➭ page 16, Installation
4.4. Redundant system architecture
Besides the already described typical safety instrumented system including an
actuator, safety can be increased by implementing a second, redundant actuator
with fail safe unit into the safety instrumented system.The decision on the appropriate
version depends on the entire system. Considering the illustrated redundant system
setup, the actuator paired with actuator controls and fail safe unit complies with
Safety Integrity Level SIL 3 for the Safe ESD function according to IEC 61508.
Figure 3: Redundant system with safe ESD for safe CLOSING
Figure 4: Redundant system with safe ESD for safe OPENING
Information
There is no reasonable option to create a redundant system for safe end position
feedback using two fail safe units. On the basis of the overall safety function of the
safety instrumented system, it must be generally verified whether a set-up with two
fail safe units actually results in HFT=1.
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with non safety-related actuators Safety instrumented system and safety functions
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