# Withstand voltage test and insulation resistance test

1、 Test principle:

a) Withstand voltage test:

The basic working principle is: compare the leakage current generated by the tested instrument at the high voltage of the test output by the voltage tester with the preset judgment current. If the leakage current detected is less than the preset value, the instrument passes the test. When the leakage current detected is greater than the judgment current, the test voltage is cut off and an audible and visual alarm is sent out, so as to determine the voltage withstand strength of the tested part.

For the first test circuit ground test principle,

The voltage withstand tester is mainly composed of AC (direct) current high voltage power supply, timing controller, detection circuit, indication circuit and alarm circuit. The basic working principle is: the ratio of leakage current generated by the tested instrument at the test high voltage output by the voltage tester is compared with the preset judgment current. If the leakage current detected is less than the preset value, the instrument passes the test, When the leakage current detected is greater than the judgment current, the test voltage is cut off momentarily and an audible and visual alarm is sent out to determine the voltage withstand strength of the tested part.

b) Insulation impedance:

We know that the voltage of insulation impedance test is generally 500V or 1000V, which is equivalent to testing a DC withstand voltage test. Under this voltage, the instrument measures a current value, and then amplifies the current through internal circuit calculation. Finally, it passes Ohm law: r = u/i, where u is the 500V or 1000V tested, And I is the leakage current at this voltage. According to the withstand voltage test experience, we can understand that the current is very small, generally less than 1 μ A 。

It can be seen from the above that the principle of insulation impedance test is exactly the same as that of withstand voltage test, but it is only another expression of Ohm law. Leakage current is used to describe the insulation performance of the object under test, while insulation impedance is resistance.

2、 Purpose of voltage withstand test:

Voltage withstand test is a non-destructive test, which is used to detect whether the insulation capacity of products is qualified under the transient high voltage. It applies high voltage to the tested equipment for a certain time to ensure that the insulation performance of the equipment is strong enough. Another reason for this test is that it can also detect some defects of the instrument, such as the insufficient creepage distance and insufficient electrical clearance in the manufacturing process.

3、 Voltage withstand test voltage:

There is a general rule of test voltage = power supply voltage × 2+1000V 。

For example: if the power supply voltage of the test product is 220V, the test voltage = 220V × 2+1000V=1480V 。

Generally, the withstand voltage test time is one minute. Because of the large amount of electrical resistance tests on the production line, the test time is usually reduced to only a few seconds. There is a typical practical principle. When the test time is reduced to only 1-2 seconds, the test voltage must be increased by 10-20%, so as to ensure the reliability of insulation in short-term test.

4、 Alarm current

The setting of alarm current shall be determined according to different products. The best way is to make leakage current test for a batch of samples in advance, get an average value, and then determine a value slightly higher than this average value as the set current. Because the leakage current of the tested instrument inevitably exists, it is necessary to ensure that the alarm current set is large enough to avoid being triggered by the leakage current error, and it should be small enough to avoid passing the unqualified sample. In some cases, it is also possible to determine whether the sample has contact with the output end of the voltage tester by setting the so-called low alarm current.

5、 Selection of AC and DC test

Test voltage, most of the safety standards allow the use of AC or DC voltage in withstand voltage tests. If AC test voltage is used, when the peak voltage is reached, the insulator to be tested will bear the maximum pressure when the peak value is positive or negative. Therefore, if it is decided to choose to use DC voltage test, it is necessary to ensure that the DC test voltage is twice the AC test voltage, so that the DC voltage can be equal to the peak value of AC voltage. For example: 1500V AC voltage, for DC voltage to produce the same amount of electrical stress must be 1500 × 1.414 is 2121v DC voltage.

One of the advantages of using DC test voltage is that in DC mode, the current flowing through the alarm current measuring device of voltage tester is the real current flowing through the sample. Another advantage of using DC testing is that voltage can be applied gradually. When the voltage increases, the operator can detect the current flowing through the sample before the breakdown occurs. It is important to note that when using DC voltage withstand tester, the sample must be discharged after the test is completed due to the charging of capacitance in the circuit. In fact, no matter how much voltage is tested and the characteristics of the product, it is good for the discharge before operating the product.

The disadvantage of DC voltage withstand test is that it can only apply test voltage in one direction, and can not apply electrical stress on two polarity as AC test, and most electronic products work under AC power supply. In addition, because the DC test voltage is difficult to produce, the cost of DC test is higher than that of AC test.

The advantage of AC voltage withstand test is that it can detect all voltage polarity, which is closer to the practical situation. In addition, because AC voltage will not charge the capacitance, in most cases, the stable current value can be obtained by directly outputting the corresponding voltage without gradual step-up. Moreover, after the AC test is completed, no sample discharge is required.

The deficiency of AC voltage withstand test is that if there is a large y capacitance in the line under test, in some cases, the AC test will be misjudged. Most safety standards allow users to either not connect Y capacitors before testing, or instead use DC tests. When the DC voltage withstand test is increased at Y capacitance, it will not be misjudged because the capacitance will not allow any current to pass at this time.

Post time: May-10-2021
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