10% Rule for Cable Length - DTX CableAnalyzer

Did the length measured exceed the limit?

Example 1:

(The display here is from DTX Version 2.74, have you updated your DTX CableAnalyzer?) 

Surely this cannot be right? The length has exceeded the limit of 90.0 m.
 
From the industry approved cabling standard ANSI/TIA-1152, we find: 
  
4.2.3.1 Physical length vs. electrical length
The pass or fail criteria is based on the maximum length allowed for the channel or permanent link as specified in ANSI/TIA-568-C.2 plus the nominal velocity of propagation (NVP) uncertainty of 10 %.
  
This means that although the limit is 90.0 m (295 ft), the tester will not fail the length parameter until the shortest pair of the four pairs has exceeded 90 m + 10 % (99 m) or 295 ft + 10% (325 ft). In the example above, the shortest pair was 92.5 meters, less than the 90 m + 10 % permitted by this standard. 
 
Many customers do not like/agree with this feature. However, your DTX CableAnalyzer is standards based. Whilst Fluke Networks may not necessarily agree with the 10 % fudge rule, the industry agreed to this. We implement the standard as is. THIS IS NOT AN EXCUSE TO GO BEYOND 90 m!!!!!!!!!!! If the cable is marginally compliant, insertion loss will fail before length fails.
 
This rule applies to channel limits as well where the length limit is 100 m (328 ft). However, because of clause 4.2.3.1 in the standard, the length test will not fail until the shortest pair of the four pairs measured exceeds 100 m + 10 % (110 m) or 328 ft + 10% (361 ft).
 
It is important to understand that we used the shortest pair to report the length of the link. Again, this is detailed in clause 4.2.3.1 of ANSI/TIA-1152.

When physical length is determined from electrical length, the physical length of the link calculated using the pair with the shortest electrical delay shall be reported and used for making the pass or fail decision. 

Example 2:

(The display here is from DTX Version 2.74, have you updated your DTX CableAnalyzer?) 

Here we see that the shortest pair is 7,8 with a length of 97.9 m and hence a PASS since it does not exceed the 90 m limit by more than 10%. The other two pairs did exceed 90 m plus 10% and are marked I for Information Only but are not used for evaluating the link. 

Example 3:

If the pair with the shortest length exceeds 90 m plus 10%, then length is reported as FAIL (shown below).