑥ High voltage test power supply is not ideal voltage source, and the output voltage is lower than its rated value under heavy load (the insulation resistance value of the tested object is small), which will lead to the decrease of measurement accuracy of insulation resistance tester with single branch direct reading method due to the change of conversion coefficient. This change varies with the load characteristics of power supply tested by insulation resistance tester.

⑦ For the insulation resistance tester with different dynamic test capacity indexes, the establishment process of test voltage on the test object (and sampling resistance) and the charging capacity of the test object are different, and the measurement results will also be different In the inertial network (including the mechanical inertia of pointer instrument), the response speed of indication value is slow, and it is too late to correctly reflect the change rule of real insulation resistance value of test object with time. Especially in the initial stage of test, the charging current of capacitor is not completely attenuated to zero, which will make the r15s and absorption ratio reading value have larger error (smaller).

⑧ The polarization of the dielectric is related to the applied voltage. Because the test voltage cannot reach the rated value quickly, or the test voltage applied to the test object is different due to the different load characteristics of the test power supply of the insulation resistance tester, the initial polarization state of the test object is different, resulting in different absorption current and different indication value of edge resistance measurement.

⑨ The test high voltage of some foreign insulation resistance testers is continuously adjustable, and it is adjusted from zero to rated value after starting. The uncertainty of the starting time of the insulation resistance tester reading and the uncertainty of the time when the high voltage reaches the rated value make the initial polarization of the test object different, and also cause the difference between the indication values.

⑩ Different insulation resistance testers have different sensitivity and resistance to field interference, and the reading value of the same sample will be different.

⑪ The difference of indication value is caused by the difference between the conventional measurement error of random fluctuation of data and the method error of insulation resistance tester.

⑫ Insufficient dielectric discharge is one of the important reasons for the difference of repeated measurement results. According to the corresponding and reversible characteristics of the charging absorption current and its reverse discharge current, if the same sample needs to be measured for the second time, the short-circuit discharge interval time of the sample after the first measurement should be longer than that of the measurement In order to fully discharge the accumulated absorbed charge, the insulating medium of the sample can be fully recovered to the original non polarization state, otherwise the accuracy of the second measurement data will be affected. In order to make the tested object have no residual charge, the measuring end should also be short circuited to the ground before each test, sometimes even for nearly 1 hour, and the wire connecting with the irrelevant equipment should be removed. In a word, the same test voltage level and wiring method should be adopted for insulation measurement of the same sample in different periods, and the same type or similar performance insulation resistance meter should be used as far as possible to ensure the comparability of measurement data.

⑬ It is especially emphasized that the instrument with low dynamic measurement accuracy and low high-voltage test power supply capacity is selected. Because the capacitor charging current has not yet completely attenuated to zero, and the indication value of the instrument can not accurately follow the change of the apparent insulation resistance value of the test object in real time, the resistance value of R15s is low, and there is a big error It should be paid special attention by testers. This may also be the main reason for the difference of absorption ratio between different types of high voltage insulation resistance tester when measuring the same sample. It also shows that the index of absorption ratio is less scientific and objective than polarization index.
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       After understanding the common questions and answers of insulation resistance tester, if you have a deeper understanding of insulation resistance tester? GDZX is a professional power testing equipment manufacturer, the production of instruments are: series resonance, high voltage test equipment, transformer test equipment, relay protection tester, high voltage switch tester, cable fault tester, battery discharge meter, AC/DC standard source, arrester tester, phase volt ampere meter, etc., with many kinds of products, welcome to buy!