Siemens S5-135U/155U - Manual (Page 53)

3-27
System Manual
C79000-G8576-C199-06
Note
For 24 V DC digital output modules with electronic short-circuit protection,
you must ensure that the reference potential of the load power supply is
connected to terminal L- of the module. If this connection is missing (e.g.
open-circuit), a typical current of 15 mA can flow at the outputs. This output
circuit is sufficient
S
to prevent energized contactors or relays from being released
S
and to energize high-resistance loads (e.g. miniature relays).
With floating modules the control circuit and load circuit are metallically
isolated.
An arrangement with floating modules is required
S
for all AC load circuits and
S
for DC load circuits which cannot be coupled. The reasons are, for
example, different reference potentials of the sensors or grounding of the
positive terminal of a battery.
Shown in the following figure is a simplified installation with floating
modules. The arrangement is independent of the grounding concept. The
connections for grounding are therefore not drawn.
1L+
1L–
PE
24 V DC Control Power Supply
2L+
2L–
24 V DC Load Power Supply
PS CPU DI DQ
Data
0 V
U
int
L1
N
230 V AC Load Power Supply
Figure 3-12 Simplified Representation of an Arrangement with Floating Modules
Installation with
Floating Modules
Installation Guidelines