Insulators do not conduct electricity only in a relative sense. As the external environmental conditions change, in reality, no insulating material is completely non-conductive. Any insulating material, when a voltage is applied at both ends, there will always be a certain current flowing through it. The active component of this current is called leakage current and this phenomenon is also known as the leakage of the insulator.
Leakage current is actually the current that flows through the insulating part of an electrical circuit or equipment when there is no fault and no voltage is applied. Therefore, it is one of the important indicators for measuring the insulation quality of electrical appliances and is also the main indicator of product safety performance. Limiting the leakage current to a very small value plays an important role in improving the safety performance of the product.
Due to the limitations of insulation resistance measurement, the measurement of leakage current was introduced in insulation tests. The equipment used for measuring leakage current is generally more complex than a megohmmeter and usually involves the use of high-voltage rectification equipment for testing. Because the test voltage is high, it is easier to expose the weaknesses of the insulation itself. The leakage current can be directly measured using an ammeter, which allows for continuous monitoring and has 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 testing method.
The direct current withstand voltage test (also known as the direct current high potential test) is when the voltage applied to the insulation exceeds the AC rated voltage of the electrical equipment. It is maintained at this high voltage for a period of time (such as 5 minutes). It is conducted based on the leakage current test. The direct current withstand voltage test can often detect some local defects that are not easily found in the AC withstand voltage test, such as the end insulation defect of a generator. It also has the characteristics of having lighter test equipment, no polarization loss, and causing much less damage to the insulation compared to AC voltage. Therefore, it is widely used. It belongs to a destructive test. It is complementary to the AC withstand voltage test and cannot be substituted for each other. The test voltage value varies for different electrical equipment.
The DC withstand voltage test has a relatively high voltage level and plays a special role in detecting certain local defects in insulation. It can be conducted simultaneously with the leakage current test. Compared with the AC withstand voltage test, the DC withstand voltage test has the advantages of lighter test equipment, less damage to insulation, and easier detection of local defects in equipment. In contrast to the AC withstand voltage test, the main disadvantage of the DC withstand voltage test is that the voltage distribution within the insulation under AC and DC is different, and the DC withstand voltage test is less realistic in testing the insulation compared to the AC test.
Post time: Mar-13-2026