As you have discovered, there are no limit lines in either the DTX or DSP to support patch cord testing with lengths greater than 20 m. This is because neither TIA or ISO support user cords in excess of 20 m. From ANSI/TIA-568-C.1 in section 7.3 it states; "The length of the cross-connect jumpers and patch cords in the MC or IC should not exceed 20 m (66 ft)." This is also stated in ISO/IEC 11801:2002; "....the length of the work area cord should not exceed 20 m"
The limit for NEXT in patch cord testing is length dependent. It relaxes as the patch length is increased. Since we do not allow users to adjust the NEXT limit in any of the tests to preserve data integrity, in theory is not possible to create a > 20 m patch cord custom test limit.
That said, the difference in the NEXT limit after 20 m is not that significant and Return Loss is fixed. You could create a custom limit based on the "TIA Patch Cord Cat6 20m" test limit and call it "XX m Patch Cord Test". Then ensure all links pass by at least 0.2 dB for NEXT.
- For Category 5e, the change in the limit line for NEXT between 20 m and 70 m is < 0.1 dB @ 100 MHz.
- For Category 6, the change in the limit line for NEXT between 20 m and 70 m is < 0.2 dB @ 250 MHz.
Are you using this cord to connect two pieces of equipment?
What you are doing is outside of the usual cabling standards but will probably work just fine. If you are using the cable to connect two pieces of equipment together directly, you could argue this is a "Channel". In which case you can use the Channel adapters and make the measurement using the TIA Cat6 Channel as a test limit. Normally this would not be recommended because we are not testing how good the RJ45 terminations are. The Channel definition excludes them. But since the DTX does a quick check on those RJ45 terminations, a Channel may be the most appropriate choice.
If you argue for doing the patch cord test, you could fail the patch cord limit, which is much tighter than the Channel limits - yet the application would still work. There is no point in raising the test limit if the application does not need it.
This article is informational only. Please look to your cabling contract for test requirements.