1、 What are Class I and Class II products?

Answer: Class I equipment refers to the connection of accessible conductor parts to grounding protective conductors; When the basic insulation fails, the grounding protection conductor must be able to withstand the fault current, which means that when the basic insulation fails, the contactable parts cannot become live parts. Simply put, devices with grounding pins on the power cord are Class I devices.

Class II equipment not only relies on 'basic insulation' to prevent electrical leakage, but also provides other safety precautions, such as' double insulation 'or' reinforced insulation '. There is no conditional requirement for the reliability of protective grounding or installation conditions.

2、 Why do products need to undergo electrical safety testing?

Answer: This is the question that many product manufacturers want to ask the most, and of course, the most common answer is 'because there are regulations in safety standards.' If you can deeply understand the background of electrical safety regulations, you will find the responsibility and significance behind it. Although electrical safety testing takes up some time on the production line, it can help you reduce the risk of product recycling due to electrical hazards. Doing it right the first time is the correct way to reduce costs and maintain goodwill.

3、 What is Electrical Shock?

Answer: There are many factors that can cause electrical injury, among which the most important is the electrical injury caused by electric current passing through the human body. This type of electrical injury has a direct impact on humans, and the severity of the injury varies depending on the size of electrical energy, humidity, contact area, etc. Imagine when you are taking a bath in the bathtub, suddenly the hair dryer in operation drops into the bathtub, causing electricity to flow from the hair dryer through your body to the ground. At this point, your heart experiences irregular palpitations and a decrease in blood pressure, causing irreversible tragedy.

4、 What are the main tests for electrical injuries?

Answer: Electrical injury testing is mainly divided into the following four types: Dielectric Withstand/Hipot Test: The voltage withstand test applies a high voltage to the power and ground circuits of the product and measures its collapse state. Insulation Resistance Test: measures the electrical insulation status of the product. Leakage Current Test: Detects whether the leakage current from the AC/DC power supply to the ground exceeds the standard. Protective Ground Test: Detects whether accessible metal structures and other parts are truly grounded.

5、 Do safety standards have special requirements for withstand voltage testing environments?

Answer: Regarding the safety of manufacturers or testing personnel in testing laboratories, it has been practiced in Europe for many years. Whether it is manufacturers and testing personnel of electronic appliances, information technology products, household appliances, mechanical tools, or other equipment, there are chapters and regulations in various safety regulations, including UL, IEC, EN, which include testing area labeling (personnel location, instrument location, DUT location) Equipment labeling (clearly indicating 'danger' or items under testing), grounding status of equipment workstations and related facilities, and electrical insulation capacity of each testing equipment (IEC 61010).

6、 What is a withstand voltage test?

Answer: 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 international safety agencies such as JSI, CSA, BSI, UL, IEC, TUV, etc.), and is also the most well-known and frequently performed production line safety test. The HIPOT test is a non-destructive test that determines that electronic insulation materials are sufficient to withstand transient high voltages. It is applicable to all equipment to ensure that the insulation materials are sufficient. The other reason for conducting HIPOT testing is that it can detect possible defects such as insufficient leakage distance and electrical clearance caused during the manufacturing process.

7、 Why do I need to perform a voltage withstand test?

Answer: Under normal circumstances, the voltage waveform in the power system is a sine wave. During the operation of the power system, due to lightning strikes, operation, faults, or improper parameter coordination of electrical equipment, the voltage in certain parts of the system suddenly increases, greatly exceeding its rated voltage, which is called overvoltage. Overvoltage can be divided into two categories based on its causes. One type is overvoltage caused by direct lightning strikes or lightning induction, which is called external overvoltage. The amplitude of lightning impulse current and impulse voltage is large, and the duration is very short, which is highly destructive. However, due to the shielding protection of factories or tall buildings, the probability of direct lightning strikes on overhead lines of 3-10kV and below in towns and general industrial enterprises is very low and relatively safe. And the discussion here is about civil electrical appliances, which are not within the above scope and will not be further discussed. Another type is caused by energy conversion or parameter changes within the power system, such as switching on no-load lines, cutting off no-load transformers, and single-phase arc grounding within the system, which is called internal overvoltage. Internal overvoltage is the main basis for determining the normal insulation level of various electrical equipment in the power system. That is to say, the design of the insulation structure of the product should not only consider the rated voltage but also the internal overvoltage of the product's operating environment. Voltage withstand test is to test whether the insulation structure of a product can withstand the internal overvoltage of the power system.

8、 What are the advantages of AC withstand voltage testing?

Answer: Usually, AC withstand voltage testing is more easily accepted by safety agencies than DC withstand voltage testing. The main reason is that most of the tested items will work under AC voltage, and AC withstand voltage testing 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 actual use. Due to the fact that AC testing does not charge capacitive loads, the current reading remains consistent from the beginning of voltage application to the end of the test. Therefore, since there is no stabilization issue required for monitoring current readings, there is no need to gradually increase the voltage. This means that unless the tested product senses a sudden applied voltage, the operator can immediately apply the full voltage and read the current without waiting. Due to the fact that AC voltage does not charge the load, there is no need to discharge the tested equipment after testing.

