The withstand voltage test, commonly known as the Hi-pot test, is sometimes referred to as the electrical strength test in the standard by applying a voltage higher than its rated value to the device and maintaining a certain time to determine whether the insulation material and space distance of the device meet the requirements. Test.
The significance of the withstand voltage test
Under normal conditions, the voltage waveform in the power system is a sine wave. In the operation of the power system due to lightning strikes, operational failures or improper coordination of electrical equipment parameters, etc., the voltage of some parts of the system suddenly rises, greatly exceeding its rated voltage, which is the overvoltage. Overvoltage can be divided into two categories according to the reasons for its occurrence: one is the overvoltage caused by direct lightning strike or lightning induction, called external overvoltage; the other is caused by energy conversion or parameter change inside the power system. For example, the combination of no-load lines, cutting off the no-load transformer, single-phase arc grounding in the system, etc., is called internal overvoltage. The 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 consider the internal overvoltage of the product use environment. The withstand voltage test is to test whether the insulation structure of the product can withstand the internal overvoltage of the power system.
Test point and test voltage value
Test points and test voltage values ​​are determined based on the relevant standards of the specific product. Due to the difference in industrial rated voltages in different regions, the calculated withstand voltage values ​​may also differ. China's national standard is basically translated from the IEC standard. The following icons are the pressure requirements of GB 7000.1-2007:
For non-SELV products, Class I and Class II lamps have large variations in withstand voltage values ​​of 2U+1000 and 4U plus 2750V. According to China's quota voltage of 220V, the pressure requirement of Class I lamps is 1440V, Class II is 3630V, and Class II is more than twice as much as Class I. Therefore, when designing the luminaire, we can use the Class I design as much as possible, which can greatly reduce the requirements for the insulation capability of the product.
The difference between DC withstand voltage test and AC power frequency withstand voltage test
There are two ways to withstand voltage test: one is the AC power frequency withstand voltage test, and the other is the DC withstand voltage test. The characteristics of the insulating material determine the breakdown mechanism of AC and DC voltage. Most insulating materials and systems contain a series of different media. When an AC test voltage is applied to it, the voltage will be based on the dielectric constant and size of the material. The ratio of the parameters is used to distribute the voltage. The DC voltage distributes the voltage only in proportion to the resistance of the material. The originally insulated medium will have a smaller resistance under the capacitive effect. Therefore, under the same withstand voltage value, the AC withstand voltage will get a larger leakage current. Therefore, we believe that the AC withstand voltage test is more stringent than the DC withstand voltage test. In actual operation, when performing the withstand voltage test, if the DC voltage test is to be used, the test voltage requirement is higher than the test voltage of the AC power frequency. The test voltage for a typical DC withstand voltage test is obtained by multiplying the effective value of the AC test voltage by a constant K. Due to daily life, most of us use AC frequency of power frequency, so we use AC voltage withstand voltage test, which has more versatile test significance.
Formal testing and factory testing
The formal test is used to determine whether the insulation structure design of the product is suitable for actual use. It is usually carried out immediately after the temperature rise test, the tide test, the abnormal test and some other tests. Generally, the test voltage is applied to the product for one minute.
Factory testing is to detect production defects in the production process, not to determine whether the design of the insulation structure is reasonable, usually before the product is ready for packaging. Defects in the insulation structure of the product usually have the following conditions: sharp parts damage the wire insulation; short-circuit between the connection line-ground or primary-secondary; the solder joint of the wire is not soldered or dropped; The distance is reduced; the insulation inside the transformer is damaged. In order to adapt to mass production, the factory test can also increase the test voltage by 20% and the test time from 1 minute to 1 second.
Leakage current setting
The leakage current here refers specifically to the leakage current generated in the withstand voltage test, which is the alarm threshold we set in the test. The human body's response to current is: perceptual current: male 1.1mA, female 0.7mA; get rid of current: male 9mA, female 6mA (probability 99.5%); lethal current (ventricular fibrillation current): 50mA (current duration exceeds heartbeat cycle) , 500mA (current duration is less than 0.1 seconds). When the current flowing through the human body exceeds 10 mA, there is a danger of electric shock. Therefore, in general, the leakage current standard of the type test is defined as 5 mA. In GB 7000.1, the type test part does not clearly specify the leakage current in the withstand voltage test, and the value of this value is clearly specified in the factory type test of the appendix.
Withstand voltage test and breakdown test
The standard specifies the specific voltage value at the time of the test. After the withstand voltage test, it can only indicate that the insulation structure of the product can withstand the test voltage, and it cannot indicate how much the insulation structure of the product can withstand. If the application of insulation materials and the design of electrical equipment are required, when it is necessary to measure the dielectric strength, a breakdown test is required. The breakdown test is to test the voltage at which the 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 conform to Ohm's law, but increases abruptly. As shown in Figure 1, the insulating material is destroyed and the insulating properties are lost. For electrical products, if the insulation breaks down, it loses its operational function.
Some customers require a one-minute test according to the type test standard, which is “one harm to the productâ€: first, it may reduce the yield of the product; second, although it passes the test, it may damage part of the insulation structure, so that Product safety is reduced; third, some components may be damaged, so that the quality of the product is reduced and the life is shortened. Passing the withstand voltage test does not mean that the lamp is intact. In many cases, the LED lamp bead can pass the pressure type test, but it will cause the lamp to be extinct, but the actual life of the lamp will be greatly reduced due to the delay.
Test Methods
Typical withstand voltage test method:
1. Check to see if the main power switch of the withstand voltage tester is in the "off" position;
2. Unless the instrument is specially designed, all uncharged 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 loop ground wire (usually black) of the withstand voltage tester to the accessible uncharged metal part of the device under test;
8. Close the main power switch of the withstand voltage tester and slowly raise the secondary voltage of the instrument to the required value. Generally, the boosting speed does not exceed 500 V/sec.
9. Maintain this test voltage for a specified period of time;
10. Slowly lower the test voltage;
11. Disconnect the main power switch of the withstand voltage tester. First disconnect the high voltage output line of the withstand voltage tester, and then disconnect the grounding wire of the withstand voltage tester circuit.
The following conditions indicate that the device under test does not pass the test:
1. When the test voltage does not rise to the specified voltage value or the voltage decreases, 2. When the voltage tester shows a warning signal.
It should be noted that due to the high voltage that is dangerous to the human body in the withstand voltage test, special care must be taken when testing.
Things to pay special attention to:
1. It must be stipulated that only trained and authorized personnel can enter the test area to operate the instrument;
2. A fixed and obvious warning slogan must be placed around the test area to prevent other personnel from entering the danger zone;
3. When conducting the test, all personnel including the operator must be away from the test instrument and the device under test;
4. When the test instrument starts, do not touch its output line.
Common ways to improve pressure resistance
· Adding an isolation transformer to effectively reduce the withstand voltage of the secondary line through the transformer;
· Increase the strength of the insulating medium, such as rubber, add insulating gaskets;
· Improve the voltage resistance of the circuit board: a, select the circuit board with high dielectric layer pressure resistance; b, increase the creepage distance of the circuit board.
As the circuit board where the lamp bead is arranged, the creepage distance and the wiring space are contradictory. In order to increase the creepage distance, the wiring space can only be compressed as much as possible. Try to avoid sharp objects to reduce the scratching of the insulation or the breakdown of the voltage due to the tip discharge. If you try to reduce the use of self-tapping screws; when designing the circuit board, apply copper at right angles and do over-treatment.
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