Siemens SIRIUS 3RT Series System Manual page 176

SIRIUS 3RT2 contactors/contactor assemblies
2.7 Accessories
RC circuit with suppression diode
Including a diode in an RC circuit ensures that switching overvoltages will no longer occur;
the diode limits the voltage to 0.7 V.
Note
However, diodes do cause the switch-off delay (the OFF time) to become 6 to 9 times
longer. This characteristic can be turned to the user's advantage if short-time voltage dips in
the range of a few milliseconds need to be bridged, for example. From a technical point of
view, it only makes sense to use freewheel diodes up to a power of 5.5 kW. For higher
power ratings we recommend an RC circuit with a diode combination.
Figure 2-30
Advantage: No overvoltages occur during the switch-off process. The diode blocks at 0.6 V.
Disadvantage: The diode can only be used for DC operation. The contactor's break time is
extended considerably, amounting to 6 to 9 times the switch-off delay. This longer break time
can be used for control purposes if required, e.g. to bridge short-time voltage dips.
Zener diodes (diode combination) can be used for shorter break times, which will then
equate to 2 to 6 times the switch-off delay.
176
RC circuit with diode (DC operation)
SIRIUS Innovations
System Manual, 01/2011, A8E56203870002-03