How is the understanding of withstand voltage and test voltage?

In practical operation, transformers and choke coils are exposed to transient overvoltages due to various reasons.

Content

●Illustrate

For instance, when a transformer's power supply is closed, it may generate a transient overvoltage twice the normal operating voltage. Similarly, when the power supply is disconnected, it can produce a transient overvoltage 8 to 10 times the normal operating voltage. Sudden open or short circuits in the load can result in extremely high transient overvoltages, and lightning can induce transient overvoltages of up to 3000 volts. The connection and disconnection of thousands of electrical devices on the same power grid may also cause mutual interference and transient overvoltages.

 Additionally, due to distributed capacitance between layers, groups, turns, and coils with the core of transformers and choke coils, the distribution of transient overvoltages is uneven, with the first and last few turns and layers of the coil typically experiencing the maximum transient overvoltage. As a result, transformers and choke coils are constantly exposed to the impact of transient overvoltages during operation.

●Electrical strength & test voltage

Although the insulation design of transformers and choke coils incorporates a significant safety margin, to ensure their long-term, safe, and reliable operation in practical settings, it is necessary to subject them to a withstand voltage test using an externally applied voltage several times higher than the normal working voltage of the product. This test is designed to evaluate the insulation system's ability to withstand transient overvoltages and to check for potential defects. In the withstand voltage test, the externally applied voltage is referred to as the test voltage. Typically, when the working voltage is below 110 volts, the test voltage is twice the working voltage plus 500 volts. When the working voltage is above 110 volts, the test voltage is twice the working voltage plus 1000 volts. Since different electronic devices have varying operational conditions, the requirements for withstanding transient overvoltages may differ. Therefore, specific test specifications are often formulated based on the individual circumstances of each type of electronic device.

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