Siemens S5-135U/155U - Manual (Page 47)

3-21
System Manual
C79000-G8576-C199-06
The electronic short-circuit protection of digital output modules only
responds when 3-times the rated current is exceeded. You should therefore so
design the load power supply units that the unit can supply the current
required for shutting down in the event of a short-circuit at one output.
In the event of short-circuits at digital outputs, if the load power supply is not
adequately rated, a current which is higher than the rated current can flow for
a longer period without responding of the electronic short-circuit protection
of the DQ module. Operation in the overload region can destroy the module.
If you use non-floating modules, you must create a common reference
potential for the internal control circuits of the PLC and for the load circuits.
The reference potential of the load power supply should therefore be
connected to the ground terminal of the PLC (PE or protective conductor
terminal). The ground terminal is permanently connected to the internal
reference potential of the controller.
Note
If you use switched-mode power supply units for floating analog modules
and BEROs, you must fit an AC system filter to the input of the power
supply.
3.4.2 Connecting the Programmable Controller and Load Power
Each of the following figures shows a circuit example for connecting the
control and load power supplies as well as the grounding concept for
operation from:
S
Grounded supplies
S
Centrally grounded supplies
S
Ungrounded supplies
When assembling the control system, observe the following explanations.
The text contains identification numbers relating to Figures 3-8 to 3-10.
S
For the programmable controller, sensors and actuators, you must fit a
main switch
À to DIN VDE 0113, Part 1, or an isolating device to DIN
VDE 0100, Part 460. If an AC plug is used as the isolating device, the
socket must be in the vicinity of the central controller and must be easily
accessible (VDE 0805, 1.7.2). These devices are not required in the case
of a subsystem where the appropriate devices are fitted at a higher level.
S
The circuits for sensors and actuators can be given group protection for
short-circuit and/or overload
Á. According to VDE 0100, Part 725,
single-phase protection is required; and all-phase protection to DIN VDE
0113, Part 1.
Ratings of Load
Power Supplies
Load Power
Supply for
Non-Floating
Modules
Load Power
Supply for
Floating Modules
Main Switch and
Protection
Installation Guidelines