Siemens SIMATIC S7 Functional Safety Manual page 102

Fail-Safe CPU and signal module (SM) installation
4.3 Control system wiring guidelines
A well-controlled turn-off transient will have a ring frequency of no more than 10KHz, with
less than 1KHz preferred. Peak voltage for AC lines should be within +/- 1200V of ground.
The data sheets list voltage thresholds for the internal suppression circuits on DC outputs.
(Refer to Appendix A: "Technical specifications" for further information.) External
suppression should limit the transient to less than this threshold to ensure that the internal
suppression circuit does not attempt to suppress an excessive load. Placing an external
suppressor circuit so that it is electrically across the load and physically located near the load
is the most effective way to reduce electrical noise.
An external suppression circuit connected directly across the load also avoids the hazard of
energizing the load if suppression circuit components fail short.
Note
The effectiveness of a suppressor circuit depends on the application and must be verified for
your particular usage. Ensure that all components are correctly rated and use an
oscilloscope to observe the turn-off transient.
Typical suppressor circuit for DC or relay outputs that switch DC inductive loads
①
②
③
④
102
1N4001 diode or equivalent
8.2 V Zener (DC outputs),
36 V Zener (Relay outputs)
Output channel F-DQ (P-switch)
or standard DQ output
Output channel F-DQ (M-switch)
or standard DQ common return
In most applications, the addition of a diode (A)
across a DC inductive load is suitable, but if your
application requires faster turn-off times, then the
addition of a Zener diode (B) is recommended. Be
sure to size your Zener diode properly for the
amount of current in your output circuit.
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA