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Causes of cabling faults for copper patch cord in structured cabling network

For each of the required TIA and ISO structured cabling measurements, you will find troubleshooting tips to help quickly pinpoint the cause of failures when they occur. In some examples, you will find suggested reasons why the measurement does not fail in cases you would expect to see a failure.

Wiremap
Test Result Possible Cause of Result
Open Wires broken by stress at connections
Cables routed to wrong connection
Wire is not punched down properly and does not make contact in the IDC
Damaged connector
Cuts or breaks in cable
Wires connected to wrong pins at connector or punch block
Application-specific cable (e.g. Ethernet using 1,2/3,6 only)
Wires connected to wrong pins at connector or punch block
Wires connected to wrong pins at connector or punch block
Short Improper connector termination
Damaged connector
Conductive material stuck between pins at a connection Damage to cable
Application specific cable (e.g. factory automation)
Reversed pair Wires connected to wrong pins at connector or punch block
Crossed pair Wires connected to wrong pins at connector or punch block
Mix of 568A and 568B wiring standards (1,2 and 3,6 crossed)
Crossover cables used (1,2 and 3,6 crossed)
Split pair Wires connected to wrong pins at connector or punch block

 

Length
Open Possible Cause of Result
Length exceeds limits Cable is too long – check for coiled service loops and remove in this case
NVP (nominal Velocity of Propogation) is set incorrectly
One or more pairs significantly shorter Intermediate break in the cable
NVP is set incorrectly
One or more pairs significantly shorter Damage to cable
Bad connection
Note: Standard practices dictate that the cabling length be defined by the length of the shortest pair. NVP varies per pair, meaning each pair could be reported with a different length. These two conditions can result in a cable with three of four pairs over the length limit, yet the link result is a PASS (e.g. a channel with 101, 99, 103, 102 meters for the four pairs). A PASS in this case is the correct interpretation.

 

Delay/Skew
Open Possible Cause of Result
Exceeds limits Cable is too long – Propagation Delay
Cable uses different insulation materials on different pairs – Delay Skew

 

Insertion Loss (Attenuation)
Open Possible Cause of Result
Exceeds limits Excessive length
Non-twisted or poor quality patch cables
High impedance connections – Use time domain techniques to troubleshoot
Inappropriate cable category – e.g. Cat 3 in a Cat 5e applications
Incorrect autotest selected for cabling under test

 

NEXT and PSNEXT
Open Possible Cause of Result
FAIL or PASS* Poor twisting at connection points
Poorly matched plug and jack (Category 6/Class E applications)
Incorrect link adapter (Cat 5 adapter for Cat 6 links) Poor quality patch cords
Bad connectors
Bad cable
Split pairs
Inappropriate use of couplers
Excessive compression caused by plastic cable ties Excessive noise source adjacent to measurement
Unexpected PASS Knots or kinks do not always cause NEXT failures, especially on good cable and far removed from the ends of the link
Incorrect autotest selected (e.g. “Bad” Cat 6 link tested to Cat 5 limits)
“Fails” at low frequency on NEXT graph but passes overall. When using the ISO/IEC standards, the so called 4dB rule states all NEXT results measured while insertion loss <4dB cannot fail

 

Return Loss
Open Possible Cause of Result
FAIL or PASS* Patch cord impedance not 100 ohms
Patch cord mishandling causes changes in impedance
Installation practices (untwists or kinks of cable – the original twists should be maintained as much as possible for each wire pair)
Excessive amount of cable jammed into the Telecom Outlet box
Bad connector
Cable impedance not uniform
Cable not 100 ohm
Impedance mismatch at junction between patch cable and horizontal cable
Poorly matched plug and jack
Use of 120 ohm cable
Service loops in telecommunications closet Inappropriate autotest selected
Defective link adapter
Unexpected PASS Knots or kinks do not always cause return loss failures, especially on good cable and far removed from the ends of the link
Incorrect autotest selected (easier to pass RL limits)
“FAILs” at low frequency on RL graph but passes overall. Due to the 3 dB rule, whereby all RL results measured while insertion loss <3 dB cannot fail

 

ACR-F and PS ACR-F
(older names: ELFEXT and PSELFEXT)
Open Possible Cause of Result
Possible Cause of Result General rule: troubleshoot NEXT problems first. This normally corrects any ACR-F (ELFEXT) problems
Service loops with many tightly coiled windings

 

Resistance
Open Possible Cause of Result
FAIL or PASS* Excessive cable length
Poor connection due to oxidized contacts
Poor connection due to marginally attached conductors Thinner gauge cable
Incorrect patch cord type
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