interprets the relationship between the megohmmeter output short-circuit current and the measured data-

When measuring capacitive load resistance, what is the relationship between the output short-circuit current of the megohmmeter and the measured data? The size of the source\'s internal resistance. When the product under test has capacitance, at the start of the test process, the high voltage source of the megohmmeter charges the capacitance through its internal resistance and gradually charges the voltage to the rated output high voltage value of the megohmmeter.

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 the load R and C (in seconds). Please note that the current charging the capacitor and the current flowing through the insulation resistance of the product under test flow together in the megohmmeter during the test. The current measured by the megger has not only the component on the resistorisolation stance, but also adds the capacitor charging current component. At this time, the measured resistance value will be too small.

For example: For a megger with a rated voltage of 5000V, if its short circuit output current is 80μA (Kyoritsu, Japan), its internal resistance is 5000V/80μA= 62 MΩ.
For example: if the capacitance of the sample is 0.15 μF, then the time constant τ=62 MΩ×0.15 μF≈9 (seconds), i.e. at 18 seconds, the charging current on the capacitor is still 11.3 μA.
It can be seen that the equivalent resistance formed by the load current alone is 5000V/11.3μA = 442MΩ. If the normal insulation is 1000MΩ, the measured insulation value displayed is only 306MΩ. This kind of test value can no longer reflect the actual situation of the insulation value, and the test value mainly changes with the change of the capacitive load capacity, that is-i.e. the small capacitance, the test resistor value is large; the capacitance is large, the test resistor value is small.

 Therefore, in order to ensure accurate measurement of the test values ​​of R15 and R60, a large capacity megohmmeter with fast charging speed should be selected. The relevant regulations of my country require the output short-circuit current of the megohmmeter to be greater than 0.5mA, 1mA, 2mA and 5mA. For high-demand occasions, try to choose a megohmmeter with a larger output short-circuit current.

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