34 en | Planning Modular Fire Panel
F.01U.028.089 | 8.0 | 2011.07 System Description Bosch Sicherheitssysteme GmbH
3.6 Achievable Cable Length with LSN 0300 A
The usable cable length depends essentially on the resistance of the cable and the quantity
and current consumption of the network elements. The way in which the elements are
distributed locally on the bus also makes a difference.
The possible cable length can be estimated using the formula below (1). This formula is also
used in the FSD (Fire System Designer) calculation program and is based on the critical
application situation where all network elements are concentrated at the end of the bus line.
The calculated cable lengths are therefore on the conservative side. The diagrams on Page 35
and Page 36 can be used to make a quick estimate.
The following applies:
L = cable length to be calculated in [m]
U
0
= bus voltage at the connection terminals = 30 volt
U
End
= bus end voltage = 15 volt (must not fall below this limit!)
q = cable cross section = 0.503 mm
2
(for cable ∅ 0.8 mm)
n
E
= number of elements
ρ= specific resistance of copper = 0.0178 Ω mm
2
/m)
R
FET
= FET resistance = 0.7 Ω (LSN classic) or 0.35 Ω (LSN improved),
I
NE
= average current consumption of elements in [A]
I
trans
= transmission current = 0.012 A
I
flash
= flash current = 0.018 A
For technical reasons the cable length must not exceed 1600 m. Note that the non-isolated
cables for network elements must not exceed a total length of 500 m and must be included in
the maximum cable length of 1600 m.
The 500 m limit applies to the following interface modules in the FLM-420 series (with non-
isolated inputs).
FLM-420-RHV, FLM-420-I2, FLM-420-I8R2, FLM-420-O8I2, FLM-420-O1I1, FLM-420-RLE.
Using the Diagram for LSN classic
FET resistance = 0.7 Ω, fire alarm cable with ∅ 0.8 mm
Example 1:
You want to know the maximum cable length for a given number of network elements and a
given current consumption (n
E
=50, I
NE
= 3 mA). On the X axis, go vertically up at 50 until you
reach the 3 mA curve. Then, from the point of intersection go horizontally to the left to the Y
axis and read off the maximum achievable cable length. In this example it is 840 m.
Example 2:
You want to know the maximum number of network elements for a given cable length and an
average current consumption of the network elements (L = 1000 m, I
NE
= 20 mA). Starting
from the Y axis at 1000, go horizontally across to the right as far as the 20 mA curve and then
from the point of intersection vertically down to the X axis. Read off the maximum number of
network elements, in this example it is 8.
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