Glossary · Functional safety
Logic solver
Also known as: safety logic solver
German: Logikverarbeitungseinheit
In process safety under IEC 61511, a logic solver is the part of a basic process control system or safety instrumented system that performs one or more logic functions, reading inputs from sensors and commanding final elements.
- Functional safety
- Integrated systems
In one sentence
A logic solver is the part of a safety instrumented system that evaluates sensor inputs and commands the final elements.
Example
In a burner management system, a safety PLC acts as the logic solver: it evaluates flame scanners and pressure switches and closes the fuel shutoff valves on loss of flame.
How it applies
- Engineering: A logic solver can be a safety PLC, a hardwired relay system or a solid-state logic system. For a SIS it must be suitable for the required SIL, which usually means a device developed and certified to IEC 61508.
- Application program: The safety application logic is part of the SIS. IEC 61511 sets requirements for its specification, development, testing and modification.
- Commissioning: Factory and site acceptance tests check each input, output and logic path against the Safety requirements specification (SRS).
- Documentation: Document the cause-and-effect matrix, application program version, safety manual conditions of the device and access restrictions. Keep the version in the manual consistent with the version installed in the plant.
Logic solver vs. BPCS controller
The basic process control system controls the process during normal operation. A SIS logic solver acts only on demand. IEC 61511 requires independence between them, or a justification where they share equipment, because a common failure could cause the demand and disable the protection at the same time.