9、 What are the drawbacks of AC withstand voltage testing?

Answer: When testing capacitive loads, the total current consists of electrical resistance current and leakage current. When the electrical resistance current is much greater than the actual leakage current, it may be difficult to detect products with excessive leakage current. When testing large capacitive loads, the total current required is much greater than the leakage current itself. Due to the operator facing higher currents, this may be a greater danger

10、 What are the advantages of DC withstand voltage testing?

Answer: When the DUT is fully charged, only the actual leakage current flows through it. This enables the DC withstand voltage tester to clearly display the true leakage current of the tested product. Due to the transient charging current, the power requirements of DC withstand voltage testers can usually be much lower than those of AC withstand voltage testers used to test the same product.

11、 What are the drawbacks of DC withstand voltage testers?

Answer: As the DC withstand voltage test does indeed charge the DUT, in order to eliminate the risk of electric shock for operators who dispose of the DUT after the withstand voltage test, the DUT must be discharged after the test. The DC test will charge the capacitor. If the DUT actually uses AC power, the DC method does not simulate the actual situation.

12、 The difference between AC withstand voltage test and DC withstand voltage test

Answer: There are two types of voltage withstand tests: AC voltage withstand test and DC voltage withstand test. Due to the characteristics of insulation materials, the breakdown mechanisms of AC and DC voltages are different. Most insulation materials and systems contain a range of different media. When applying AC test voltage to it, the voltage will be distributed in proportion to the dielectric constant and size of the material. The DC voltage is only distributed in proportion to the resistance of the material. Moreover, in fact, the breakdown of insulation structures often occurs in multiple forms such as electrical breakdown, thermal breakdown, and discharge, making it difficult to completely separate them. AC voltage increases the possibility of thermal breakdown compared to DC voltage. So, we believe that AC withstand voltage testing is more stringent than DC withstand voltage testing. In practical operation, if DC is used for voltage withstand testing, the test voltage should be higher than the AC power frequency test voltage. The test voltage for general DC withstand voltage testing is achieved by multiplying the effective value of the AC test voltage by a constant K. Through comparative testing, we have the following results: for wire and cable products, the constant K is selected as 3; In the aviation industry, the constant K is chosen from 1.6 to 1.7; CSA generally uses 1.414 for civilian products.

13、 How to determine the test voltage used for withstand voltage testing?

Answer: The test voltage for determining the withstand voltage test depends on the market in which your product is intended to be put, and you must comply with the safety standards or regulations that are part of the country's import control regulations. The safety standards specify the test voltage and test time for withstand voltage testing. The ideal scenario is to ask your client to provide you with relevant testing requirements. The general test voltage for withstand voltage testing is as follows: for a working voltage between 42V and 1000V, the test voltage is twice the working voltage plus 1000V. This test voltage needs to be applied for 1 minute. For example, for a product operating at 230V, the test voltage is 1460V. If the time for applying voltage is reduced, the test voltage must be increased. For example, in UL 935, the production line testing conditions are:

condition

Application time (seconds)

Applied voltage (V)

A
 
60
 
1000+（2 × V）
 

B
 
1
 
1200+（2.4 × V）
 

Note: V=maximum rated voltage 

14、 What is the capacity for withstand voltage testing and how should it be calculated?

Answer: The capacity of a withstand voltage tester refers to its power output. The capacity of the withstand voltage tester is determined by the maximum output current x maximum output voltage. For example: 5000Vx100mA=500VA

15、 Why do the leakage current values measured using AC withstand voltage test and DC withstand voltage test differ?

Answer: The stray capacitance of the tested object is the main reason for the difference in the measured values between AC and DC withstand voltage tests. When testing with AC, it may not be possible to fully charge these stray capacitors and there will be a continuous current flowing through them. When using DC testing, once the stray capacitance on the tested object is fully charged, the remaining is the actual leakage current of the tested object. Therefore, the leakage current values measured using AC withstand voltage testing and DC withstand voltage testing may differ.

16、 What is the leakage current in withstand voltage testing

Answer: Insulators are non-conductive, but in reality, almost no insulation material is absolutely non-conductive. Any type of insulation material, when a voltage is applied at both ends, will always have a certain amount of current passing through it. The active component of this current is called leakage current, and this phenomenon is also called insulation leakage.

For the testing of electrical appliances, leakage current refers to the current formed between metal parts with mutual insulation, or between live parts and grounded parts, through the surrounding medium or insulation surface, without the application of voltage due to faults. According to the US UL standard, leakage current refers to the current that can be conducted from the accessible part of household appliances, including capacitive coupling current. The leakage current consists of two parts, one is the conduction current I1 through the insulation resistance; The other part is through the displacement current I2 of the distributed capacitor, which has a capacitance impedance of XC=1/2pfc 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 using thyristor power supply, the harmonic component increases the leakage current.

