Posted on October 24, 2020
What is Dielectric Testing and How is it Done?
Like any other material, electric components can deteriorate with time due to factors like aging or environmental changes. This can result in problems like component failures, rusting, dust accumulation, condensation, among others. On top of that, there could be load changes or circuit alterations. This is where periodic dielectric testing comes in.
What is dielectric testing?
Dielectric testing helps to determine the durability of insulating materials in electronic devices. It ensures the peak level of voltage doesn’t exceed that of the insulating barrier. Because it’s a non-destructive method, it verifies the adequacy of electrical insulation. And depending on the environmental conditions the device is subjected to, the voltage can be adjusted accordingly. This test helps to check if an electrical product has complied with safety standards. Also, it verifies whether the device protects the user adequately from potential hazards.
A higher dielectric test voltage means that the electrical device has been subjected to environmental degradation. Keep in mind that this type of test shows how long the material has before it breaks down.
Since dielectric testing is part of electrical compliance regarding the safety of the equipment, it should be performed to prevent a breakdown. Unfortunately, the real goal for this test is often misunderstood and may give misleading results. This post seeks to clarify how the test is done.
How is dielectric testing done?
It involves placing high voltage (1000V) across the insulation barrier of the device for 60 seconds- the duration will depend on your requirements. Generally, the test time for materials covered under IEC 60950 is less than a minute.
In most production tests, the test time is usually one second while in design tests, it can go for up to a minute. While most devices follow the standards of the dwell time, a general rule of thumb is raising the test time by 20%.
If the device holds a voltage, then it has passed the test. But if there’s a sudden breakdown of the material, the insulation is deemed to be insufficient.
Unless otherwise stated, the dielectric testing voltage should be 2X the nominal input voltage plus 1000 Watts. For example, if the nominal voltage for a product is 100V, then the testing voltage should be (100 x 2) + 1000 = 1200V. But there’s a caveat to this. In some devices, you may be required to use a specific voltage.
Most double insulated products like medical equipment require a higher voltage than the one described above.
Let’s say you have two conductors- an insulated wire and a metal enclosure. Since they are isolated from one another, a high voltage will not allow the current to flow between the conductors and hence the term, dielectric test. The goal of this test is:
- Helping to find crushed insulation
- Identifying tolerance errors in cables
- Finding corrosive contaminants around conductors
- To monitor changes and aging
- Checking the quality of the electrical equipment
- Detecting the manufacturing defects
This test consists of two phases. The first one checks if there’s excess current flow of the insulating material that separates the two conductors while the second one checks the electrical discharge.
During dielectric testing, the electronic devices may exhibit current analogous. But since the dielectric test is much higher, the current increases with the voltage. For example, if a device has 5.2mA, you should use a dielectric current of 1240V.
Electrical safety when performing dielectric testing
While some manufacturers recommend that you run the test for about 10 seconds, one second could be enough to obtain the correct reading. The resistance limit also matters. Generally, the idea of measuring the resistance limit is to determine how low the current will flow. Depending on the product’s ground wire, the majority have a maximum resistance value. Be sure to follow the standards to ensure you follow proper testing procedures.
If the resistance is too low, the test will fail. If there’s no product specification, then a test at 1000V could be ideal.
Trip current during dielectric testing
During the test, the insulating material loses resistance and allows the current to flow to a much higher level. If the current exceeds the testing equipment, then it trips out. This explains why this method has potential limits to its use.
So, what’s the problem with false failures? When the current is above the trip-out threshold, it indicates the natural dielectric current of the tested device. While a trip out may indicate a current breakdown, it doesn’t mean that the test results are unacceptable. Do you get the picture?
Think about it- there’s nothing worse than believing that you’ve applied the correct voltage on your device only to realize that part of the cable is broken. This is the case if you didn’t detect the fault on time. Having said that, you should set the proper low limit.
AC and DC dielectric Testing
While dielectric testing can be met by both AC and DC voltages, there’s a slight difference between the two. Both use a metered voltage equal to 4.14 times the test voltage. However, a DC test voltage should be equal to the highest AC voltage.
With a DC test, the dielectric current is usually lower. Unlike AC dielectric testing, the test voltage raises slowly and once the voltage stabilizes, there’s very little current flow. Because there’s no trip out limit, you can adjust the dielectric tester without false alarms. If the voltage ramps up too quickly, the charging current can lead to false failure.
Some products may have an electromagnetic filter that can add to capacitance. To accommodate them, you should set the current leakage to 3.5 mA. But with this kind of heavy loading, the tester may not deliver enough power to cross the terminals.
In the DC test, the capacitive current is a one-time charging current. So, in the event of high leakage limits, the tester can deliver a high current load.
The only downside with this type of test is that the device under test has the potential of a shock hazard. When testing some products, you may have to discharge them, which adds the complexity of the test.
