Q & a summary and sharing of safety inspection and certification knowledge

Date：2022-01-25 16:32:00Views：1101

Q: Classification of contact current

A: For electronic products with different structures, the measurement of contact current also has different requirements, but in general, the contact current can be divided into ground leakage current, surface to line leakage current and surface to surface leakage current.

Q: Why do contact current tests?

A: For the accessible metal parts or enclosures of electronic products of class I equipment, good grounding lines shall also be provided as an anti electric shock protection measure other than basic insulation. However, we often encounter that some users arbitrarily use class I equipment as class II equipment, or directly unplug the grounding terminal (GND) at the power input end of class I equipment, so there are certain potential safety hazards. Even so, as a manufacturer, it is obliged to avoid the danger to users caused by this situation. This is why the purpose of contact current test is.

Q: Why is there no standard for setting leakage current in voltage withstand test?

A: During AC voltage withstand test, there is no standard because of different types of tested objects, stray capacitance in the tested objects and different leakage current due to different test voltage.

Q: How to determine the test voltage?

A: The best way to determine the test voltage is to set it according to the specifications required for the test. Generally speaking, we will set the test voltage as twice the working voltage plus 1000V. For example, if the working voltage of a product is 115VAC, we will take 2 x 115 + 1000 = 1230 volt as the test voltage. Of course, the test voltage will also be set differently according to the level of insulation layer.

Q: What is the difference between dielectric voltage withstand testing, high potential testing and hipot testing?

A: These three nouns all have the same meaning, but they are often used interchangeably in the testing industry.

Q: What is the insulation impedance (IR) test?

A: The insulation resistance test is very similar to the withstand voltage test. Apply a DC voltage up to 1000V to the two points to be tested. The IR test usually gives the resistance value in megaohms, rather than the pass / fail representation from the withstand voltage test. Typically, the test voltage is 500V DC, and the value of insulation resistance (IR) shall not be less than a few megaohms. The insulation impedance test is a non-destructive test and can detect whether the insulation is good. In some specifications, the insulation impedance test is conducted first and then the withstand voltage test. When the insulation impedance test cannot pass, the withstand voltage test often fails.

Q: What is the ground bond test?

A: Grounding connection test, which is called ground continuity test, measures the impedance between the frame of the DUT and the grounding column. The grounding connection test determines whether the protection circuit of the DUT is competent to handle the fault current if the product is broken. The grounding connection tester will generate a DC current or AC root mean square current up to 30A through the grounding circuit (CSA requires measuring 40a), so as to determine the impedance of the grounding circuit, which is generally below 0.1 ohm.

Q: What is the difference between withstand voltage test and insulation resistance test?

A: IR test is a qualitative test, which gives a representation of the relative quality of the insulation system. 500V or 1000V DC voltage is usually used for test, and the result is measured by megohm resistance. The withstand voltage test also applies high voltage to the DUT, but the applied voltage is higher than that of IR test. It can be carried out at AC or DC voltage. The results were measured with milliampere or microampere. In some specifications, the IR test is carried out first, and then the withstand voltage test is carried out. If a DUT cannot pass the IR test, the DUT cannot pass the withstand voltage test at a higher voltage.

Q: Why is there an open circuit voltage limit in the grounding impedance test? Why is alternating current (AC) recommended?

A: The purpose of grounding impedance test is to ensure that in case of abnormal conditions of equipment and products, the protective grounding wire can withstand the flow of fault current to ensure the safety of users. The standard test voltage of safety regulations requires that the maximum open circuit voltage shall not exceed the limit of 12V, that is, based on the safety consideration of users, in case of test failure of the tested object, it can reduce the risk of electric shock to operators. The general standard requires that the grounding resistance should be less than 0.1ohm. It is recommended to adopt AC current test with frequency of 50Hz or 60Hz to meet the actual working environment of the product.

Q: What is the difference between the leakage current measured by voltage withstand test and power leakage test?

A: There are some differences between voltage withstand test and power leakage test, but in general, these differences can be summarized as follows. The withstand voltage test is to use high voltage to pressurize the insulation of the product to determine whether the insulation strength of the product is sufficient to prevent excessive leakage current. Leakage current test is to measure the leakage current flowing through the product under normal and single power failure conditions.

