Pre-insulated tubular terminalsThe difference between withstand voltage and insulationWithstand voltage test standards
Between the primary circuit and the metal casing of the device under test. If the insulation between them is good enough, the voltage difference applied will only produce a small leakage current. Another scenario is testing the insulation between the primary and secondary circuits of the test power supply. In this case, short-circuit all output terminals and connect them to the low-end terminal of the withstand voltage tester, then short-circuit the L and N lines of the device's power input and connect them to the high-voltage output terminal. During the test, it is important to remember that the device under test is not connected to its working power supply and is in a non-operating state, but its power switch must be turned on. Practice shows that without turning on the power switch, the withstand voltage test is very easy to pass. If the test voltage is too low, the insulation material may not have sufficient voltage applied, causing substandard insulation to pass the test; if the voltage is too high, it can cause permanent damage to the insulation material during the test. However, there is a general rule: for example, if the product's power supply voltage is 120V, then the test voltage = 120V × 2 + 1000V = 1240V. In practice, this method is also adopted by most safety standards. The reason for using 1000V as part of the basic formula is that the insulation performance of any product is subjected to transient high voltages daily. Laboratory and research studies have shown that this high voltage can reach up to 1000V. Since a large number of products undergo electrical withstand tests on the production line, the test time is usually reduced to only a few seconds. A typical practical principle is that when the test time is reduced to only 1-2 seconds, the test voltage must be increased by 10-20% to ensure the reliability of insulation during short-time testing. The method of using a withstand voltage tester is to pre-test a batch of samples for leakage current, obtain an average value, and then set a current slightly above this average as the set current. Since the device under test inevitably has some leakage current, it should be ensured that the set alarm current is large enough to avoid false triggering by leakage current, while also being small enough to avoid passing substandard samples. In some cases, a lower limit alarm current can also be set to determine whether the sample is in contact with the output terminal of the withstand voltage tester. For the selection of AC/DC test voltage, most safety standards allow the use of AC or DC voltage in withstand voltage tests. If AC test voltage is used, when the voltage reaches its peak, whether positive or negative, the insulation under test bears maximum stress. Therefore, if you decide to use DC test voltage, you must ensure that the DC test voltage is a multiple of the AC test voltage, so that the DC voltage can be equivalent to the AC voltage peak. For example: 1500V AC voltage, to produce the same amount of electrical stress, the DC voltage must be 1500 × 1.414, i.e., 2121V DC. One advantage of using DC test voltage is that in DC mode, the current flowing through the alarm current measurement device of the withstand voltage tester is the actual current flowing through the sample. Another advantage of DC testing is that the voltage can be applied gradually. By monitoring the current through the sample as the voltage increases, the operator can detect before breakdown occurs. It should be noted that when using a DC withstand voltage tester, due to the charging of capacitors in the circuit, the sample must be discharged after the test is completed. In fact, regardless of the test voltage or product characteristics, it is beneficial to discharge the product before handling it. The disadvantage of DC withstand voltage testing is that it can only apply test voltage in one direction, unlike AC testing which can apply electrical stress in both polarities, and most electronic products operate under AC power. Additionally, since DC test voltage is more difficult to generate, DC testing is more costly than AC testing. The advantage of the AC withstand voltage tester produced by Dingsheng Electric is that it can detect all voltage polarities, which is closer to actual practical conditions. Furthermore, since AC voltage does not charge capacitors, in most cases, there is no need to gradually increase the voltage; directly outputting the corresponding voltage can obtain a stable current value. Moreover, after AC testing, there is no need to discharge the sample. The disadvantage of AC withstand voltage testing is that if there are large Y capacitors in the circuit under test, in some cases, AC testing may cause false judgments. Most safety standards allow users to either not connect Y capacitors before testing or switch to DC testing. DC withstand voltage testing will not cause false judgments when applying high voltage to Y capacitors because the capacitors will not allow any current to pass. Consideration of the maximum permissible error of the withstand voltage tester: Since the output voltage of the withstand voltage tester is not 100% accurate, when determining the test voltage, the error of the instrument's output voltage should be considered. This can be based on the technical specifications of the withstand voltage tester itself or the calibration certificate from a higher authority. For example, when performing a withstand voltage test at AC 3000V, if the maximum permissible error of the withstand voltage tester at this point is 3%, i.e., 90V, then to ensure sufficient test voltage, the output test voltage should be adjusted to 3090V. Of course, if the high-voltage withstand voltage tester itself outputs high, there is a risk of voltage being too high, and the tester should fully consider this. Precautions for accurate testing: New programmable withstand voltage testers often provide many functions not available in older models, such as voltage presetting, current presetting, ground continuity test, arc detection, and current zeroing. These functions can bring convenience to testing, but the correct use of these functions is often overlooked. AC small current testing: Some instrument manufacturers claim that their withstand voltage testers can achieve leakage current resolution at the nanoampere level. However, actual AC measurements make it difficult to truly test such small currents. There is a certain amount of capacitance in any circuit; even a simple transformer has capacitance between its windings and core. Capacitance not only produces leakage current due to a certain resistance, but under AC voltage, the capacitor itself is also an impedance device. These currents are independent of the current the user wishes to test, and their magnitude depends on the capacitance value, frequency, and applied voltage. Most withstand voltage testers often have the voltage circuit terminal grounded, and sometimes the sample under test is also grounded at the low end. Therefore, in this case, the leakage current measured by the withstand voltage tester is necessarily the sum of the leakage current through the sample and the leakage current of the withstand voltage tester itself. The leakage current of the withstand voltage tester itself is usually very small, but when measuring nanoampere-level leakage currents, it becomes a major issue.