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

Answer: Voltage withstand 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 leakage current of the power supply (contact current) is measured under the most unfavorable conditions (voltage, frequency) of the measured object under normal operation. Simply put, the leakage current of the withstand voltage test is the measured leakage current without a working power supply, and the leakage current of the power supply (contact current) is the measured leakage current under normal operation.

18、 Classification of contact current

Answer: For electronic products with different structures, there are also different requirements for measuring contact current. However, in summary, contact current can be divided into three types: Ground Leakage Current, Surface to Line Leakage Current, and Surface to Surface Leakage Current.

19、 Why do we need to conduct a contact current test?

A: For electronic products of Class I equipment, metal parts or shells that can be touched should also have a good grounding circuit as a protective measure against electric shock in addition to basic insulation. However, we often encounter some users randomly using Class I devices as Class II devices, or simply unplugging the ground terminal (GND) from the power input of their Class I devices, which poses certain safety hazards. Nevertheless, as a manufacturer, it is an obligation to avoid the danger that this situation poses to users. This is why the purpose of conducting contact current testing is to do so.

20、 Why is there no standard for setting leakage current in withstand voltage testing?

Answer: During the AC withstand voltage test, there is no standard due to the different types of tested objects, the presence of stray capacitors in the tested objects, and the different test voltages leading to different leakage currents.

21、 How to determine the test voltage?

A: The best way to determine the test voltage is to set it according to the required specifications for testing. Generally speaking, we will set the test voltage at twice the working voltage plus 1000V. For example, if the working voltage of a product is 115VAC, we will use 2 x 115+1000=1230 Volt as the test voltage. Of course, the test voltage will also have different settings depending on the level of insulation layer.

22、 What is the difference between Dielectric Voltage Withstand Testing, High Potential Testing, and Hipot Testing?

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

23、 What is the insulation impedance (IR) test?

Answer: The insulation resistance test and voltage withstand test are very similar. Apply a DC voltage of up to 1000V to the two points to be tested. IR testing typically provides resistance values in megaohms, rather than Pass/Fail representations obtained from withstand voltage testing. Typically, the test voltage is 500V DC and the insulation resistance (IR) value should not be less than a few megaohms. The insulation impedance test is a non-destructive test that can detect whether the insulation is good. In some specifications, the insulation impedance test is performed first and then the withstand voltage test. When the insulation impedance test cannot pass, the withstand voltage test often cannot pass.

24、 What is the Ground Bond test?

Answer: The grounding connection test, also known as the Ground Continuity test, measures the impedance between the DUT frame and the grounding column. The grounding connection test determines whether the DUT's protection circuit is capable of handling fault currents effectively if the product breaks down. The grounding connection tester will generate a maximum DC current or AC root mean square current of 30A through the grounding circuit (CSA requires measuring 40A) to determine the impedance of the grounding circuit, which is generally below 0.1 ohm.

25、 What is the difference between voltage withstand test and insulation resistance test?

Answer: IR testing is a qualitative test that provides a representation of the relative quality of an insulation system. Usually, a DC voltage of 500V or 1000V is used for testing, and the results are measured using a megohm resistor. The withstand voltage test also applies high voltage to the DUT, but the applied voltage is higher than that of the IR test. It can be operated at AC or DC voltage. The results are measured in milliamperes or microamperes. In some specifications, IR testing is performed first, followed by voltage withstand testing. If a DUT cannot pass the IR test, then the DUT cannot pass the withstand voltage test at a higher voltage.

26、 Why is there an open circuit voltage limit for grounding impedance testing? Why is it recommended to use alternating current (AC)?

Answer: The purpose of grounding impedance testing is to ensure that when abnormal conditions occur in the equipment product, the protective grounding wire can withstand the flow of fault current to ensure the safety of the user. The safety standard test voltage requires that the maximum open circuit voltage should not exceed the limit of 12V, which is based on user safety considerations. In the event of a test failure of the tested object, the risk of electric shock to operators can be reduced. However, the general standard requires a grounding resistance of less than 0.1ohm. It is recommended to use alternating current testing with a frequency of 50Hz or 60Hz to meet the actual working environment of the product.

27、 What is the difference between the leakage current measured by the withstand voltage test and the power leakage test?

Answer: There are some differences between withstand voltage testing and power leakage testing, but generally speaking, these differences can be summarized as follows. Voltage withstand testing is the use of high voltage to pressurize the insulation of a product to determine whether its insulation strength is sufficient to prevent excessive leakage current. Leakage current testing is the measurement of the leakage current flowing through a product under normal and single power failure conditions during use.

28、 How to determine the discharge time of capacitive loads during DC withstand voltage testing?

Answer: The difference in discharge time depends on the capacitance of the tested object and the discharge circuit of the withstand voltage testing machine. The larger the capacitance, the longer the discharge time required.