Modular Fire Panel Installation/Configuration Notes | en 59
Bosch Sicherheitssysteme GmbH Product Information F.01U.028.089 | 5.4 | 2009.08
3.4 Achievable cable length with the LSN 1500 A
The usable cable length depends essentially on the resistance of the cable and the number
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 (1). This formula is also used in the
FSD (Fire System Designer) calculation program and assumes the critical application in which
all network elements are concentrated at the end of the bus line. As a result, the calculated
cable lengths are on the safe side.
The diagrams on pages Page 60 and Page 61 can be used to make a quick estimate.
The following applies:
L = the cable length in [m] that is to be calculated,
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 Ohm mm
2
/m),
R
FET
= FET resistance = 0.7 Ohm (classic LSN element) or 0.35 Ohm (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 3000 m. It should be remembered
that the non-electrically isolated lines running from network elements to, for example, all
FLM-420 series interface modules with optically unisolated inputs must not exceed a total of
500 m (1640 ft) and must be included within the maximum cable length of 3000 m (9842 ft).
Using the diagram for LSN improved
FET resistor = 0.35 Ohm, fire detector 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
= 120, IN
E
= 0.5 mA). On the X axis, go vertically up at 120 as
far as the 0.5 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 1950 m.
Example 2:
You want to know the maximum number of network elements for a given cable length (L =
1000 m, I
NE
= 2 mA). Starting from 1000 on the Y axis, go horizontally across to the right as far
as the 2 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 73.
q
U
0
− U
END
− n
E
× R
FET
n
E
− 1
⁄
2 × I
NE
+ I
trans
+ I
flash
2 Ó (n
× I
+ I
trans
+ I
=
(1
flash
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