ABB AC 800M

Section 2 Transmission Technology MBP power and line length calculation
3BDS009029R5001 B 37
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:
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
= i
1
*R
L1
or U
RL1
= i
1
* l
1
* r; where r = the resistance coating of the line
For the second station, this appears as follows:
U
RL2
= i
2
* (R
L1
+ R
L2
) or U
RL2
= i
2
* l
2
* r
In general, the equation for Ex applications is then as follows:
– U
RL
= r * Σ (i
x
* l
x
) < U
out
- 9 V
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.
Figure 6. Voltage calculation and line length
Linking
Device
R
L1
R
L2
R
Lx
= Line resistance of line segment x
I
x
= Current consumption of PA station x
i
1
i
2
i
n
> 9 VU
out
l
1
l
2
l
n