Conditions that could contribute to, or worsen, the effects of this problem:
Relative Humidity substantially greater than 60%.
Wire insulation other than Teflon/TFE, and PVC such as rubber and neoprene, or even PVC if special additives have been used.
Soft wire insulation such as is found in highly flexible cables.
Root Cause:
Wire insulation that absorbs moisture creating IR failures.
Understanding the Failure Mechanism:
When wire insulation absorbs water, electricity begins to leak between wires that are next to each other or to the shield. The more water that is absorbed, the worse the problem. In addition, if the voids within the insulation that absorb water molecules connect from one to the next, the problem gets even worse. These voids sometimes look like little tree structures.
Confirmation of Root Cause:
One or more of the following might be used to verify the defect.
Use a heat gun or oven to “dry out” the insulation and then confirm that it passes. To confirm that you can eliminate the problem with heat on wires, use this on the wiring and not on the terminated connector ends where possible.
Soak the insulated wires, but not the terminated connector ends of the cable, in water overnight. 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.
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.
General Recommendations:
If you are using wire with a soft insulation, you may want to evaluate the effects of moisture. This can be done by placing a sample length or whole spool of cable in a garbage bag with a bowl of water (to create a humidity chamber).
Case Study
Problem
A cable assembly house experienced sporadic failures with a 1000 volt 100 meg IR test on a cable assembly that used very fine wires that were highly flexible and had an outer jacket that felt rubbery. The failures occurred on humid days but not on dry days. Was the problem the terminations, connectors, rubbery cable, fixturing, or tester?
Discovery
A sample cable placed in a humidity chamber began to exhibit IR failures after about an hour. To further isolate where the moisture absorption was occurring, the cable (but not the connectors) was submersed in water overnight. Subsequent testing verified the absorption was through the insulation, not at the connector ends. The insulation material used on the outer jacket was the same as used on the internal wires. It absorbed moisture, which caused IR failures.
Solution
The assembly house confronted the wire supplier with the IR performance problem of their bulk cable and requested to return the defective cable so that it could be replaced it with an acceptable one.
