Short-Circuit Protection And Fusing - Siemens S5-155U System Manual

e.g. 430 DI 0 V e.g. 431
DI
L _
L I I L _ I L _ L _
I I
+
+
AC
230V
_
DC
24V
Figure 8-13 Feeding of Isolated Input/Output Modules from Two Power Supply Units
8.2.6
Short-Circuit Protection and
Fusing
System Manual
C79000-B8576-C199-03
With isolated input/output modules the supply voltage is applied
separately to the individual modules.
e.g. 451 DQ
0 V
L Q Q Q L Q Q
L _ L _ L _
I I
+ +
_
1 L
1 L +
2 L +
_
2 L
+
AC
230V
_
DC
24V
With isolated modules, the inputs or outputs can be fed from two
separate power supply units in isolation groups created by the internal
distribution in the module.
Note that when inputs or outputs from two isolated groups are
connected to one power supply unit, isolation between the groups is
defeated.
With digital output modules for DC voltage, fuses are provided on the
module, in addition to the electronic short-circuit protection, for the
wiring cables and to protect the module. The fuses also provide
protection against reversal of supply voltage connections and can only
be replaced at the factory.
The electronic short-circuit protection guaranteed in the technical
specifications applies to a resistance which is lower than the specified,
maximum permissible line resistance.
In the event of a short-circuit, two to three-times the rated output
current flows briefly at the output before the switched electronic
short-circuit protection becomes effective. When selecting the load
power supply units, therefore, allow for the increased short-circuit
current as well as all connected output loads (observe the coincidence
factor). With unregulated load power supplies, this overcurrent is
generally ensured. With regulated load power supplies, especially at
low output power levels (up to 20 A), you must allow for a
corresponding overcurrent.
Installation and Startup
Reference Potential
0 V e.g. 453 DQ 0 V
Q
L L Q L Q L Q
+ + + +
and Con-
nection
to Protec-
tive Con-
ductor (PE)
8
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