The basic principle of voltage withstand test:
The product under test in the high-voltage machine output test under high voltage leakage current and set the judgment, current comparison, if the leakage current is less than the preset value of the check, then the product is judged to pass the test. If the leakage current is greater than the judgment current, the test voltage will be cut off instantaneously and sound and light alarm will be issued, and the test program will show that the test has failed. The test program will show the test failure.
Voltage withstand test (a) Dielectric strength test, also known as hipot test, is probably the most known and frequently performed production line safety test. In fact, demonstrating its importance is part of every standard. The hipot test is a non-destructive test that determines the adequacy of electrical insulation to resist instantaneous high voltages. It is a high voltage test applied to all equipment to ensure that the insulation is adequate. Other reasons for performing the hipot test are that it can detect possible defects such as insufficient leakage distances and electrical clearances caused during the manufacturing process.
When type testing is performed, the hipot test is performed after certain tests (e.g., failure, moisture, and vibration tests) to determine if there is any degradation of the insulation as a result of these tests. However, hipot testing for routine production is an in-process test to determine if the construction of the product is the same as that used in the type test. Some defects caused by the production process can be detected by in-line hipot testing, e.g. a reduction in the electrical clearance and creepage distance of the transformer windings. Such faults can be caused by a new operator in the winding department. Other examples include checking for pinhole defects in the insulation or finding an oversized solder joint.
Most safety standards use the convention of 2xU + 1000 V as the basis for basic insulation testing, where U is the operating voltage (rms value). This convention is only a guide, and for individual standards, especially IEC 60950, a specific table is provided to define the exact test voltage based on the actual operating voltage measured.1 The reason for using 1000 V as the basic convention is that the insulation of a product may be subjected to momentary overvoltages in everyday use. Experiments and studies have shown that these overvoltages are often as high as 1000 V.
Test Methods:
High voltage is usually applied between two components that span the insulation under test, such as the primary circuit of the test equipment (EUT) and the metal enclosure. If the insulation between the two components is sufficient, then the large voltage applied between the two conductors separated by the insulator can only produce a very small current flow through the insulator. Although this small current is acceptable, no breakdown of the air or solid insulation should occur. Therefore, it is important to note that this current is the result of a partial discharge or breakdown, and not due to a capacitive connection.
Another example is to test the insulation between the primary and secondary circuits of the power supply. This is when all outputs are shorted together. The earth probe of the withstand voltage tester is connected to the outputs that are shorted together, while the high voltage probe is connected to L and N (L and N are shorted) (see Figure 1). The EUT does not operate during the hipot test. It is important to note that during a type test, it is ideal to apply less than 1/2 of the specified voltage, then gradually increase it and reach the specified voltage in 10 seconds and maintain it for 1 minute. Most test instruments, however, output the specified voltage directly or use an electronic control circuit to achieve voltage creepage.
Test Duration:
If the test is to represent part of the certification process, then the test duration must be in accordance with the safety standards used. For example, most standards, including IEC 60950, have a test duration of 1 minute. However, when testing products on a production line, it is often impractical to perform a 1 minute hipot test on each product and manufacturers will often reduce the test duration, say by a few seconds, but use a higher voltage. A typical rule of thumb is 110-120% (2xU + 1000 V), 1-2 seconds. The test duration and procedure should be agreed with the relevant test institute. It is important to note that although the reduced times and increased voltages are approximate, experimental and manufacturer's' data indicate that each insulation material has its own specific voltage-time characteristics.
Ground test:
The ground loop impedance is relatively smaller than the human body impedance model. If there is a leakage, the shunt from the human body will be so small that it will not cause ventricular confession.
Test Principle: Ground resistance test is a very important test of safety performance testing. Tester for the test product to provide a constant current, the current does not change with the change of the load. i given, measure u, according to Ohm's law to calculate the resistance R.
Test method: in the test of the product grounding or input socket ground contact, the product shell or metal parts, apply a constant current to test the magnitude of the impedance between the two points, the current should be equal to the test of the product's rated current The current shall be equal to 1.5 times the rated current of the test product or 25A (whichever is greater). Measure the voltage drop between the ground point of the test product or the ground contact of the input jack and the product's case or metal part. From the current and the voltage drop to calculate the resistance, the resistance value should not exceed 0.1 Ω. This test can detect the grounding point screws are not locked, grounding wire diameter is too small, grounding wire breakage and other safety issues.