Q: How to determine the discharge time of capacitive load during DC withstand voltage test?

A: The difference of discharge time depends on the capacitance of the tested object and the discharge circuit of the withstand voltage tester. The larger the capacitance, the longer the discharge time.

Q: What is withstand voltage test?

A: Voltage withstand test or high-voltage test (hipot test) is a 100% production line test used to verify the quality and electrical safety characteristics of products (such as standards required by JSI, CSA, BSI, UL, IEC, TUV and other international safety agencies). It is also a production line safety test that is most known and frequently executed. Hipot test is a non-destructive test to determine that the electronic insulating material is sufficient to resist the instantaneous high voltage. It is a high-voltage test applicable to all equipment to ensure that the insulating material is sufficient. Another reason for hipot testing is that it can detect possible defects, such as insufficient leakage distance and electrical clearance during the manufacturing process.

Q: Why do you have to do a pressure test?

A: Under normal circumstances, the voltage waveform in the power system is a sine wave. During the operation of power system, the voltage of some parts of the system rises suddenly and greatly exceeds its rated voltage due to lightning strike, operation, fault or improper parameter coordination of electrical equipment. This is overvoltage. Overvoltage can be divided into two categories according to its causes. One is overvoltage caused by direct lightning stroke or lightning induction, which is called external overvoltage. The amplitude of lightning impulse current and impulse voltage are very large, and the duration is very short, which is very destructive. However, because the overhead lines with 3-10kV and below in cities and towns and general industrial enterprises are shielded by factories or tall buildings, the probability of direct lightning stroke is very small and relatively safe. Moreover, what is discussed here is civil electrical appliances. If it is not within the above scope, there will be no further discussion. The other is caused by energy conversion or parameter changes within the power system, such as connecting with no-load line, cutting off no-load transformer, single-phase arc grounding in the system, etc., which is called internal overvoltage. Internal overvoltage is the main basis for determining the normal insulation level of various electrical equipment in power system. In other words, the design of the insulation structure of the product should consider not only the rated voltage, but also the Internal Overvoltage of the product service environment. Withstand voltage test is to detect whether the insulation structure of the product can withstand the Internal Overvoltage of the power system.

Q: What are the advantages of AC voltage withstand test?

A: Generally, AC voltage withstand test is easier to be accepted by safety authorities than DC voltage withstand test. The main reason is that most of the tested items will work under AC voltage, and AC withstand voltage test provides the advantage of alternating two polarities to apply pressure to the insulation, which is closer to the pressure that the product will encounter in practical use. Since the AC test will not charge the capacitive load, the current reading is consistent from the beginning of voltage application to the end of the test. Therefore, since there is no stabilization Q required to monitor the current reading, there is no need to gradually increase the voltage. This means that unless the product under test senses a suddenly applied voltage, the operator can immediately apply the full voltage and read the current without waiting. Since the AC voltage will not charge the load, there is no need to discharge the tested equipment after the test.

Q: What are the disadvantages of AC voltage withstand test?

A: When testing capacitive load, the total current consists of electrical resistance current and leakage current. When the electric resistance current is much larger than the real leakage current, it may be difficult to detect products with excessive leakage current. When testing large capacitive load, the total current required is much greater than the leakage current itself. This may be a greater danger as the operator faces a greater current.

Q: Difference between AC voltage withstand test and DC voltage withstand test

A: There are two voltage withstand tests: AC voltage withstand test and DC voltage withstand test. The breakdown mechanism of AC and DC voltage is different due to the characteristics of insulating materials. Most insulating materials and systems contain a range of different media. When AC test voltage is applied to it, the voltage will be distributed according to the proportion of dielectric constant, size and other parameters of the material. The DC voltage is distributed only in proportion to the resistance of the material. Moreover, in fact, the breakdown of insulation structure often occurs in the form of electric breakdown, thermal breakdown, discharge and other forms at the same time, which is difficult to be completely separated. AC voltage increases the possibility of thermal breakdown than DC voltage. Therefore, we believe that AC voltage withstand test is more stringent than DC voltage withstand test. In practice, if DC is to be used for withstand voltage test during withstand voltage test, the test voltage shall be higher than that of AC power frequency. Generally, the test voltage of DC withstand voltage test is by multiplying the effective value of AC test voltage by a constant K. Through the comparative test, we have the following results: for wire and cable products, the constant k is 3; In the aviation industry, the constant k is 1.6 to 1.7; CSA generally uses 1.414 for civil products.

