Withstand voltage test of electrical products
In the CSA, UL and IEC standards, almost all electrical safety standards require that the product undergo a withstand voltage test. This shows that the withstand voltage test is an important part of electrical safety standards. Dielectric Voltage Withstand Test is also commonly known as High Voltage Test. It is a test to determine whether the insulation material and space distance of a device meet the requirements by applying a voltage higher than its rated value and maintaining it for a certain period of time. This article introduces the characteristics of the withstand voltage test of the US and Canadian standards, and describes in detail the withstand voltage test procedures and their indicators and methods to help Chinese companies manufacture electrical equipment that truly meets the North American market standards.
Why do you need a withstand voltage test?
Normally, the voltage waveform in a power system is a sine wave. During operation of the power system due to lightning strikes, operations, faults, or improper parameter coordination of electrical equipment, the voltage in some parts of the system suddenly rises, greatly exceeding its rated voltage, which is overvoltage. Overvoltage can be divided into two categories according to the cause of its occurrence. One type is overvoltage caused by direct lightning strike or lightning induction, which is called external overvoltage.
The amplitude of the lightning impulse current and the 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 direct lightning strikes is very small and relatively safe. Moreover, the electrical appliances discussed here are outside the above-mentioned range and will not be discussed further. The other type is caused by the energy conversion or parameter changes in the power system, such as switching on no-load lines, cutting off no-load transformers, and single-phase arc grounding in the system. They are called internal overvoltages.
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. The withstand voltage test is to check whether the insulation structure of the product can withstand the internal overvoltage of the power system.
Test points and test voltage values
The test points and test voltage values are determined according to the relevant standards of the specific product. The United States and Canada have new standards based on I EC in addition to the standards of their North American system. Here we use the standards of "Motor-Operated Appliances (Household and Commercial)" CAN / CSA-C22.2 No. 68-92 and "Portable Electrical Motor-Operated and Heating Appliances: General Requirements" C22.2 NO. 1335.1-93 To introduce the characteristics of the withstand voltage test of the United States and Canada.
1.CAN / CSA-C22.2 No. 68-92
Requirement: Appropriate frequency AC voltage should be applied between the live part of the product and the non-conductive conductor that may be grounded for 1 minute. The specific test voltage is as follows:
a For equipment with a rated voltage of 31 ~ 250V, the test voltage is 1000V.
b For equipment with a rated voltage of 251 ~ 600V, the test voltage is 1000V + twice the rated voltage.
c Equipment with a rated voltage of 31 ~ 250V, no grounding and can be touched by the human body, the test voltage is 2500V.
d For low voltage circuits of 30 volts or less, the test voltage is 500V.
2.Double insulated products
The difference between DC withstand voltage test and AC power frequency withstand voltage test There are two types of withstand voltage test: one is AC power frequency withstand voltage test, and the other is DC withstand voltage test. Due to the characteristics of the insulating material, the breakdown mechanisms of AC and DC voltages are different. Most insulation materials and systems contain a range of different media. When an AC test voltage is applied to it, the voltage will be distributed according to the ratio of the dielectric constant and size of the material.
The DC voltage only distributes the voltage in proportion to the resistance of the material. And in fact, the breakdown of the insulation structure often occurs in the form of electrical breakdown, thermal breakdown, discharge, etc., and it is difficult to separate them. The AC voltage increases the possibility of thermal breakdown than the DC voltage. Therefore, we believe that the AC withstand voltage test is more stringent than the DC withstand voltage test. In actual operation, when conducting a withstand voltage test, if a DC is used for a withstand voltage test, the test voltage is required to be higher than the test voltage of the AC power frequency. The test voltage of the general DC withstand voltage test is by multiplying the effective value of the AC test voltage by a constant K. Through comparative testing, we have the following results: wire and cable products, the constant K is selected.
3. aviation industry, the constant K is selected from 1.6 to 1.7; CSA is generally used for civil products 1.414.
Formal test and factory test Formal test is used to judge whether the design of the insulation structure of the product conforms to actual use. It is usually performed immediately after temperature rise test, tide test, abnormal test and other tests. The test voltage is usually applied to the product for one minute.
Factory testing is to detect production defects in the production process, not to check whether the design of the insulation structure is reasonable. This is usually done after the product is finished and ready for packaging. Defects in the insulation structure of the product usually have the following situations: sharp parts damage the insulation layer of the wire, the electronic components between the connection line-ground or the primary-secondary are shorted, and the welding spot of the wire is not good or falls , The creepage distance is reduced, the insulation inside the transformer is damaged, and so on. In order to adapt to mass production, the factory test can also increase the test voltage value by 20% and reduce the test time from 1 minute to 1 second.
