Siemens S5-155U System Manual page 328

Installation and Startup
Two-wire switches
Common Diode,
only on 453/457
Digital Output
Contactor,
Solenoid Valve, etc.
L-
Quenching Device
(Diode, Zener Diode, R/C Network or Varistor)
Figure 8-15 Quenching of Inductive Loads when Switching the Supply Line L+ and L.
8 - 30
With two-wire switches and AC voltage outputs:
Disconnection of the supply voltage during operation is not
permissible without additional protective circuitry. Quenching of the
inductive load is achieved here via the load power supply unit or
system. Since, as two-wire switches, the outputs have no L-/N (0 V
ground) terminal, the inductive load cannot be quenched on the
module when the supply line is disconnected.
With the 453 and 457 modules using two-wire switches, the
overvoltages can result in destruction of the module.
If the load voltage must be switched for safety reasons, you must
provide external quenching circuitry at each inductive load (see
Figure 8-15).
Instead, with the 453 and 457 modules using two-wire switches, you
can insert a quenching device (diode) with adequate ratings after
contact K from the L+ line to L-.
DC Voltage
L+
Contact, Switch
AC Voltage
L
Digital Output
ELR
ELR
System Manual
C79000-B8576-C199-03
ELR
N