Siemens SIMATIC S5-115F - Page 82

Installation Guidelines S5-115F 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.
Figure 3-34. Simplified Representation for Installation with Floating Modules
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3-40 EWA 4NEB 811 6148-02