Power repeater / power supply according to IEC 61158-2
FOUNDATION FieldbusGeneral
Subject to reasonable modifications due to technical advances.
Copyright Fuchs, Printed in Germany
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15
Voltage calculation and line length
The distribution of field devices on the field bus segment can have a negative effect
on the maximum possible line length under certain circumstances. The following ex-
ample is intended to clarify this point:
The case in question is an Ex application. The result of the current calculation is that
a maximum direct current including I
FDE
of 100 mA is flowing. Cable type A is used,
with a resistor coating of 44
Ω
/km.
A requirement for problem-free functioning of a field device is that the input voltage
on the bus line be at least 9 V. The following therefore applies to the maximum voltage
drop over the lead:
U
Lmax
= U
out
- 9 V
for example, U
out
for the power repeater KLD2-PR-Ex1.IEC1 is at least 12.8 V (12.8
V to 13.4). For the worst case consideration, U
out
must be set to 12.8 V. This results
in U
Lmax
of 3.8 V.
Since all field devices are connected to the end of the lead, the maximum line resis-
tance is 38
Ω
(U
L
/i
max
). This corresponds to a line length of 863 m for a resistor coat-
ing of 44
Ω
/km.
Since in practice the field devices are not all connected compactly at the end of the
transmission line, the resulting structure is as follows:
Power
Repeater
R
L
R
L
= Lead resistor of lead segment x
I
x
= current consumption of PA field device x
i
1
i
2
i
n
> 9 V
U
L
U
out
i
max
Power
Repeater
R
L1
R
L2
R
Lx
= Lead resistor of lead segment x
I
x
= current consumption of PA field device x
i
1
i
2
i
n
> 9 V
U
off
l
1
l
2
l
n
