Siemens SIMATIC S5-90U System Manual page 79

S5-90U/S5-95U
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 digital output 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 or relays 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 3-30 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 3-30. Simplified Representation for Installation with Floating Modules
EWA 4NEB 812 6115-02b
CPU DI
Installation Guidelines
U int
Data
M int
DO
L1
N
230 V AC load power
supply
3-31