If you use a DC test in an AC circuit, then the voltage should be twice the peak. And depending on the applicable standard, the leakage current should be less than the allowable current. For double-insulting products, a higher voltage may be required for the testing products.
The defects like dirt and other contaminants can often be detected. If this problem is not corrected, it may cause an unnecessary level of leakage.
The nominal voltage line is 60V (cycles per second) refers to the frequency of the voltage. Depending on the cycle, you may experience instantaneous voltage. The voltage may start from zero, and then it rises to the peak.
AC Dielectric Testing
While a standard transformer can be used to do the test, an electrical surge could occur. The higher the voltage, the better the results. Generally, AC testing goes from a positive peak to a negative every second.
When running the test, you’re simply creating a capacitor that incorporates line and neutral. The reason behind this test is to verify whether the insulation can withstand high voltage for a specific period.
One of the advantages of AC dielectric testing is checking the voltage polarity. On the downside, the test may indicate a failure.
How can you tell if a breakdown has occurred?
Just like other types of tests, an electrical breakdown can happen at the peak. And the time the voltage takes to drop from peak takes seconds. It’s worth mentioning that breakdown can occur any time, so both AC and DC voltages can appear the same.
Setting low and high current limits
Any current above the tester is considered a fail. On the other hand, if the current is below the tester, that’s a pass. For proper dielectric testing, the current should flow through the device under test. If the device is broken, then the integrity of your test is compromised. Of course, the use of a low limit can help to detect any faults that may occur.
If the test voltage is too high, this may cause permanent damage to the insulation material. Similarly, if the voltage is too low, the insulation material won’t pass the test. From an expert point of view, a 120V wiring should be tested using a voltage of 1000V. The best way to identify the test level is to test products with an average current.
The results of dielectric testing is determined by:
- Electrical factors – direct current, voltage, and alternating current
- Physical factors- humidity, temperature, and mechanical deformation
- Chemical factors – oxidation and impurities
Understanding the mechanism of the insulating materials
Different materials require various levels of the electrical field for a dielectric breakdown to happen. For example, metals have free electrons and have a high electrical field that allows the current to flow. If the current is sufficient, the electrons of the insulating material will cross the bandgap. This, in turn, results in dielectric breakdown.
Where is the high voltage applied?
As mentioned above, high voltage is used in dielectric testing. Typically, one side of the electric supply is connected to the ground while the other is connected to the device being tested. What if you test something more complicated? This is where the high voltage comes in. If the working voltage of the product in question is 240 VAC, then you need a tester that can withstand the high volts.
Current leakage during dielectric testing
All electrical devices produce a leakage due to the internal capacitance. Sometimes, the leakage may not be enough to be detected by a human body. But because of the manufacturing defects of products, an electrical gadget could break down.
A breakdown is referred to as failure to prevent the insulating device to stop the flow of current. As you perform the dielectric test, the leakage could be higher than the nominal measurement. However, the actual leakage will depend on the product being tested.
To understand proper leakage settings, you should calculate the average leakage current. The high leakage setting is determined by calculating the average value and then add 25%. If you want to know the lower limit, you should subtract 25% from the average value.
How to choose the best dielectric testing equipment
The dielectric equipment should have specific features to ensure you get the best results. This include:
- A programmable multi-level testing feature
- AC and DC outputs
- An adjustable output voltage
- Capacitive coupling detection
- Great regulation – line and load
Modern testers use electronic source technology that detects incorrect results when there’s a leakage. Also, they will indicate the current used during the test.
Safety precautions during dielectric testing
Before you start the test, the important thing to keep in mind is your safety and those around you. The most important thing to consider is the operators’ health. Because high voltage can be dangerous, you should avoid touching the cables. Also, you should have some knowledge of the effects of high voltage on your health. Plus, only a qualified person should be allowed to do the job.
Another important consideration is the source of power. It should be reliable and must meet the code for grounding and polarization. To ensure your overall protection, you should use an outlet that has been tested for safety.
The working area also matters. You should ensure that there’re non-metallic materials in your working area. Furthermore, there should be no materials placed between the device under test and the operator. Other things to keep in mind include:
- Use a safety interlock system to ensure you don’t contact the cable as you perform the test
- The current should not exceed the 5MA peak
- Total energy should not exceed 350MJ
- The total charge should not exceed 45UC
- Wear insulating gloves
- Always verify the safety circuits
- Don’t do the test if you have any electronic implants
Can two consecutive dielectric tests give the same results?
After the first test, the insulating material must return to their previous state. If you intend to do the second dielectric testing, the discharge times will be longer.
Dielectric testing is a requirement for most electronic devices. While this may look like a daunting task, there are many high-tech devices with all the features and functionalities to get the job done. You should keep in mind that the importance of this test is to evaluate the electrical strength of the insulation.
Upon the completion of the test, the equipment will indicate a pass or fail. If it passes, you should record the leakage current. If it fails, then you can record the failure voltage.