Pepperl+Fuchs KLD2-PR-Ex1.IEC. Manual page 17

Power repeater / power supply according to IEC 61158-2
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:
Power
Repeater
The case in question is an Ex application. The result of the current calculation is that
a maximum direct current including I
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:
for example, U
V to 13.4). For the worst case consideration, U
in U of 3.8 V.
Lmax
Since all field devices are connected to the end of the lead, the maximum line resis-
tance is 38 Ω (U
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
Subject to reasonable modifications due to technical advances.
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R
i
L
max
U
L
U
out
R = Lead resistor of lead segment x
L
I = current consumption of PA field device x
x
U
Lmax
for the power repeater KLD2-PR-Ex1.IEC1 is at least 12.8 V (12.8
out
/i ). This corresponds to a line length of 863 m for a resistor coat-
L max
l
2
l
1
R
L1
U
off
R = Lead resistor of lead segment x
Lx
I = current consumption of PA field device x
x
FOUNDATION FieldbusGeneral
i
i
1
2
> 9 V
of 100 mA is flowing. Cable type A is used,
FDE
= U - 9 V
out
must be set to 12.8 V. This results
out
l
n
R
L2
i
i
2
1
Copyright Pepperl+Fuchs, Printed in Germany
i
n
i
n
> 9 V
15