Withstand voltage test and breakdown test
The standard specifies the voltage value at the time of the test. After the withstand voltage test, it can only show that the insulation structure of the product can withstand the test voltage, but not how high the voltage can be withstood by the insulation structure of the product. If the application research of insulation materials and the design of electrical equipment are required to determine the insulation strength, a breakdown test is required.
A breakdown test is a test of the voltage when a dielectric is broken down. When the electric field strength exceeds a certain limit, the relationship between the current passing through the medium and the voltage applied to the medium does not comply with Ohm's law, but suddenly increases. As shown in Figure 1. At this time, the insulation material is destroyed and the insulation performance is lost. For an electrical product, if its insulation breaks down, it will lose its operational function. This result is completely different from the withstand voltage test.
The withstand voltage test is to ensure that the product is free from defects and can work normally and safely through testing. Therefore, the test voltage of the withstand voltage test is proved to be safe, reliable and effective after practice, and it should be strictly implemented. When we went to the factory for factory inspection, some factories increased the voltage of the withstand voltage test in order to show the quality of their products, and some even doubled the required voltage. There is no harm in doing so. Because first, it may reduce the qualification rate of the product. Second, although the test is passed, it may damage a part of the insulation structure and reduce the safety of the product. Third, it may damage some components, reduce the quality of the product and shorten the life.
It’s often mentioned the question of "how much leakage current is considered to withstand voltage breakdown?" In fact, this leakage current is difficult to determine accurately. Because when high voltage is applied to the product, a stable, small leakage current is generated between the insulation. Due to the complexity and insulation structure of each product, this leakage current will change accordingly. When the insulation is broken down, this leakage current will increase rapidly. As shown in Figure 1. Therefore, in general, as long as the overcurrent relay operating current (ie, leakage current) of this withstand voltage test instrument is set to be slightly higher than the leakage current generated when the product normally applies high voltage.
Please note that the leakage current and the leakage current in the Leakage Current Test are essentially the same. They are very small currents generated by the insulation system under the action of voltage. The difference is that the leakage current in the withstand voltage test is generated under high voltage, while the leakage current in the leakage current test is generated at rated voltage.
Elimination of factory withstand voltage conditions
1. The power input of the product is the low-voltage (ELV) or extra-low voltage (SELV) of the secondary output of other products, such as power supplies or transformers.
2. When finished products need to use another power supply as a component, and this power supply is CSA certified, with a complete housing and input and output connectors. Since this power supply is CSA certified, it has already undergone a withstand voltage test, so you can consider waiving the entire product's withstand voltage test. However, care must be taken to check the power supply for damage before installing it.
3. When the signal transformer is used in the line of the same voltage, and it is not used as a safety device, you can consider eliminating the voltage test between the primary and the core.
Test equipment requirements
For products below 600V, the U.S. and Canadian standards usually require the transformer capacity of the withstand voltage test instrument to be at least 500VA, the output current can reach 95mA, and the capacitance value between the live part and ground should not exceed 0.1mF. Correct it once. The instrument must be inspected before the formal test, and the instrument must be inspected regularly in the factory test. This can avoid misjudgment or damage to the product due to the abnormality of the instrument.
AC withstand voltage test instruments generally include high-voltage test transformers, voltage regulators, voltage measurement systems, and control and protection devices. Figure 2 is a circuit diagram of a manual boost test device for reference.
Method and sequence of withstand voltage test:
1. Check that the main power switch of the withstand voltage tester is in the "off" position
2. Except for the special design of the instrument, all non-live metal parts must be reliably grounded.
3. Connect the wires or terminals of all power input terminals of the device under test
4. Close all power switches, relays, etc. of the device under test
5. Adjust the test voltage of the withstand voltage tester to zero
6. Connect the high voltage output line (usually red) of the withstand voltage tester to the power input of the device under test
7. Connect the circuit ground wire (usually black) of the withstand voltage tester to the accessible non-live metal part of the device under test
8. Close the main power switch of the withstand voltage tester and slowly increase the secondary voltage of the instrument to the required value. Generally boost speed does not exceed the speed of 500V / sec
9. Maintain this test voltage for a specified time
10. Slowly drop the test voltage
11. Turn off the main power switch of the withstand voltage tester. Disconnect the high-voltage output line of the withstand voltage tester first, and then disconnect the circuit ground wire of the withstand voltage tester.
When the test voltage cannot rise to the specified voltage value or the voltage drops instead
When the withstand voltage tester displays a warning signal, it should be noted that due to the high voltage that is dangerous to the human body during the withstand voltage test, special care must be taken when testing.
The following points need special attention:
It must be stipulated that only trained and authorized personnel can enter the test area to operate the instrument
Fixed and obvious warning signs must be placed around the test area to prevent other people from entering the danger zone
When conducting tests, all personnel, including operators, must be kept away from the test equipment and equipment under test
When the test instrument is started, do not touch its output cable.