Insulation Compression Failures
in Cables

Test Results:

• Shorts both intermittent and constant
• Failed Hipot - DWV and "Over Current"

Necessary Conditions:

• Cable Ties
• Clamps
• Routing of wires through holes in sheet metal where the holes do not have soft grommets.
• Tight fitting strain relief's
• Wrapping bare wire around exposed shielding to make a soldered connection.
• Insulated wires on the factory floor that get stepped on with hard soled shoes, run over with wheeled carts, etc. during the assembly process.

Aggravating Conditions:

• Heat from soldering shields.
• Unusually soft wire insulation such as is found in highly flexible cables or coax cable with "foam" insulation
• Sharp edges on components brought in contact with wire insulation.
• Thin-wall wire insulation

Root Cause:

Understanding the Failure Mechanism:

• Insulation on a wire has "plastic characteristics" that under pressure will cold flow and reduce the insulation wall thickness. If there is another wire, a metal connector shell, or other metal components close by, the insulation between wires or between a wire and another metal component will likely breakdown with hipot testing. Hipot testing is a good method to predict shorts that may appear later (a latent defect). The more "soft" the insulation, the higher the susceptibility to this type of failure.
  
  
• If heat is applied to components under compression like when adding heat shrink coverings or soldering to shields, the problem often occurs in PVC insulations too and even in very "hard" insulations such as TFE.

Confirmation of Root Cause:

1. A solid short usually can be isolated with a VOM or low voltage continuity tester. Sometimes this requires cutting the failed wire at a point before and after the compression to prove that the failure occurs at the compression point.
2. If the wire insulation cannot be easily inspected for problems or none are visible you can,   a.) divide and conquer or,    b.) make the problem more solid.
   

• A.  Divide and Conquer: Perform a destructive test by cutting a cable in two and separating the cut wires. Perform a shorts and/or a hipot test on each separate side of the cable. (on Cirris testers you can do a "no connects" shorts/hipot test.) Be sure to use, "apply HV to all pins" with "Single Net Failure" set to "Fails Hipot" or "High Cap Shield" set to NO. Continue this process discovering which end had the problem and at what point the problem is detected or eliminated.
  
• B.  Make the problem more solid: Heat (bake) the cable, (don't exceed the acceptable storage temperature of the cable) to increase the rate of cold flow. A heat gun is less controlled but could also be used. After the application of heat, the problem should be more consistent. Another way to surface this problem is to apply salt water in the area of a compression, (if it is not near any uninsulated areas such as connector contacts). The wire insulation should be impermeable to salt water. Wires that are insulated, including the shield which may not be insulated, should still read off-scale for resistance with a VOM. If the resistance drops then begin the divide and conquer approach with resistance measurements.
  
  After confirming the point of failure you may want to inspect the damage under a microscope by removing the wire(s) from the bundle in the compression area.

General Recommendations:

1. If you are using wire with a soft insulation, do not rely on wire insulation in any compression zone. If the compression cannot be eliminated or appropriately reduced, insulated tubing over the wire might add to the normal insulation.
   
   
2. If you are using heat near wire insulation where wires are bundled together, carefully evaluate the application to avoid insulation damage.