Nominal Velocity of Propagation (NVP)

No other parameter is discussed that often as NVP as plays as it is important for testing the correct length of a link and thus for billing the customer correctly on it. Read on to find out what NVP is all about and how it is determined in accordance with the relevant standards.

NVP states the speed of a data signal on a cable. It is noted as the percentage of the speed of light in a vacuum (symbol “c“) or as the corresponding decimal number. The speed of light in a vacuum is about 300.000 km/s or 300.000.000 m/s. An NVP of “78 % c“ or “0,78 c“ means 78 % of 300.000.000 m/s or 300.000.000 m/s times 0.78 respectively, which is about 234.000.000 m/s.

NVP is not a test parameter but a material property. The field tester send a pulse down the link and measures the time until it comes back. Then it calculates the link length according to the well-known formula “length = speed • time“.

NVP is selected at the tester or typed into it. Only when the NVP is correct the tester can determine the link length correctly. To give an example: When 0.77 instead of 0.78 is typed in, the length of a 90 m link is calculated as 88.8 m. In a project with 500 duble outlets (1,000 links), a mistake of just 1 m per link leads to a total mistake of more than 1 km! The larger the difference between the NVP that is typed in and the correct NVP is, the larger the total mistake becomes. This is particularly important when the link lenghts determined by the tester are used to bill the customer for the cable installed.

There are several possibilities to selecte the proper NVP of a cable:

• The data of the installed cable is in the data base of the field tester and can be selected before testing. However it is recommended to double-check with the data sheet of the cable manufacturer and change it if necessary.
• In case the cable is not in the field tester’s data base, the NVP can be typed in manually using the data sheet of the cable manufacturer