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10.2 Appendix "B" – Rules about cables
The following rules must be obeyed when using long cables in the DREAM system:
Never mix in the same cable two lines of separate 2w channels.
Never mix in the same cable a 2 wired channel and an RS485 communication line.
Always maintain a distance of at least 20 cm between the cables of different 2W channels and
RS485 when laid in the ground.
For laying cable in the ground always used double-coated NYY cables.
The recommended cable thickness is 1.5 mm2.
The resistance of the cable should be reasonably low, with no leakage between the wires and
between each wire to the ground. (see testing procedure below).
10.2.1 CABLE RESISTANCE TESTING
The cable must be tested for continuity of its wires, and for having good isolation between the
wires and between each wire and the ground.
Disconnect both ends of the cable under test (including from any RTU in the middle) and make
sure the wires are not touching each other.
Check the resistance between the wires. Use the highest range available on the ohmmeter (tens
or hundreds of KΩ). The resistance should be infinite or at least not lower than 1 MΩ.
Check the resistance between each wire to the ground. Use the highest range available on the
ohmmeter (tens or hundreds of KΩ). The resistance should be infinite or at least not lower than
1 MΩ.
Make a short circuit between the wires at one end of the cable and test the resistance between
the wires at the other end. This time use the lowest range of your ohmmeter (tens or hundreds
of Ω). The resistance between a pair of wires increases with the length of the wires and
decreases with their thickness. For 1 Km distance and with a pair of wires with 1.5 mm2 cross
section the resistance should be about 22Ω.
The formula for calculating the expected resistance of a pair of copper wires is the following:
Resistance (in Ω) = 0.017 x Length (in meters)
Cross section (in mm2)
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