Q: How to determine the test voltage used in withstand voltage test?

A: The determination of the test voltage of the withstand voltage test depends on the market in which your product is to be put, and you must comply with the safety standards or regulations of the components of the import control regulations of that country. The safety standard specifies the test voltage and test time of withstand voltage test. The most ideal situation is the customer's relevant test requirements. The test voltage of general withstand voltage test is as follows: if the working voltage is between 42V and 1000V, the test voltage is twice the working voltage plus 1000V. This test voltage is applied for 1 minute. For example, for a product operating at 230V, the test voltage is 1460v. If the time of applying voltage is reduced, the test voltage must be increased.

Q: What is the capacity of voltage withstand test and how to calculate it?

A: The capacity of the withstand voltage tester refers to its power output. The capacity of the withstand voltage tester depends on the maximum output current x the maximum output voltage. For example: 5000vx100ma = 500VA

Q: Why is the leakage current measured by AC voltage withstand test different from that measured by DC voltage withstand test?

A: The stray capacitance of the measured object is the main reason why the measured values of AC and DC voltage withstand tests are different. These stray capacitors may not be fully charged when tested with AC, and a continuous current will flow through these stray capacitors. For DC test, once the stray capacitance on the tested object is filled, the rest is the actual leakage current of the tested object. Therefore, the leakage current measured by AC voltage withstand test and DC voltage withstand test will be different.

Q: What is the leakage current of voltage withstand test?

A: Insulators are non-conductive, but virtually no insulating material is absolutely non-conductive. When voltage is applied to both ends of any insulating material, there will always be a certain current passing through. The active component of this current is called leakage current, and this phenomenon is also called insulator leakage. For the test of electrical appliances, leakage current refers to the current formed between metal parts with mutual insulation or between live parts and grounding parts through the surrounding medium or insulating surface without fault applied voltage, which is called leakage current. According to UL standard, leakage current is the current that can be conducted from the accessible part of household appliances, including capacitive coupling current. The leakage current includes two parts, one part is the conduction current I1 through the insulation resistance; The other part is the displacement current I2 through the distributed capacitor. The capacitive reactance of the latter is XC = 1 / 2PFC, which is inversely proportional to the power frequency. The distributed capacitor current increases with the increase of frequency, so the leakage current increases with the increase of power frequency. For example, when thyristor is used for power supply, its harmonic component increases the leakage current.

Q: What are the advantages of DC withstand voltage test?

A: When the DUT is fully charged, only the real leakage current flows. This enables the DC withstand voltage tester to clearly show the real leakage current of the tested product. Because the charging current is short, the power requirement of DC withstand voltage tester can usually be much smaller than that of AC withstand voltage tester used to test the same product.

Q: What are the disadvantages of DC withstand voltage tester?

A: Since the DC voltage withstand test does charge the DUT, in order to eliminate the risk of electric shock to the operator handling the DUT after the voltage withstand test, the DUT must be discharged after the test. The DC test will charge the capacitor. If the DUT actually uses AC power supply, the DC method does not simulate the actual situation.

Q: What is the difference between the leakage current of voltage withstand test and the leakage current (contact current) of power supply?

A: The withstand voltage test is to detect the leakage current flowing through the insulation system of the tested object and apply a voltage higher than the working voltage to the insulation system; The power leakage current (contact current) is to measure the leakage current of the measured object under the most unfavorable conditions (voltage and frequency) under the normal operation of the measured object. In short, the leakage current of the withstand voltage test is the leakage current measured without the working power supply, and the power supply leakage current (contact current) is the leakage current measured under normal operation.