Application 6: Wiring A Directly-Connected Sil-Rated Actuator; Application 7: Wiring A Directly-Connected Sil-Rated Actuator, Switching Both Load Conductors - Siemens SIMATIC S7 Functional Safety Manual

Fail-Safe signal module (SM) applications
3.2 Digital output applications
3.2.7

Application 6: Wiring a directly-connected SIL-rated actuator

A short-circuit from 1L to A or equivalent fault can lead immediately to a dangerous failure.
You must prevent this failure mode by proper separationand protection of conductors.
①
3.2.8 Application 7: Wiring a directly-connected SIL-rated actuator, switching both
load conductors
By switching both conductors to the load, this arrangement provides extra protection for
faults to power or ground in the external wiring that might energize the load device.
①
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External fuse
External fuse
S7-1200 Functional Safety Manual
Manual, 02/2015, A5E03470344-AA