ABB AC 800M User Manual page 37

Section 2 Transmission Technology
The maximum line length of the PROFIBUS PA segment is calculated as follow:
Max. line length = 38 Ohm / 44 (Ohm/km) = 0.863 km
Max. line length = 863 m
Since in practice the stations are not all connected compactly at the end of the
transmission line, the resulting structure is as follows:
Figure 6. Voltage calculation and line length
Each station causes a voltage drop on the length segment through which its power
supply current flows. For the first station, this would be:
U
RL1
For the second station, this appears as follows:
U
RL2
In general, the equation for Ex applications is then as follows:
If the condition described above is not fulfilled,
a) the line has be shortened or
b) a line with reduced resistance coating has to be used.
3BDS009029R5001 B
l
1
Linking
Device
U
out
R
I
x
= i *R or U = i
1 L1 RL1
= i * (R + R ) or U
2 L1 L2
= r * Σ (i
U * l
–
RL x
MBP power and line length calculation
l
n
l
2
R
R
L2
L1
i
1
= Line resistance of line segment x
Lx
= Current consumption of PA station x
* l * r; where r = the resistance coating of the line
1 1
= i * l * r
RL2 2 2
) < U - 9 V
x out
i
i
n
2
> 9 V
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