The methods and characteristics of DC leakage current measurement and DC withstand voltage test

Methods and characteristics of DC leakage current measurement and DC withstand voltage test
Due to the limitations of insulation resistance measurement, the measurement of leakage resistance was introduced in insulation tests. The equipment used for measuring leakage current is more complex than a megohmmeter and is generally tested using high-voltage rectification equipment. Because the test voltage is high, it is easy to expose the weaknesses of the insulation itself. Measuring the leakage current directly with an ammeter allows for continuous monitoring at any time, with high sensitivity. Moreover, the relationship curves between voltage and current, and current and time can be used to determine the insulation defects. Therefore, it belongs to a non-destructive test.
The voltage is applied to the insulator in stages, allowing for control over the voltage. As the voltage increases, the weak insulation will experience a significant leakage current, resulting in a lower insulation resistance.
The wiring and principle of the DC leakage current test are the same as those of the DC withstand voltage test, and they are often conducted simultaneously in on-site tests. The principle is the same as that of insulation resistance measurement. The difference lies in that when measuring the leakage current or the DC withstand voltage test, the power supply mostly uses adjustable DC high-voltage devices – DC high-voltage generators, and the current flowing through the test specimen is directly measured using an ammeter.

The DC leakage current test and the DC withstand voltage test are usually carried out simultaneously using the same set of DC high-voltage generator equipment.
Test method: Apply high voltage and measure current. A DC high-voltage generator can be used to adjust the required high voltage, and an ammeter can be used to measure the leakage current with greater accuracy.
Test Characteristics:
(1) The test voltage is high and can be adjusted at will.
When measuring leakage current, a corresponding test voltage is applied to the tested equipment at a certain voltage level. This test voltage is much higher than the rated voltage of the megohmmeter, which makes it easy to expose the inherent weaknesses of the insulation. Because certain defects or weaknesses in the insulation can only be exposed under a higher electric field intensity.
(2) The test voltage is stable. The microampere-level measuring instrument directly measures the leakage current, with high sensitivity. It also exhibits good repeatability under the same test conditions.
To make the conversion, you first need to know the voltage applied to the test equipment. Although the megohmmeter has the specified voltage value marked on its nameplate, the actual voltage applied to the tested equipment is not necessarily this value. It is related to the insulation resistance of the tested equipment. When the insulation resistance of the tested equipment is very low, the voltage applied to the tested equipment is also very low. Only when the insulation resistance tends to infinity, the voltage applied to the tested equipment will approach the value marked on the nameplate. This is because when the insulation resistance of the tested equipment is too low, the internal resistance voltage drop of the megohmmeter causes a significant decrease in the voltage at the “line” terminals.
(3) During the leakage current test, a relationship curve between the leakage current and the application time of voltage, as well as a relationship curve between the leakage current and the applied voltage can be plotted. This enables the assessment of the insulation condition. Under direct current voltage, when the insulation is damp or has defects, the current decreases relatively slowly with the increase of the application time, and the final steady-state value is also larger, indicating that the insulation resistance is smaller.
(4) The DC high-voltage generator of the test instrument is lightweight and suitable for carrying during on-site tests.
The DC withstand voltage test and the DC leakage current test have the same methods but serve different purposes.
DC withstand voltage test has a high voltage level and lasts for a long time – it tests the insulation strength.
The test voltage for the DC leakage current is relatively low – used to check the insulation condition.