Siemens S5-135U/155U System Manual page 339

Two-Wire Switches
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-.
Common Diode
only on 453/457
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
System Manual
C79000-G8576-C199-06
DC Voltage
L+
Contact, Switch
Digital Output
Digital Input/Output Modules
AC Voltage
L
Digital Output
ELR
ELR ELR
N
8-27