When performing insulation testing, most engineers focus on the voltage range and resistance range of an insulation megohmmeter or insulation resistance tester. Choosing the correct test voltage and resistance range can ensure the safety of the test and obtain more accurate test values. But there is another parameter that is often overlooked by the majority of engineers, and more importantly, it has a great impact on the test results.
The magnitude of the output short-circuit current of the insulation resistance tester (megohmmeter ) can reflect the size of the internal resistance of its internal output high-voltage source.
To understand the role of short-circuit currents, first need to understand the principle of insulation testing. The insulation resistance is the value calculated by Ohm's law of the DC voltage on the device under test and the leakage current flowing through the device under test. Therefore, what needs to be tested is the leakage current flowing through the device under test when a certain voltage is applied.
But in fact, the [leakage current I total] measured by the insulation test includes three parts:
(1) I1 capacitor current, in the process of applying voltage, is equivalent to charging the capacitor of the device under test, the initial charging stage, the capacitor current is large, and then the capacitor current will decay to zero as the charging is completed.
(2) I2 absorbs current, and during the process of applying voltage to stabilization, the current formed when the charge is rearranged due to the polarization of the dielectric will also decay to zero with stability.
(3) I3 leakage current, the current generated due to poor insulation, generally remains unchanged, and is also a key parameter in insulation testing.
In the early stage of applying voltage, such as 15s, the existence of I1 and I2 will increase the overall [leakage current I total], resulting in the measured R15s being smaller than the actual value, unable to reflect the true resistance, and too small R15s will also make the dielectric absorption more serious than R60S/R15S, causing engineers to misjudge.
At this time, it is necessary to talk about [short-circuit current]. A larger short-circuit current can make the capacitor charging process faster and stabilize earlier, so the faster the capacitor current I1 and the absorb current I2 will decay to zero, reducing the impact, and the final resistance value will be closer to the true insulation resistance.
The relevant standard DL/T845.1-2004 of China's power industry stipulates: "The output short-circuit current of an insulation megohmmeter shall not be less than a definite value in the sequence of 0.1、0.2、0.3、0.5、0.6、0.8、1、1.2、1.5、2、2.5、3、4、5、8、10mA.
At present, most of the insulation megohmmeter short-circuit current in the world is between 1mA ~ 5mA, and the occasions with high requirements should try to choose the insulation resistance tester with a large output short-circuit current. Kingrun newly developed JYM insulation resistance tester, Short circuit current: not less than 30mA (this index far exceeds the traditional megohmmeter, making the test faster and the results more accurate.
