Wiring the S7-400
4-4 AutomationSystem S7-400 Hardwareand Installation
A5E00850741-01
4.3 Choosing the Load Current Power Supply

Choosing the Load Current Power Supply

The input and output circuits (load current circuits)as w ellas sensors and
actuators are powered by the load current power supply.Listed in the following are
the characteristics of the load current power supplies required in special
applications for choosing the load current power supplies.
Characteristics of
theLoad Current
PowerSupply
Requiredfor ... Remarks
Safe isolation Modules which must be
supplied with voltages
≤60 VDC
or ≤25 VAC.
The Siemens SITOPpower load current
supplies have this characteristic.
24 VDC load circuits
Tolerancesof the
output voltages:
20.4V to28.8 V
40.8V to57.6 V
51Vto72V
24 VDC load circuits
48 VDC load circuits
60 VDC load circuits
If the output voltage tolerances are
exceeded, you should provide an energy
storage capacitor.Rating: 200 μF per1 A
load current (with bridge rectification).

Load Current Power Supplies

The DC load current power supply must meet the following requirements:
Only a safe, isolated extra-low voltage of ≤60 VDC may be used as the load
current supply.Isolation may be implemented according to the requirements of the
following, amongst other publications:
VDE 0100-410 / HD 384-4-41 S2 / IEC 60364-4-41
(as a functional extra-low voltage with isolation) or
VDE 0805 / EN 60950 / IEC 60950
(as a safety extra-low voltage SELV)or VDE 0106 Part 101.

Determining the Load Current

The required output load current is determined by the accumulated current of all
sensors and actuators connected to the outputs.
IN the case of a short-circuit, a current of two to three times the rated output
current flows briefly at DC outputs before the switched electronic short-circuit
protection becomes effective. When selecting the load currentpower supply,
therefore, you must ensure that the increased short-circuit current is available. With
unregulated load current power supplies, this excess current is generally ensured.
With regulated load current power supplies, particularly at low output levels (up to
20 A), you must ensure a suitable excess current.