Banner DF-G1-PR-Q7 Istruzioni per gli Interventi Pagina 216

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C – Cables and Tones
DPN 402197 © TSS (International) Ltd Page 3 of 4
Figure C–1: Tone injection – Short cables
You must connect the tone generator v
f
between the near end of the cable and a
good ground point. At the far end of the cable, you must connect the tone-carrying
conductor to a good ground point to provide an effective signal return path.
C.1.2.2 Long cables
As shown in Figure C–2, the conductors possess some small capacitance to the envi-
ronment that surrounds the cable. If the cable is long (greater than approximately
100km) then the tone signal will find a return path through the distributed capacitance
C
c
of the cable. The impedance of this path reduces as the tone frequency increases.
Figure C–2: Tone injection – Long cables
Under these circumstances, it is not always necessary to make a separate ground
connection at the far end of the cable.However, you will reduce the effects of tone
leakage by connecting the far end of the cable to a good grounding point.
The capacitance of the conductors extends throughout the length of the cable. This
represents a progressive short circuit that means less tone current flows at the far end
of the cable than at the near end. The detection range of the 350 System depends upon
the current flowing at the tone frequency. It follows therefore that the measurement
range of the 350 System decreases with the distance from the point of tone injection.
C.1.2.3 Fibre-optic Cables
In most cases, fibre-optic cables carry at least one conductor to supply power for the
repeaters or to act as a dedicated tone-carrying facility.
Alternatively, when there is no other conductor available, the armoured covering of a
fibre-optic cable can be used to carry the tone, provided it is insulated from ground.
The owner of the fibre-optic cable will usually specify the maximum level of tone cur-
rent that the cable can tolerate. This is to limit the amount of noise that may be gener-
ated within repeaters along the cable.
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