UL certification withstand voltage test methods and precautions

In UL standards, almost all electrical safety standards require products to be tested for withstand voltage. Dielectric Voltage Withstand Test, also known as High Voltage Test, determines whether the insulation material and space distance of the equipment meet the requirements by applying a voltage higher than its rated value to the equipment and maintaining it for a certain period of time. test.

This article describes in detail the methods and precautions for UL certification withstand voltage testing to help Chinese companies manufacture electrical equipment that truly meets North American market standards.

Why does UL certification require voltage resistance testing?
Under normal circumstances, the voltage waveform in the power system is a sine wave. During the operation of the power system, due to lightning strikes, operations, faults, or improper parameter coordination of electrical equipment, the voltage of 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 is overvoltage caused by direct lightning strike or lightning induction, which is called external overvoltage. The amplitude of lightning impulse current and impulse voltage is very large, and its duration is very short, and it is extremely destructive. However, since overhead lines of 3-10kV and below in towns and general industrial enterprises are shielded and protected by factories or tall buildings, the probability of being struck by direct lightning is very small and is relatively safe.

Moreover, what is discussed here is civilian electrical appliances, which are not within the above scope and will not be discussed further. The other type is caused by energy conversion or parameter changes within the power system, such as connecting no-load lines, cutting off no-load transformers, single-phase arc grounding in the system, etc., which is called internal overvoltage.

Internal overvoltage is the main basis for determining the normal insulation level of various electrical equipment in the power system. In other words, the design of the product's insulation structure must not only consider the rated voltage but also the internal overvoltage of the product's usage environment. The withstand voltage test is to detect whether the product insulation structure can withstand the internal overvoltage of the power system.

UL certified withstand voltage test equipment requires
products below 600 V. U.S. and Canadian standards usually require that the transformer capacity of the withstand voltage test instrument be at least 500 VA, the output current can reach 95 mA, and the capacitance value between the live part and the ground No more than 0.1 mF. The instrument should be calibrated at least once a year. Instrument inspection must be carried out before form testing, and instruments must be checked regularly during factory testing. This can avoid misjudgment or damage to the product due to abnormal instrumentation.

AC withstand voltage test instruments generally include high-voltage test transformers, voltage regulators, voltage measurement systems, and control and protection devices. Figure 1 is a circuit diagram of the manual voltage boost test device for reference.

Figure 1 Circuit diagram of manual voltage boost test device

UL certification withstand voltage test method
The following introduces the typical withstand voltage test method:
1. Check to confirm that the main power switch of the withstand voltage tester is in the "off" position;
2. Unless the instrument is specially designed, all non-energized metal parts It must be reliably grounded;
3. Connect the wires or terminals of all power input terminals of the equipment under test;
4. Close all power switches, relays, etc. of the equipment under test;
5. Adjust the test voltage of the withstand voltage tester to zero;
6. Connect the high-voltage output wire (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 terminal of the device under test. Touch the non-energized metal part;
8. Turn on the main power switch of the withstand voltage tester and slowly increase the secondary voltage of the instrument to the required value. Generally, the voltage boost speed does not exceed the speed of 500 V/sec;
9. Maintain the test voltage within the specified time;
10. Slowly lower the test voltage;
11. Turn off 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 loop ground wire of the withstand voltage tester.

The following situations indicate that the device under test fails the test:
· When the test voltage cannot rise to the specified voltage value or the voltage drops instead When;
· When a warning signal appears on the withstand voltage tester;
it should be noted that because there is a high voltage that is dangerous to the human body during the withstand voltage test, special care must be taken during the test.

Precautions for UL certification withstand voltage test:
· It must be specified 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 personnel from entering the dangerous zone;
· When When testing, all personnel, including operators, must stay away from the test instrument and the equipment under test;
· When the test instrument is started, never touch its output wires.