Short-Circuit Protection And Fusing - Siemens S5-135U/155U System Manual

Digital Input/Output Modules
With isolated input/output modules the supply voltage is applied separately
to the individual modules.
e.g.430 DI 0 V
e.g.431
DI
L _ I L _ I L _
I
L I I L _
+
+
AC
230V
_
DC
24V
Figure 8-13 Feeding of Isolated Input/Output Modules from Two Power Supply Units
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.
8.2.6

Short-Circuit Protection and Fusing

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.
8-24
e.g.451 DQ
0 V
L _ I L _ L Q Q Q L _ L Q Q
I
+ +
1L _
1L+
2L+
_
2L
+
AC
230V
_
DC
24V
Reference Potential
0 V
0 V
e.g.453 DQ
and Connection
L Q L Q L Q L Q
to Protective
+ + + +
Conductor (PE)
System Manual
C79000-G8576-C199-06