Length

19.02.2024 | Legth testing of a link serves two purposes: To make sure the length of the cable does not exceed the limits specified by the standards and to use the measured length to bill the customer for the cable installed. There are a few points to be minded to make sure the tested lenght is accurate. Read on to find out about them.

To calculate the length of a cable, a field tester typically sends an electric pulse down the link and measures the time it takes until the pulse comes back to it. It then calculates the length using the well-known formula “length = velocity • time“. The velocity is determined by the NVP which has to be selected correctly. To give an example: Should 0.77 be selected instead of 0.78, the length of a 90 m cable is calculated to just 88.8 m resulting in an error of 1.2 m for this link. Knowing the impact of a slightly wrong NVP resulting in a wrong length is mandatory when it comes to using the test results for billing a customer on the installed cable.

IEC 61935-1:2009 specifies a resolution of the test results of 0.1 m but recognizes a tolerance of ±(1 m + 4 %). High-quality field testers have a much smaller tolerance, e.g. ±0.5 m.

Due to the different twisting, the four wire pairs in a twisted pair cable have different lengths. In case the NVP is not in the data base of the field tester, it has to be determined manually (refer to the previous issue for details). IEC 61935-1:2009 and TIA-1152-A specifiy that the shortest wire pair has to be used for determining the NVP. For testing the length of the cable, this pair (e.g. brown-white) always has to be used then.

The wire pairs of a cable have different lengths due to the different twist. ISO/IEC 11801-1:2017 recognizes a maximum of 90 m for the permanent link and 100 m for the channel. However, it is not a pass/fail criteria according to this standard but is a pass/fail criteria according to the American standards TIA-1152-A.

Even though length is not a pass/fail criteria accoring to ISO/IEC 11801-1:2017, it has to be minded that long cable lengths lead to high insertion loss which in turn might exceed the specified values.

Trouble-shooting tips if lenth tests fail

Typical cause of the problem	This might help (without engagement)
Wrong NVP selected	Select proper NVP
Field tester not calibrated	Calibrate field tester and test agein
Cable slack	Eliminate slack by cutting the cable, re-route the cable using a shorter cable pathway
Link in total too long	Cut the horizontal cable if possible, re-route the cable using a shorter cable pathway, use shorter patch cords in case there is a consolidation point installed: use a shorter consolidation point cable

Typical causes of length problems and what might help to solve them.

Author:

Dirk Traeger

Technical Solutions Manager DataVoice