The logic solver reads the sensors, applies voting and logic, and drives the final elements. For SIL 1 to 3 this is typically a certified safety PLC; simple functions may use certified safety relays.
Requirements
- Certification for the target SIL, used within the conditions of its safety manual (configuration, diagnostics settings, proof test interval, environmental limits).
- Separation from the basic process control system: separate hardware, or a combined system only where the integrity of the separation is demonstrated.
- De-energise-to-trip outputs with monitoring of field circuits where energise-to-trip is used.
- Diagnostics configured and acted on (out-of-range inputs, line monitoring).
- Access security and change control for the application program.
Contribution to PFDavg
Certified safety PLCs usually contribute a small share of PFDavg thanks to internal redundancy and diagnostics. The input and output modules count too; the PFD figures in the safety manual normally state what is included.
In First Out AI
The logic group has "Internal fault tolerance" for redundant controllers, a long proof test interval (often 10 years) and high coverage by default. The proof test draft checks voting, delays and the program version.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.