Making Measurements
LT51 User Manual
61
In a vacuum, electricity, like light, travels at about 300,000 km/second. In cables,
it travels slower than this and we refer to the speed as a % of the speed of light.
This is usually expressed as the Propagation Velocity Factor or PVF. You may
also see it referred to as Velocity of Propagation or VOP. Typical PVFs for
electric cables are from 50 to 68 (50 % to 68 % of the speed of light). Water in
cable will increase the value of PVF.
If the PVF is unknown, you can start with a typical value for that type of cable. To
obtain a more accurate value, test a known length cable of the same type with a
discontinuity at the end (either S/C or O/C). Adjust the PVF until the cursor,
which is placed at the beginning of the reflection due to the discontinuity at the
end of the cable, reads the known length. Another possible method, involves
measuring the fault from both ends of the cable, add the distances together and
either adjust the PVF or use a ratio calculation to find the position.
Some commonly used cables and there PVFs are listed below:
Cable
PVF (%)
Coaxial - 50
, RG58C/U (PE), RG178/U (PTFE)
69
Coaxial - 75
, RG59B/U (PE), RG179/U (PTFE)
67
Coaxial - 50
, LMR-200,HDF-200,CFD-200 (PF)
83
Coaxial
– 92
, RG-62/U (PF)
84
UTP
– Polyethylene (PE)
67
UTP
– Air PIC, 19 AWG / 0.90mm
69
UTP
– Air PIC, 22 AWG / 0.64mm
68
UTP
– Air PIC, 24 AWG / 0.50mm
67
UTP
– Air PIC, 26 AWG / 0.40mm
66
UTP
– PTFE (Teflon)
71
UTP
– Pulp, 19 AWG / 0.90mm
70
UTP
– Pulp, 22 AWG / 0.64mm
69
UTP
– Pulp, 24 AWG / 0.50mm
68
UTP
– Pulp, 26 AWG / 0.40mm
67
UTP
– Jelly PIC, 19 AWG / 0.90mm
66
UTP
– Jelly PIC, 22 AWG / 0.64mm
65
UTP
– Jelly PIC, 24 AWG / 0.50mm
64
UTP
– Jelly PIC, 26 AWG / 0.40mm
63
FTP
– Air PIC, 24 AWG / 0.50mm
78
Power
– Cross Linked Polyethylene (XLPE)
52
– 58
Power
– Paper Oil Filled (PILC)
49 - 54
Table 6.3: PVF for common cable types
where: UTP
– Unshielded Twisted Pair, FTP – Foil-screened Twisted Pair
PIC
– Plastic Insulated Cable
Pulp
– Paper slurry filled cable
PE
– solid polyethylene, PF - polyethylene foam
PTFE - polytetrafluoroethylene
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