That not so dry feeling in the summer
Summertime arrives and along with it high humidity. To cable and harness assemblers, summertime humidity means a hipot testing storm is brewing. When humidity is high, such as happens in summertime, the increased moisture can affect wire insulation. If you are experiencing a high number of insulation resistance (IR) failures during summertime, your cables and harnesses may be suffering from humidity.
Humidity and Insulation
Occasionally in humid conditions insulation will absorb the humidity in the atmosphere and can cause IR failures. When wire insulation absorbs water or water vapor, electricity begins to leak (water is a conductor) between wires that are next to each other or to a shield. The more water absorbed, the worse the problem.
Insulation may absorb water in the following conditions:
- High relative humidity.
- Insulation made of materials other than Teflon/TFE or PVC such as nylon, rubber, neoprene, or PVC with special additives. Each of these respond differently in humidity.
- Cables that are highly flexible with soft wire insulation.
- Residue such as no-clean flux, cleaning alcohol, or even oils from your hands that contaminate the materials.
What do humidity problems look like?
Humidity related IR failures will usually report themselves as an IR or HV leakage failure. High humidity can allow moisture paths for small amounts of current to flow between unintended connections. This will cause a lower IR value.
For example, your 1 GigaOhm IR test fails with a value of 998 MegaOhms. If you repeat the test, the measured value may get higher until it starts to pass the test. This is a tipoff that the problem has to do with an atmospheric problem. The change in value comes from the slight amount of heat that is generated through the wires during hipot testing, thereby drying out the insulation one test at a time. If this is the case, we have some suggestions to further verify the problem, and possible solutions to get you back to passing consistently good product.
How to Test for Humidity Problems
If you suspect humidity is causing IR failures in your cables, there are a few tests that can help you know.
- Use a heat gun or oven to "dry out" the insulation and confirm that it passes. Use this on the wiring, not on the terminated connector ends where possible, to confirm that you can eliminate the problem with heat on wires. Make sure just to warm the wires. Too much heat may damage them and the hotter they are the higher the resistance.
- Soak the insulated wires, but not the terminated connector ends of the cable, in water overnight. Then retest soon after removal from the water. If the insulating material does not absorb moisture the cables will still pass. Normal PVC and TFE/HFE type insulation will always pass without any problem. (As an alternative to soaking the cables in water, you can create a humid environment by placing a sample length or a whole spool of cable in a garbage bag or a sealable container with a bowl of water.)
- Experiment with different HV dwell times. Shorter dwell times should make the problem worse; longer dwell times should make it better. This is because the longer High Voltage is applied the more "drying" takes place on the cable.
How to Solve Humidity Problems
Clean and Neat
To decrease humidity problems, keep your materials clean. If residue is left on your connectors or circuit boards, humidity will more likely creep into your insulation and cause IR failures. The best solution is to keep things clean rather than try to clean once a problem is discovered.
Choose Materials Carefully
One of the easiest ways to prevent humidity problems is by using the right materials. For example, nylon is very absorbent. Using nylon insulation or connector housings in humid temperatures will cause IR problems in your cables. Use a material such as Teflon or PVC. They may cost more than nylon, but they will save you money by giving you cables that work in humid conditions.
“Soak” Them to Dry Them
“Soak” is a special command available in some of the Cirris hipot testers. Similar to a dwell time, Cirris testers are capable of soaking the device under test with electricity to help evaporate the humidity from the insulation. During soak time, voltage is raised on the net to prepare the net for an IR test. Like a warm up before the big game, soak time will help wires pass that would otherwise have failed during the IR test. In humid conditions a longer soak time will dry cables more thoroughly. During the humid months many tests require longer soak times to consistently pass the IR test.
Air Conditioning – AKA Dehumidifying
We get calls about failing IR tests every year from customers in some provinces of China during monsoon season. To solve the problem, our customers put their testers and finished cables in an air conditioned office space overnight. This gave the customers a few hours in the morning to test.
If people are uncomfortable in the heat and humidity, then it is likely the devices to be tested are too. If you have the ability and space, move the test area to an air conditioned environment. You will have to leave them in the conditioned space for a while to let them dry out, but your tests will be more consistent.
Cirris’s Easy Touch, CH2, and 1100H+ are all able to test cables even in 0 to 95% humidity. However, IR test results will degrade when relative humidity exceeds 75% non-condensing (meaning moisture does not bead). Though many factors could contribute to IR failures, Cirris testers are built to withstand humidity better than any other we have found.
If your cables start failing the IR test when they used to consistently pass, check your wires’ insulation to find out if you are susceptible to humidity before spending time and money chasing a nonexistent tester problem.