Siemens S5-135U/155U - Manual (Page 337)

8-25
System Manual
C79000-G8576-C199-06
8.2.7 Arc-Quenching for Inductive Loads
Note
Digital output modules have integrated circuits on the module to quench
inductive loads (refer to the technical specifications of the output modules).
Exception: The 458 digital output module allows the switching of inductive
loads with contact protection submodules.
The integrated circuits on the modules for quenching inductive loads are
disabled if the load circuits are interrupted, either
S
operationally by contacts and switches,
S
or by fuses in the event of a fault.
This can result in excessive, inductive breaking voltages which put fault-free
operation at risk.
You can prevent this with additional, external quenching circuitry for the
inductive loads.
L– N
Digital Output Digital Output
AC Voltage
DC Voltage
L+ L
Contact, Switch
Contactor,
Solenoid Valve, etc.
Quenching Device
(Diode, Zener Diode, R/C Network or Varistor)
ELR ELR ELR
Figure 8-14 Quenching of Inductive Loads when the Load Circuit is Opened
Digital Input/Output Modules