Common Problems in Withstand Voltage Test

Q: Why do products need to undergo electrical safety testing?

A: This is a question that many product manufacturers want to ask the most. Of course, the most common answer is "because it is stipulated in the safety standards." If you can deeply understand the background of electrical safety regulations, you will find the responsibilities and significance behind it. Although electrical safety testing takes up a little time on the production line, it can help you reduce the risk of product recalls due to electrical hazards. Doing it right the first time is the right way to reduce costs and maintain your reputation.

Q: What is electrical shock?

A: There are many factors that cause electrical damage, the most important of which is electrical damage caused by current passing through the human body. This type of electrical damage has a direct impact on humans, and the severity of the damage varies depending on the amount of electrical energy, humidity, contact area, etc. Imagine that when you are taking a bath in a bathtub, a hair dryer suddenly falls into the bathtub. In this case, the current from the hair dryer passes through your body and flows to the ground. At this time, your heart will have irregular palpitations and your blood pressure will drop, causing an irreversible tragedy.

Q: What are Class I and Class II products?

A: Class I equipment refers to the contactable conductive parts connected to the grounding protective conductor; when the basic insulation fails, the grounding protective conductor must be able to withstand the failure error current, that is, when the basic insulation fails, the contactable parts cannot become live electrical parts. Simply put, equipment with a grounding pin on the power cord is Class I equipment.

Class II equipment not only relies on "basic insulation" to prevent electric shock, but also provides other safety precautions, such as "double insulation" or "reinforced insulation". There are no conditions for the reliability of protective grounding or installation conditions.

Q: What are the main tests for electrical damage?

A: There are four main types of electrical damage tests: Dielectric Withstand / Hipot Test: The Dielectric Withstand / Hipot Test applies a high voltage to the power and ground circuits of the product and measures its breakdown state. Insulation Resistance Test: Measures the electrical insulation state 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 the accessible metal structures and other parts are actually grounded.

Q: Does the safety standard have any special requirements for the withstand voltage test environment?

A: The safety of testers in manufacturers or test laboratories has been practiced in Europe for many years. Whether it is manufacturers and testers of electronic appliances, information technology products, household appliances, mechanical tools or other equipment, there are chapters to stipulate in various safety regulations, whether it is UL, IEC, or EN. The content includes test area marking (personnel position, instrument position, DUT position), equipment marking (clearly marking "danger" or the item under test), the grounding status of equipment workbenches and other related facilities, and the electrical insulation capacity of each test equipment (IEC 61010).

Q: What is the withstand voltage test?

A: The explanation given by the Electronic Components Technology Network is: Withstand voltage test or high voltage test (HIPOT test) is a 100% production line test used to verify the quality and electrical safety characteristics of the product (such as the 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 to determine whether the electronic insulation material is sufficient to withstand instantaneous high voltage. It is a high-voltage test applicable to all equipment to ensure that the insulation material is sufficient. Another reason for conducting the HIPOT test is that it can detect possible defects such as insufficient creepage distance and electrical clearance caused during the manufacturing process.

Q: Why do we need to do a withstand voltage test?

A: 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, failures or improper matching of electrical equipment parameters, the voltage of some parts of the system suddenly rises and greatly exceeds its rated voltage. This is overvoltage. Overvoltage can be divided into two categories according to the cause of its occurrence. One is the overvoltage caused by direct lightning strikes or lightning induction, which is called external overvoltage. The amplitude of lightning impulse current and impulse voltage are both large, and the duration is very short, which is extremely destructive. However, since the overhead lines of 3-10kV and below in towns and general industrial enterprises are shielded and protected by factory buildings or tall buildings, the probability of being directly struck by lightning is very small and relatively safe. Moreover, what is discussed here is civil electrical appliances, which are not within the above scope, so no further discussion will be made. The other type is caused by energy conversion or parameter changes within the power system, such as cutting off unloaded lines, cutting off unloaded transformers, and single-phase arc grounding in 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 product's insulation structure must not only consider the rated voltage but also the internal overvoltage of the product's use environment. The withstand voltage test is to detect whether the product's insulation structure can withstand the internal overvoltage of the power system.

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

A: Generally, AC withstand voltage tests are more easily accepted by safety agencies than DC withstand voltage tests. The main reason is that most items under test will operate under AC voltage, and AC withstand voltage tests offer the advantage of stressing the insulation with two polarities alternating, which is closer to the stress that the product will encounter in actual use. Since AC testing does not charge the capacitive load, the current reading remains consistent from the beginning of the voltage application to the end of the test. Therefore, there is no need to gradually increase the voltage because there is no stabilization problem required to monitor the current reading. This means that unless the product under test senses a sudden application of voltage, the operator can immediately apply full voltage and read the current without waiting. Since AC voltage does not charge the load, there is no need to discharge the device under test after the test.

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

A: When testing capacitive loads, the total current is composed of reactive current and leakage current. When the reactive current is much larger than the true leakage current, it may be difficult to detect products with excessive leakage current. When testing large capacitive loads, the total current required is much larger than the leakage current itself. This can be a greater hazard as the operator is faced with a larger current.