Withstanding Voltage Test, also known as Hipot Test or Dielectric Test, is perhaps the most familiar and widely used in product safety testing processes. AC withstanding voltage {test transformer, variable frequency series resonance, ultra-low frequency high voltage generator}
The withstanding voltage test is a non-destructive test used to check whether the insulation capability of a product is sufficient under frequently occurring transient high voltages. It applies high voltage to the device under test for a certain period to ensure that the insulation performance is strong enough. DC withstanding voltage {DC high voltage generator}
Test voltage: Most safety standards allow the use of AC or DC voltage in withstanding voltage tests. If AC test voltage is used, when the voltage reaches its peak, whether positive or negative, the insulation under test bears maximum stress. Therefore, if you decide to use DC voltage for testing, you must ensure that the DC test voltage is N times the AC test voltage so that the DC voltage can be equivalent to the AC voltage peak. For example: 1500V AC voltage, to produce the same amount of electrical stress, DC voltage must be 1500 × 1.414, i.e., 2121V DC voltage.
One advantage of using DC test voltage is that in DC mode, the current flowing through the alarm current measuring device of the withstanding voltage tester is the actual current flowing through the sample. Another advantage of DC testing is that voltage can be applied gradually. By monitoring the current through the sample as the voltage increases, the operator can detect before breakdown occurs. It should be noted that when using a DC withstanding voltage tester, due to the charging of capacitance in the circuit, the sample must be discharged after the test is completed. In fact, regardless of the test voltage or product characteristics, it is beneficial to discharge the product before handling it.
The disadvantage of DC withstanding voltage testing is that it can only apply test voltage in one direction, unlike AC testing which can apply electrical stress in both polarities, while most electronic products operate on AC power. Additionally, because DC test voltage is more difficult to generate, DC testing is more costly than AC testing.
Differences and functions of AC withstanding voltage and DC withstanding voltage tests The advantage of AC withstanding voltage testing is that it can detect all voltage polarities, which is closer to actual practical conditions. In addition, since AC voltage does not charge capacitors, in most cases, there is no need to gradually increase the voltage; directly outputting the corresponding voltage can yield a stable current value. Moreover, after AC testing, there is no need to discharge the sample.
The disadvantage of AC withstanding voltage testing is that if there are large Y capacitors in the circuit under test, in some cases, AC testing may cause misjudgment. Most safety standards allow users to either not connect Y capacitors before testing or switch to DC testing. DC withstanding voltage testing will not cause misjudgment when applying high voltage to Y capacitors because the capacitors will not allow any current to pass through at that time.
AC withstanding voltage test is the most effective and direct method to identify the insulation strength of electrical equipment.
During operation, the insulation of electrical equipment is gradually degraded by the long-term effects of electric fields, temperature, and mechanical vibration, including overall degradation, forming defects. For example, local defects exist due to concentrated electric fields in some areas or weak local insulation. Various preventive test methods each have their strengths and can detect some defects, reflecting the insulation condition, but the test voltage of other test methods is often lower than the working voltage of electrical equipment, which is not strong enough to guarantee safe operation. Although DC withstanding voltage test has a relatively high test voltage and can detect some insulation weaknesses, since most insulation of electrical equipment is composite dielectric, under DC voltage, the voltage is distributed according to resistance, so weaknesses under AC electric fields may not be detected by DC testing. AC withstanding voltage test conforms to the electrical conditions that electrical equipment withstands during operation, and the AC withstanding voltage test voltage is generally higher than the operating voltage. Therefore, after passing the test, the equipment has a larger safety margin, making this test an important means to ensure safe operation.
However, because the test voltage used in AC withstanding voltage test is much higher than the operating voltage, excessively high voltage can increase insulation loss, heat, and discharge, accelerating the detection of insulation defects. Therefore, in a sense, AC withstanding voltage test is a destructive test.
Before conducting the AC withstanding voltage test, various non-destructive tests must be carried out first, such as measuring insulation resistance, absorption ratio, dielectric loss factor tgδ, DC leakage current, etc., and a comprehensive analysis of the test results should be conducted to determine whether the equipment is damp or contains defects. If problems are found, they must be pre-treated, and the AC withstanding voltage test can only be conducted after the defects are eliminated, to avoid insulation breakdown during the AC withstanding voltage test, expanding insulation defects, prolonging maintenance time, and increasing maintenance workload.
