NVP incorrectly set in tester - does it matter

Do not get hung up on length, attenuation is the critical factor. Please read the following carefully.

  • The NVP of the cable can also vary in manufacturer by up to 2%. 
  • The NVP setting affects the length measurement and NO other parameter. 
  • The table below gives you an indication of the additional inaccuracy created on a maximum length with the incorrect setting for NVP.

Cable Length vs. NVP

If you have selected a TIA Standard, the length test will not fail until the prescribed length plus ten percent has been exceeded. 
  
If you are measuring a Permanent Link which has a limit of 90 meters, the length test will not fail unless the measured length is greater than 99 meters (90 meters + 10%). 

ANSI/TIA-1152

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 %.
  
 
If you have selected an ISO or EN Standard, length is not required for compliance. You cannot therefore fail on length. 
  
The active equipment is not concerned about the physical length, only that the attenuation, propagation delay and delay skew are within specification. This is why length is not a requirement in ISO/IEC 11801:2002. 

From ISO/IEC 11801:2010:

Transmission parameter b

Testing for

Reference conformance testing

Installation conformance testing

Возвратные потери

Нет

Нет

Вносимые потери

Нет

Нет

Pair-to-pair NEXT

Нет

Нет

PS NEXT

С

С

Pair-to-pair ACR-N

С

Нет

PS ACR

С

С

Pair-to-pair ACR-F

Нет

Нет

PS ACR-F

С

С

Direct current (d.c.) loop resistance

Нет

Нет

Direct current (d.c.) resistance unbalance

Нет

I

Задержка распространения

Нет

Нет

Разница задержек

Нет

Нет

Unbalance attenuation, near-end (TCL)

Нет

I

Unbalance attenuation, far-end (ELTCTL)

Нет

I

Coupling attenuation

Нет

I

PS ANEXT

Нет

NS

PS ANEXTavg

С

С

PS AACR-F

Нет

NS

PS AACR-Favg

С

С

Схема разводки кабелей

Нет

Нет

Continuity of conductors, screens (if applicable), short and open circuits

Нет

Нет

Lengtha

I

I

 

where
C is the calculated value;
I is the informative (optional) testing;
N is the normative (100 %) testing, if not met by design;
NS is the normative (sampled) testing, if not met by design. The sample size to
be tested should be in accordance with ISO/IEC 14763-2.

NOTE The term “met by design” refers to a requirement which may be met by the selection of appropriate materials and installation techniques.

a Length is not a pass/fail criterion.
b Only those parameters specified for each Class of cabling need to be tested, as required in Amendment 1:2008 and Annex A.

 
In Table B.1 above, the type of test to be carried out for each channel or permanent link is indicated by an "I" (informative) or "N" (normative). Parameters, which are computed from the measured parameters, are indicated by a "C". The tests indicated by I may be carried out as part of an acceptance test. The tests indicated by N shall be carried out as part of an acceptance, reference or compliance test. 

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