Difference Between AC and DC Voltage Withstand Tests

Both AC withstand voltage and DC withstand voltage tests are crucial methods for assessing the insulation strength of electrical equipment. These tests help evaluate the device’s ability to withstand high voltage, monitor its insulation condition, identify potential hidden defects, and ensure safe operation.

1. What is AC Withstand Voltage?

AC withstand voltage is the most effective and direct method for assessing the insulation strength of electrical equipment. The test involves applying an AC voltage higher than the equipment’s operating voltage, simulating the electrical environment the device will encounter during actual operation. This exposes potential insulation defects, ensuring the equipment’s safety. Since the test current is capacitive, the required test equipment has a larger capacity, and the damage to the insulation is more noticeable.

2. What is DC Withstand Voltage?

DC withstand voltage tests the device’s ability to withstand high voltage by applying DC voltage. Unlike AC withstand voltage, DC voltage does not cause dielectric losses in the insulation, so it causes less damage to the insulation. The equipment needed for DC withstand voltage tests is smaller, portable, and can simultaneously measure leakage current to further assess the device’s insulation condition.

3. Main Differences Between AC and DC Withstand Voltage Tests

Test Nature:
- AC Withstand Voltage: Simulates the operating conditions of the equipment under an AC electric field, directly reflecting the electrical environment the device will experience during actual operation. The test voltage is usually higher than the equipment’s operating voltage.
- DC Withstand Voltage: Tests the equipment’s ability to withstand DC voltage, and the voltage distribution differs from actual operating conditions.
Damage to Insulation:
- AC Withstand Voltage: The test current is capacitive, which causes more significant damage to the insulation, especially under high voltage, where insulation defects are more apparent.
- DC Withstand Voltage: Causes less damage to the insulation because there is minimal dielectric loss under DC voltage. DC tests are also more portable and suitable for on-site testing, especially for detecting local insulation defects.

4. Insulation Preventive Testing

Insulation preventive testing is a key measure to ensure the safe operation of electrical equipment. These tests allow for the timely identification of the device’s insulation condition and the detection of internal defects, enabling preventive measures to be taken to avoid faults caused by insulation issues during operation.

There are two main types of insulation preventive tests:

Non-destructive tests: These measure insulation resistance, leakage current, dielectric loss, and other parameters to assess the insulation status of the device without causing damage.
Destructive tests (withstand voltage tests): These apply a voltage higher than the equipment’s operating voltage to rigorously test insulation strength. This includes AC and DC withstand voltage tests.

5. What is an AC Withstand Voltage Test?

An AC withstand voltage test involves applying an AC voltage higher than the equipment’s working voltage to rigorously assess its insulation strength. This type of test effectively reveals concentrated insulation defects, especially in regions with high electric field concentration and weak local insulation. AC withstand voltage tests are indispensable for factory acceptance and commissioning tests.

Since AC withstand voltage tests are destructive, they must be preceded by various non-destructive tests, such as measuring insulation resistance, absorption ratio, dielectric loss factor (tgδ), and DC leakage current. These results must be analyzed to determine if the device is damp or defective. If any problems are found, they must be addressed before proceeding with the AC withstand voltage test. Otherwise, insulation breakdowns might occur during the test, worsening defects, extending repair times, and increasing workload.

AC withstand voltage tests are the most direct method to assess insulation strength, and they effectively identify concentrated insulation defects. These tests play a decisive role in determining whether electrical equipment can be safely put into operation and are essential for ensuring insulation levels and preventing insulation failures.

6. What is a DC Withstand Voltage Test?

A DC withstand voltage test applies high DC voltage to the equipment to assess its insulation performance. During the test, leakage current can be measured to identify potential local insulation problems. DC withstand voltage testing equipment is lightweight and suitable for on-site testing, especially for detecting local insulation defects.

Although DC withstand voltage tests cause minimal damage to the insulation, the voltage distribution is different from the actual operating conditions, so it cannot fully reflect the device’s performance during operation. Therefore, DC tests are typically used in combination with other testing methods.

One advantage of DC testing is that the voltage can be gradually applied. By monitoring the current flowing through the sample as the voltage increases, operators can detect potential breakdowns before they occur. After the test, the sample must be discharged to avoid any residual charge from affecting subsequent operations.

However, DC withstand voltage testing can only apply voltage in one direction, unlike AC tests, which apply voltage stress in both polarities. Additionally, DC testing equipment is harder to generate and thus more expensive than AC testing equipment.

7. Why Can't AC and DC Withstand Voltage Tests Replace Each Other?

AC and DC withstand voltage tests cannot replace each other because of their different voltage distribution mechanisms. DC voltage distributes according to conductivity in the insulation layer, while AC voltage distributes according to the resistance and capacitance ratio. As a result, AC voltage more closely mimics the actual voltage distribution in equipment during operation, providing a more accurate reflection of the equipment’s electrical environment.

Moreover, the insulation strength under DC voltage is usually higher than that under AC voltage, meaning that DC tests cannot fully reveal insulation weak points in an AC electric field. Therefore, AC and DC withstand voltage tests are complementary, not interchangeable.

Difference Between AC and DC Voltage Withstand Tests