Siemens FS20 - Page 72

Planning detector lin
es
Planning an FDnet detector line
7
72 | 242
Building Technologies
008843_v_en_
--
Fire Safety
2015
-
12
-
15
Defining and calculating the cable resistance (R)
The cable resistance R is the resistance value over the complete wire length. This
corresponds to the resistance value for both cable conductor strings.
R = l * R'
R Cable resistance [Ω]
l Cable length (both conductor strings) [km]
R' Cable resistance per km [Ω/km]
If the cable resistance per km (R') is not known, it can be calculated with the
following formula:
R' = ρ * 2000 / A
ρ (rho)
Specific electric resistance for copper
(constant: 0.0178 Ωmm/m)
2000 2 km in m (both conductors for 1 km cable)
A Cable cross-section [mm
2
]
The cross-section A can be determined from the wire diameter with the following
formula:
A = π * (d / 2)
2
π Constant (3.1416)
d Diameter of the wire [mm]
Calculation example for R' of a copper cable with Ø 0.8 mm
R' = (0.0178 Ωmm/m * 2000) / ((0.8 mm / 2)
2
* 3.1416) = 70.8 Ω/km