Permanent Link Passes but CAT6A Channel Fails
An installer is terminating a 48-port CAT6A shielded patch panel in a new office. After termination, the copper tester shows that all 48 permanent links pass the CAT6A permanent-link test. When 10 m factory patch cords are added at each end and the channels are retested, six channels fail insertion loss and near-end crosstalk on pair 3,6. Link lengths are 55 m to 70 m. The installer used a utility knife to strip about 50 mm of jacket, untwisted the pairs about 25 mm at the jack, used CAT6 patch cords instead of CAT6A, and folded the foil shield back without connecting the drain wire. Shield continuity fails on 10 links. The contractor argues that the permanent-link result proves the cable is fine and that the channel failures must be tester error.
1. Why can the permanent links pass while the channels fail?
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A permanent-link test measures only the fixed cabling from the patch panel to the outlet; it excludes the equipment cords. A channel test includes the patch cords, jacks, and everything in the end-to-end path. The six channels fail because the added cords and the poor terminations contribute insertion loss and near-end crosstalk. The 10 m cords do not violate the 100 m channel limit, but the CAT6 patch cords are not built to CAT6A performance, and 25 mm of untwisted pair at the jack is far beyond the 13 mm maximum for CAT6A. The permanent-link pass does not prove the channel is good; the channel failure is the correct result for the installed system.
2. What termination defects should be corrected, and how should they be retested?
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Re-terminate the failed ends. Strip only enough jacket to reach the jack, keep the pair twists as close to the termination as possible, and untwist no more than about 13 mm for CAT6A. Use CAT6A-rated jacks and maintain the pair geometry shown by the manufacturer. Replace the CAT6 patch cords with CAT6A cords of the correct length. Re-run both permanent-link and channel tests. The 25 mm untwist and wrong cord category are enough to explain NEXT and insertion-loss failures; a tester error is unlikely when the failures repeat on pair 3,6. Check all 48 ports for the same workmanship issues, not only the six failed channels.
3. What is required for the shield continuity failures on the shielded system?
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Shielded cabling needs the foil/drain wire bonded to the grounded patch panel or jack at both ends so the shield is continuous and grounded through the telecommunications bonding system. Folding the foil back leaves an open shield, which is why continuity fails on 10 links. Terminate the drain wire per the manufacturer, use shielded jacks and shielded patch cords, and verify shield continuity with the tester. Do not ground only one end unless the manufacturer specifically requires it; an open or one-end-only shield can act as an antenna and may worsen noise. After correction, retest shield continuity and the channel performance.