Siemens SIMATIC S5-115U User Manual page 107

Simatic s5 series cpu 941-7ub11 cpu 942-7ub11 cpu 943-7ub11 and cpu 943-7ub21 cpu 944-7ub11 and cpu 944-7ub21
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S5-115U Manual
Note
It is imperative that you connect the reference potential of the load power supply unit
with the L-terminal of the module in the case of 24 V DC DQ modules. If this
connection is missing (e.g. wirebreak), a current of typically 15 mA can flow at the
outputs. This output current can be sufficient to ensure that
Energized contactors do not drop out
and
High-resistance loads (e.g. miniature relays) can be driven.
Installation with floating modules
Control circuit and load circuit are galvanically isolated in the case of floating modules.
Installation with floating modules is necessary in the following cases:
All AC load circuits
and
Non-connectable DC load circuits.
The reasons for this are, e.g. different reference potentials of the sensors or the grounding of
the plus poles of a battery, ...
Figure 3-34 shows the simplified representation of an installation with floating modules. The
installation is independent of the grounding concept. The connections for grounding measures
are therefore not shown.
PS
1L+
-
1L
PE
24 V DC control power
supply
+
2L
-
2L
24 V DC load power
supply
Figure 3-34. Simplified Representation for Installation with Floating Modules
EWA 4NEB 811 6130-02b
CPU
DI
Installation Guidelines
U int
Data
M int
DQ
L1
N
230 V AC load power
supply
3-41

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